Podcasts by VK6FLAB
There is a perception in the community that the hobby of amateur radio is an expensive way to have fun. While it's entirely possible to spend thousands of dollars on equipment, in much the same way that it's possible if your preferred hobby is golf, getting started does not have to require that you start planting money trees.
Lots of fun can be had using cheap amateur radio transceivers that are used all around the world. If you do start with such a radio, the chances are good that you'll come across amateurs who make disparaging remarks about the lack of compliance of such radios.
When I say compliance, I'm talking about specific measurements specified by the International Telecommunications Union, the ITU. When you transmit on a specific frequency, there are rules about how much that signal is allowed to be unintended, or to use the official term, spurious emissions.
Specifically, the signal you transmit has to meet the requirements for the mode you're using and it must also stay within limits on other frequencies. For example, if you have a 2m handheld radio that uses FM, the transmitter must stay within the required width for FM and it's not allowed to transmit above a certain level on any of the harmonic frequencies.
When someone claims that all cheap radios are non-compliant, they're saying that such radios are either not transmitting a valid FM signal, or that the levels of the signal exceed the limits specified by the ITU.
Given that such radios are in wide use, Randall VK6WR, Glynn VK6PAW and I got together to see if we could come up with something a little more scientific in the way of comment about such radios. With access to Randall's HP 8920A RF Communications Test Set we came up with a repeatable way to test a radio and then went to the local HAMfest where we subjected a pile of radios to our tests. In total we did 75 tests. Overall we tested 39 distinct models across 12 brands.
So, what did we learn?
All so-called "name brand" radios were fully compliant.
All radios that were sold in Australia by Australian distributors were compliant.
Baofeng radios made up the largest sample of inexpensive radios. Seven out of the 26 tested were compliant, eight were non-compliant and the rest, 11 were borderline. More on that shortly.
We also tested many radios that had been purchased online. We didn't test enough of each model to make specific comments, but it's worth pointing out that half of all the online radios were compliant.
Now, I mentioned borderline compliance. What we learnt was that there were some radios that fell within 6 dB of being compliant. The HP test set hasn't been calibrated for a while and we felt that allowing for a 3 dB random measurement error and a 3 dB systematic error would prevent us from marking a radio non-compliant when in fact it was. We categorised 16 radios as borderline.
Our report is of course public. You can find it on my GitHub page as both a PDF and a markdown document.
Whilst we were writing our report, we discovered uncorroborated suggestions that some radios might fail an emissions test after suffering unspecified damage in the output filtering stages. We looked at the schematic of one radio that suggests that a simple capacitor failure might cause a filter to fail without preventing the transmitter from operating.
This might mean that a non-compliant transmitter might be made compliant again by replacing the faulty capacitor. We haven't tried and we don't know if a failed capacitor actually makes a radio non-compliant or not, or even if such a failure could occur and if-so, how.
In other words, this might be a red-herring, we just don't know.
One other comment worth pointing out is that it was suggested that some radios might use a specifically designed antenna to suppress the second harmonic. Given that some radios failed only on the second harmonic spurious emission requirement, but not the third, this seems plausible. All radios we tested had removable antennas and were tested without an antenna, since compliance relates to the transmitter, not the antenna. It does raise a more interesting question. What happens if you fit a different antenna to the radio?
One adage that stands is that "you get what you pay for", but given the amount of cheap testing equipment available, it's relatively easy to test every handset in your shack.
I'm Onno VK6FLAB
Amateur radio is an activity enjoyed by many people around the world. How many exactly is cause for debate. The most recent official figure we have is from the IARU, the International Amateur Radio Union. In 2020 it counted over 3 million people, but an article written a year later puts that figure at 1.75 million.
In Australia there's a common narrative that the total amateur population is in undeniable decline, some think that it's on a stark decline. Interested in hard data, for years I've been collecting information around the amateur population in Australia and I can report that across the nine years that I have data for the total variation is within two percent and it's not a straight line down either. There was a dip in 2020, potentially associated with training and callsign allocation being moved from the Wireless Institute of Australia to the Australian Maritime College, something which is going to change again shortly when amateur licensing in Australia will revert to the regulator, the Australian Communications and Media Authority. If you're familiar with amateur licensing in Australia, that's not the only change, but that's not what I'm looking at today, mainly because the available information associated with the upcoming changes are limited at best, seemingly buried in invective at worst.
Back to the topic at hand. One of the often heard responses in relation to the decline of our hobby is recruitment of new amateurs. It's a topic that I've spent plenty of time over the past decade contemplating. How do you share the joy of amateur radio with a general public who is apathetic to the preconceived ideas associated with this hobby, you know, old white men sitting in the dark with Morse keys.
For the record, I prefer a shack with light and I still don't know how to use a Morse key, other than to make my radio beep. The rest is genetic.
In the quest for spreading the word there's a repeated emphasis on the young, often coalescing around the annual Jamboree on the Air, or JOTA, as organised between Scouting groups and radio amateurs. I have previously said that JOTA was how I first came across amateur radio, but at the time, aged 15 or so, I had no money for such endeavours and the experience didn't resonate with me until decades later. So, you could argue that this is what changed me into an amateur, but the reality is that I had to come across the hobby a few more times before I got interested enough to investigate, something which I have spoken about before, in short, Meg, then VK6LUX introduced me to the concept of controlling a 2.4 GHz drone using higher power than was permitted with standard Wi-Fi equipment. I was hooked and got my license less than a month later. I then discovered that I needed more permissions and set about studying, only to get distracted with everything I could already do. I'm still being distracted today.
So, JOTA is a potential touch point, but I see little evidence that the initial spark goes anywhere in a hurry. I'm not dismissing it as a way to have amateur radio gain relevance outside our own community, but perhaps there are other ways to make this happen. In the early days of my journey I attended country fairs with my club and we'd set-up a radio or six to show and tell. There was talk of doing this in a shopping centre, at the local hardware store and even brief discussions about doing this at the local electronics store. As enjoyable as this was, none of it ever appeared to generate any permanent interest and I don't recall seeing new amateurs suddenly appear at the club after any outings.
Last week Glynn VK6PAW and I, set-up at the local airport, YPPH, that's Perth International Airport if you're not familiar with the designation allocated by the United Nations arm, ICAO or the International Civil Aviation Organization. Perth has a public viewing area. It's situated at the south western end of runway 03/21. It's an elevated position with minimal shade, some seating and you're 320 m from the runway centreline. It's a place where plane spotters congregate and now a few radio amateurs.
One thing we have in common is an interest in radio. We were told that the plane spotters often listen to one or two frequencies and if they're into video, they might record one radio channel to include on their YouTube videos. When Glynn and I visited we had a few radios with us. When I say few, in amateur terms we only had about five or so, but I suppose that comes with the territory. As it happens, admittedly not by accident, our radios could receive airband frequencies, so we could tune to Perth Tower, Perth Arrival, Perth Ground, Perth Departure and Melbourne Central, all at the same time. Next time we'll likely bring some HF gear so we can also listen to HF aviation frequencies as well.
While I was hosting F-troop, the weekly net for new and returning amateurs, midnight UTC, every Saturday morning for an hour, Glynn was busy talking and sharing with the plane spotting community. There were conversations around what radios and antennas to use, how you could tune to more than one frequency at the same time, how you could use software defined radios, how to set-up radios so you could have different channels appear at the left or the right, in the middle, or somewhere in between, which will allow you to focus on a particular radio call as it happens. Also, I should mention a piece of software called rtl-airband which allows you to use an RTL-SDR dongle to do this at home, but I digress.
There was a steady stream of people looking at planes and their age was surprising, aged 3 to 93 or so. Of course not all were into the radio, but plenty were.
Afterwards it occurred to us that there might be other venues like this, attracting people who are interested in radio for their own purposes. I have no doubt that we'll be back to Perth Airport, but I suspect we'll also see if we can find some other spotters. Train, ship and other airports come to mind immediately. I can't wait to learn about other people's uses and interests in radio, even if radio isn't the main attraction in their hobby. Perhaps you can think of some that you'd like to share.
Getting on air and making noise is one way to get outside, but publicly listening to frequencies that others are interested in is a perfect excuse to play with radios.
I'm Onno VK6FLAB
For years I've been hosting a weekly net called F-troop. It's a one hour opportunity for new and returning amateurs to get together and share their questions, and sometimes answers, about anything and everything amateur radio, with side trips into astronomy, electronics, circuit boards, testing gear and whatever else takes our fancy on the day. The net runs for an hour every Saturday morning starting at midnight UTC, which for some is a time when they're fast asleep, though truth be told, several of our regulars are night owls.
In VK6 where I am, midnight UTC is a more reasonable 8am, unless we have another referendum when we can decide if we want daylight saving, or not. So far we've had four of those, yes, really, in 1975, 1984, 1992, and 2009, and each time daylight saving or summer time was rejected. All I'm saying is that the chances are good that midnight UTC is going to be 8am in VK6 for a while yet.
Anyway, that time of the morning affords me the luxury of getting out of bed at a sensible hour, having a shower, making a cup of coffee with my Significant Other, or SO, and ambling into my shack to get ready. It's a comfortable process, something I've done for over 12 years with very little in the way of variation with the exception of the 500th and 600th episodes which I hosted outdoors at a local radio club, complete with BBQ and many visitors. That and the Friday Night Technical Net with Reg VK6BQQ, but that's a story for another day.
Last week a good friend, Glynn VK6PAW, asked me if I wanted to go out and have some fun, and having been pretty much cooped up for several years now, of course I said "yes". We're going to the viewing platform at the Perth International Airport, that's airport code YPPH, where I'll host the net in whatever way we figure out at the time. It's not an event, we haven't told anyone about it, and telling you now won't ruin the surprise for anyone, since this weekly rambling hits the airwaves after F-troop concludes. I knew there was a reason.
Anyway, at this point you have every right to ask me, "Onno, why should I care?"
Indulge me and let me see if I can explain.
Most, if not all, of my amateur radio activities are planned. From time-to-time I might get in my car and drive to a nearby park and get on HF, but truth be told, I haven't done that for several years. I have regularly told you about contests I've done, often whilst operating portable, often with friends, but sometimes alone. I have activated all manner of things, climbed summits, played in parks, gone to lighthouses and other such places. Every, single, time, those activities were planned, often to within an inch of their life. What should I bring? Where am I going to set-up? What gear do I need? What spares are required? What logging tool is needed? Will I need food and water? You know, a typical 7p activity, Proper Planning and Preparation Prevents Piss Poor Performance.
This time the plan consists of: "Do you want to go to the airport?" and "Sure!"
Mind you, that's in the context of Glynn normally having several radios in his car and me not having a clue what to expect. The other day I actually had my first ever look at the location in Google Street View, only to discover that there's a shelter there, so hopefully we won't fry in the forecast 38 degrees Celsius, that's 100 degrees in Ray Bradbury's temperature scale, if you're wondering.
Now, on the whole, this is a pretty low risk activity. Nobody is going to die if I don't manage to get the net going, though I do have Echolink on my phone, which reminds me, I should probably check if that still works. I'll put a pencil and a notepad in my pocket for logging and I'll bring a bottle or six of water and probably some coffee. Sorry, I can't help myself.
In other words, it's entirely possible to get on air and make noise without having to go to the Nth degree of planning and still have fun. As it happens, fun is something that's been in short supply of late, so, that's also a welcome change.
As an aside, in a completely unrelated and random observation, I recently installed a new font on my computer, called Hack. It's mono-spaced, sans-serif, intended for source code, and licensed under the MIT License. I'm using it right now and I'm in love. So secretly, between you and me, that's what goes for fun around here. Oh, in case you're wondering, no, I did not get paid to say that, the authors have no idea I exist, unless they're unexpectedly radio amateurs, I'm just a happy user.
Also, if you're wondering about Echolink, no need to fret. I just tested and it just works straight out of the box. Gotta love that.
Now, here's a question for you. When was the last time you spontaneously got on air to make noise?
I'm Onno VK6FLAB
During the week I started a new project. If you know me at all, this is not unusual. Having worked in the IT industry for nearly 40 years it's also not unusual that projects have a way of surprising you and this project was no different.
Recently I've been talking about antennas, a topic close to the heart of many amateurs and one that garners a lot of opinion and in my experience, much less in the way of facts, so being a firm believer of facts, I set out to add some of those to the discussion.
After having described that the environment is not often discussed in the context of antenna behaviour, coupled with the personal experience that it has by far the biggest influence, I set out to discover if I could use my computing skills to simulate this problem to build a picture that would speak a thousand words. Prompted by a friend who shared with me a link to an opinion that stated that dipole antennas didn't have 2.15 dBi gain, but in fact, apparently, had 8.5 dBi gain, I was energised to find out where this number came from.
I figured I'd spin up some antenna modelling software, use a standard model of a dipole, then simulate it at various heights above the ground and see what I could learn. Any good journey starts with a single step, so I started with looking for a generic model of a dipole antenna. I've played in this space before, so I was familiar with the fact that most, but not all, antenna modelling tools use a piece of software called NEC2 to do the actual calculations. Its models are described using text files ending in the .NEC extension. This software goes back to punch card days, so the format for the NEC2 files is essentially a stack of punch cards, so much so that every line in the text file is called a card and any software that uses the underlying NEC2 tool describes it in that way.
I won't bore you with the syntax, it's, let's put it this way. If you've been around computers for as long as I have, you're familiar with a tool called "sendmail", which is known to be user-friendly, just very particular with whom it makes friends. The NEC2 card format is much the same. It's not that surprising, and for added nostalgia, NEC2 was written in FORTRAN, originally in 1981 at the Lawrence Livermore Labs by Jerry Burke and Andrew Poggio. It was later released to the public. There's translations to C and C++, but they use the same notion of cards, so no magic progress there.
I started learning the syntax, and eventually came across a text-book with an example of cards that describe a dipole. Mind you, there were plenty of disclaimers around how poorly the ground was simulated and wouldn't you know it, the file format uses meters as the dimension, rather than wavelengths, so as far as I can tell, you can't simulate a quarter wave antenna, you have to simulate one of a specific length, so much for using a standard model of a dipole.
I found a tool that uses Python to issue NEC2 commands and as a surprise to nobody, it too uses cards. I used it to discover that for a particular type of ground, at some unknown height, the optimum length for a 10m WSPR dipole antenna is 5,225.87 millimetres long, apparently. You know it's true, it says so right there on the screen. I'm skipping over having to compile the software that was supposed to be a ready made Python library, but I digress.
There was a tool, written in TCL, that visualised NEC2 output, last updated 18 or so years ago and I unsuccessfully tried to make it work. Then there were those who suggested to fire up some random Windows tool on my Linux box, but as far as I can tell, I'd have to do the height adjustments manually, not ideal if you want to visualise from say, ground to geostationary orbit, one millimetre at a time and output an image at every step.
I searched the net for others who would surely have trodden this path long before I came along, only to discover that my search-fu is clearly broken, or any website with promising information has long ago been booted off the Internet, leaving "For Sale" signs on the domain name.
I came across one file which simulated a dipole in free space. It had, to use the NEC2 terms, 11 cards. When I run that through "nec2c", it generates a 12 megabyte file with over 104-thousand lines of output. Only takes 650 milliseconds to generate. If only I could visualise it.
I also came across a whole range of physics programs, which is not that surprising, since essentially antenna design is physics, but those tools require that I start learning a whole new way of building antennas, apparently from electrons, preferably whilst getting a degree in physics with a specialisation in computational electromagnetics. Then there was the Wolfram Alpha notebook model for a simple dipole, only 3,200 lines of code, so, you know, trivial to use.
So, here's the thing. Has nobody in living memory simulated a dipole at more than three heights and documented the process? Am I really the first human on the planet to think of this?
Oh, yes, I did find a project that simulated different lengths of dipoles, but I'll leave those for another day. And finally, I also found "xnecview", which does generate images, but it too is very particular whom it makes friends with and I've yet to discover if it can generate what I'm looking for.
As for the 8.5 dBi, I'm still looking. My current best guess is that at some specific height a dipole has an ugly spike that has 8.5 dBi gain and that someone used that number without looking at the detail, but, who knows, there's plenty of opinion to go around.
I'm Onno VK6FLAB
Let's compare the same antenna in different locations...
Over the years I've spent many hours building and testing antennas. I've talked about this and discussed how there is essentially an infinite variety of antennas that can exist. To give you a sense of this, picture a basic dipole antenna, two bits of wire, same length, connected to a feed-point. We're doing this experiment in space, so we're not concerned with trees or rope, or the ground for that matter, more on those shortly.
We can make this dipole straight, or we can make it into a V-shape, or bend over the edges, or make each side into a half-circle and join them, or make them into a spiral, or kink the wires, or bend them over, or any number of variations. Every time you change something, the antenna radiation pattern changes and the antenna behaves differently. While at its heart the antenna might still be considered a dipole, essentially a change in radiation pattern effectively means a different antenna.
In those changes or wire orientation alone we have already defined an infinite number of antennas, but that only scratches the surface. We can build an infinite variety of physical antennas. Consider the design of vertical antennas, loop antennas, log periodic antennas, yagi antennas, slot antennas, and beverage antennas to name a few.
Once you start investigating antennas you'll discover just how many options there are and once you've acquired the antenna of your dreams, the work is only just beginning.
To explain why this is the case, consider the process of finding an antenna to buy or build. You'll find breathless reports of how amazing an antenna is and how it allowed the operator to hear a mosquito land on the back of a container ship in the middle of a tropical cyclone whilst the sunspot activity was at an all time low. Right next to those reports you'll find another amateur describing how their dummy load performed better and cost less.
If not those specific examples, you'll have no doubt found both positive and negative reviews for the very same antenna, often side-by-side and if you don't, you're not looking hard enough.
Leaving aside the notion that someone is trying to discredit a commercial competitor or that the antennas are inadvertently physically different, because someone put it together incorrectly, there's plenty of opportunity for other reasons for this wide range of opinion.
Let's take the popular G5RV antenna, invented in 1946 by Louis G5RV, who became a silent key on June 28, 2000. The antenna is a multi-band HF antenna and there are plenty of people offering plans and kits for this antenna. Ignoring the differences in plans, let's imagine that two amateurs purchased the exact same G5RV from the same batch from the same supplier. Both erect their antennas at their home shack, or QTH and get on air to make noise. At a local BBQ they get together and compare notes only to discover that the two antennas are behaving completely differently.
How is this possible? What other factors might cause this experience?
You're not going to like my answer, but "it depends". The height at which the antenna is erected, how tight you pull it between two trees, how you feed it, the type of coax you use, how much power your transmitter uses, how close it is to another object like a fence or a house, what type of ground is below the antenna, what the local noise floor is like, which direction it's oriented, which day you use it and finally, what colour clothes you're wearing at the time.
That last one isn't strictly true, but it serves to highlight that some differences exist that are so innocuous as to be laughable, for example, have you considered the type of tree and how much foliage there is, when the lawn below the antenna was last watered, etc. My point is that some differences aren't obvious, but they can, and do, make an antenna behave differently.
In other words, the environment around two identical antennas is hardly ever the same and thus the antenna system as a whole, since the environment and the antenna together combine into a system, are never the same.
This means that when you go about finding an antenna that's suitable for you, the reviews you read are only part of the story. If the antenna needs ground radials that are physically not possible at your site, then that antenna is unlikely to be suitable for your situation, regardless of the glowing reviews.
As I said, in my time I've built and bought plenty of antennas. I've also tried several by way of my local amateur radio club. I've operated a mobile station from my car, set-up a portable station in numerous locations using the exact same antenna, and learnt that while the environment is almost never discussed, it has by far the biggest influence on the performance of your antenna.
My recommendation is to pick an antenna, any antenna, cheap is good, and start. Play with it, change how you erect it, set it up in different locations and I'd highly recommend that you do this with a friend. Between the two of you the shared installation can be used as a baseline to compare your own antenna against and if you're both comparing notes against what you built together, you'll both have a better chance of understanding what particular difference matters in your own setup.
I'm Onno VK6FLAB
All antennas have a radiation pattern that charts on a sphere where it radiates more and where it radiates less than the theoretical isotropic radiator. This comparison is expressed as dBi antenna gain.
There is a fundamental concept in antenna design called "reciprocity". Essentially it means that transmit and receive behaviour of an antenna is identical. In other words, the radiation pattern of an antenna applies for both transmitting and receiving of signals.
Unfortunately, this does not mean that if two stations are communicating and one can hear the other, the reverse is also true. Let me explain why.
Let's set the scene. Imagine two stations, me, VK6FLAB at Lake Monger, in Perth, Western Australia and Charles NK8O in the Lake of the Ozarks state park within the Ozark Mountains in central Missouri. We're both on the 10m HF band and in this story I've finally managed to learn Morse code and I'm "talking" to Charles, mind you, Charles apparently does have a microphone, so perhaps this might actually happen one day.
To simplify things, we both have the same antenna, the same radio, the same power level, we both love low power or QRP operation, and while we're keeping it simple, we have the same ground conductivity and we're both experiencing the same very low noise levels. While we're constructing this fantasy, the communication paths for both our stations are identical. Note that I said paths, more on that shortly.
In that situation, both Charles and I have the same experience. We can hear each other at the same level, our S-meter has the same reading, and apart from my current inability to actually use Morse code, our readability is identical. You might think this is "reciprocity", but it's not as simple as that.
I'm parked near a lake in the middle of a city and often other vehicles come and go. One new arrival has a solar panel on the roof with a noisy inverter and suddenly the local noise in my location jumps from S0 to S6.
The vehicle arrives whilst I'm transmitting, so at first nothing happens. Charles continues to hear my signal at the same level and at my end I'm blissfully unaware of any change, until I stop transmitting, when I hear the noise. Meanwhile, Charles starts his transmission and I cannot hear him because the local noise in my location is louder than his wanted signal. At this point, Charles still has the ability to hear me, but I can no longer hear him, even though our equipment is identical. The only change is the local noise floor at my location which interferes with my ability to receive the signals coming from Charles.
This means that I can still send "again, again, local QRM" and I can do so as often as I want. Charles will hear this without any issue, but I won't hear his reply until the local noise stops.
What this highlights is that two-way communication between two stations involves two signal paths. They might, or might not, follow the same journey through the ionosphere and be between two identical antennas, but the experience for either station can be, and almost always is, completely different.
Because the ability to transmit isn't affected by local noise at the transmitter, it doesn't figure into the total path loss when you're calculating it for the receiving station. However, when the roles are reversed, it does. So when you're receiving, you need to take into account your local noise, but when you're transmitting, you don't.
So, when Charles is transmitting to me, I need to take into account my local noise and ignore his, and when I'm transmitting to Charles, he needs to take into account his local noise, but not mine.
This is how you can have so-called "alligator" stations, all mouth, no ears. The station is likely using high power with a high gain antenna in a noisy environment. This means that everyone can hear them, but because their local noise is so high, they can often only hear other alligators, but not the rest of the world who can perfectly hear them. If you encounter a station on-air that keeps calling CQ, regardless of how many people are calling back, that's an "alligator".
So, the takeaway is that even if you can hear a station, it doesn't mean that they can hear you and the reverse is also true. You can be transmitting and heard all over the place, but if you're in a high noise environment, you might not be able to hear them. It's one reason that QRP stations prefer to work in low noise environments where they can hear many more stations.
It reminds me of a funny story told by Wally VK6YS, now SK. In his early amateur radio days he operated from Cockatoo Island, an island off the north coast of Western Australia, near Yampi Sound, which is where his callsign came from. With a new radio and transverter for 6m, Wally had been calling CQ for weeks, but nobody would talk to him. Occasionally he'd hear a faint voice in the background. Meanwhile it transpired that amateurs across Japan were getting upset that he wasn't responding to their 20 and 40 over 9 signal reports. His transmission was getting out just fine, his receiver wasn't working nearly as well. Turns out that during manufacturing, a pin on the back of his transverter hadn't been soldered correctly. Once he fixed that, he worked 150 Japanese stations on the first day and a lifelong love of the 6m band was born.
In other words, just because someone can hear you, doesn't mean that you can hear them, sometimes it's noise and sometimes its a faulty connector.
I'm Onno VK6FLAB
After recently talking about noise, today I want to discuss gain, specifically antenna gain. When you say that your antenna has 18 dBi gain, what does that mean?
This entire discussion starts with an isotropic radiator or antenna. It's often described as the perfect antenna, but rarely is there any description on how that actually works, so I'd like to start there.
Before we dig in too much, it's worth remembering that an isotropic antenna is a thought experiment, it cannot physically exist, but it's a useful tool for comparing antennas.
Antennas have a physical size. There's often a direct relationship between the size of the antenna and the frequencies for which it works best. A lower frequency means a longer wavelength and corresponding large antenna to handle that radio frequency. In contrast, an isotropic antenna is infinitesimally small and responds equally well for all frequencies.
Similarly, unlike an actual antenna, an isotropic antenna is symmetric in all directions, that is, there's no difference between the back or the front, the top or the bottom, the left or the right. You can position an isotropic antenna in any orientation and there's no difference, not just no detectable difference, no actual difference. The radiation pattern is a perfect sphere.
As I said, the isotropic antenna is an imaginary, let's call it, ideal antenna, that's used as the base reference to measure all antennas against.
When you use the word gain in relation to an antenna, you're using the unit dBi and in doing so, you're comparing the antenna against this imaginary perfect isotropic antenna.
When you see that the gain of an antenna is 2.15 dBi, you're saying that this antenna performs better than the isotropic antenna and does so by 2.15 dB.
There's one "minor" detail missing in that statement.
The full statement, often completely overlooked, is that this antenna performs better than the isotropic antenna and does so by 2.15 dB, in some directions, but not all.
Said differently, antenna gain comes from distorting the ideal, perfect sphere into different shapes. For example, the 2.15 dBi gain of a horizontal dipole antenna distorts into a squashed doughnut on its side.
In other words, there are directions where a dipole radiates better and has an increased gain when compared to an isotropic antenna, but there are also directions where it radiates worse, much worse, if at all. In the case of a dipole, it receives best from the side and worst in line with the antenna and I'll point out that the doughnut is also an idealised shape that in turn gets distorted by proximity to other objects, like the ground.
Consider that a dipole has 2.15 dBi gain over an isotropic antenna. This means that, for some directions the gain is increased and for some directions it's decreased, perhaps even eliminated. In other words, in some direction, the antenna amplifies the signal and in other directions it attenuates the signal, potentially even to zero at a so-called null in an antenna radiation pattern.
As I've said before, an antenna receives a combination of both wanted signal and unwanted noise. For an isotropic antenna all signals, from any direction, both wanted and unwanted, are treated the same. This is not true for an antenna that has gain.
Consider an antenna that exhibits gain in one specific direction and loss in all other directions. If you were to point that antenna at a wanted signal, the incoming signal would be amplified in that direction and attenuated in all other directions. If noise comes from all directions equally, most of the noise would be attenuated and only a little bit of noise coming from the same direction as the wanted signal is amplified.
Overall, this means that the total amount of incoming noise is reduced in comparison to the wanted signal. In other words, the noise floor is reduced and the signal level is increased, making the signal more audible above the noise.
This means that the impact of antenna gain is that the Signal to Noise Ratio is improved for an incoming signal in comparison to local noise.
Notice also, that the antenna gain works in multiple ways. It serves to improve the local signal to noise ratio, by attenuating noise and amplifying a wanted signal, but it also increases the transmitted signal that's sent towards the other station.
Both affect your station's performance, but do so at different sides of the communication link and because we're talking about two separate signals, an incoming one and an outgoing one, the optimal direction might not be the same for both.
So, now what do you think the impact might be of adding an 18 dBi Yagi to your station?
I also have a supplementary question. If a commercial antenna is compared with a dipole, using the dBd unit, is the antenna compared to the entire radiation pattern of a dipole and if so, at what height from what type of ground and is that a useful comparison, or hiding the true performance of such an antenna?
I'm Onno VK6FLAB
Today I'd like to talk about noise, but before I do, I need to cover some ground. Recently I explored the idea that, on their own, neither antenna, nor coax, made a big difference in the potential for a contact when compared to the impact of path loss between two stations.
I went on to point out that you'd be unlikely to even notice the difference in normal communications. Only when you're working at the margins, when the signal is barely detectable, would adding a single dB here or there make any potential difference.
In saying that, I skipped over one detail, noise.
Noise is by definition an unwanted signal that arrives together with a wanted signal at the receiver. In HF communications, noise comes from many sources, the galaxy, our atmosphere, and man-made noise from things like electrical switches, motors, alternator circuits, inverters and computers.
The example I used was my 10 dBm beacon being reported by an Antarctic station. My signal report was about 5 dB above the minimum decode level and based on signal path calculations, -129 dBm, or around an S0 signal level. What that statement hides is that this is in the context of a noise level that's lower than -129 dBm. Remember, a negative dBm value means a fraction of a milliwatt.
While you're considering that, think of the reality of an Antarctic station. This particular station, "Neumayer III" has three 75 kW diesel generators, a 30 kW wind turbine generator, 20 caterpillar trucks, 10 snowmobiles and 2 snow blowers and computers and technology to support 60 people, in other words, plenty of local noise.
This makes it all the more remarkable that my 10 dBm beacon was heard and that there was an amateur there to set-up the receiver in the first place.
Before I continue, picture mountain tops peaking through the top of a cloud layer as viewed from the window of an aeroplane. If the cloud layer increases in height, less and less mountain tops are visible, until at some point, only clouds are visible. Alternatively, if the cloud layer descends, more and more of the peaks are visible, until at some point no cloud remains and you see the mountains in all their magnificent glory.
In that analogy, mountains represent signals and the cloud layer is the equivalent of the noise floor, and in a similar way, signals can be heard or not, depending on the relationship between the level of noise in comparison to the level of the signal. There's a name for this, it's called the signal to noise ratio or SNR, where a value of 0 dB means that noise and signal are at the same level, negative SNR values mean that the signal is weaker than the noise, positive SNR values means that the signal is stronger than the noise. If you know the power level in dBm for both the noise and the signal, you can subtract the two and end up with the signal to noise ratio.
In reality, all receiving stations have to contend with noise.
If I arbitrarily set the local noise floor at -100 dBm, somewhere halfway between S4 and S5, I'll mostly get laughed at by many stations, either because it's too high or too low. In case you're wondering, I've worked my station in both S0 noise and S9 noise environments and it's fun trying either and comparing. It's one of the reasons I often use a mobile station, to get away from urban noise around me, and you don't have to go far, a local park might be far enough from local noise to whet your appetite.
Besides, -100 dBm is a nice round number to play with.
You might recall that a typical path loss number for a 2,500 km contact on HF on the 10m band is about 129 dB. With a noise floor of -100 dBm, we immediately know how much output power is required to be heard above the noise. If the received signal has to be at least more than -100 dBm and we know that the path loss is 129 dB, then our transmitted signal needs to at least be enough to make up the difference.
Said differently, if our output power is too low, the signal at the receive station will fall below the noise and they won't be able to hear us.
So, if we start at say 30 dBm, have a path loss of 129 dB, we'll end up at -99 dBm, which is 1 dB above -100 dBm. Said in another way, the SNR for this is 1 dB.
I'd like you to notice something.
I've said nothing about the noise floor at the transmitter. We could have low noise, or horrendous noise, either way, it makes no difference to the receiver. What it hears is entirely dependent on the noise floor at the receiving station.
I wonder if that observation changes anything about what you think the impact might be of adding an 18 dBi Yagi to your station?
I'm Onno VK6FLAB
Recently I explained some of the reasons why I've shifted to using dBm to discuss power. You might recall that 1 Watt is defined as 1,000 mW and that's represented by 30 dBm. 10 Watts is 40 dBm, 400 Watts, the maximum power output in Australia is 56 dBm and 1,500 Watts, the maximum in the USA, is just under 62 dBm. My favourite power level, 5 Watts, is 37 dBm.
I mentioned that using dBm allows us to create a continuous scale between the transmitted power and the received signal. On HF, an S9 report is defined as -73 dBm. Between each S-point lies 6 dB, so an S8 signal is -79 dBm, S7 is -85 dBm and so-on to S0, which is -127 dBm. Said differently, to increase the received signal by one S-point you need to quadruple the power output.
Now, let's consider a contact with a 100 Watt station, 50 dBm. Let's imagine that the receiver reports an S8 signal. That means that between a transmitter output of 50 dBm and the received signal at -79 dBm, there's a loss of 129 dB. If we dial the power down to 5 Watts, our 37 dBm will be received at -92 dBm, and earn a S6 report, which, in my experience, is pretty common. If we instead use the maximum power permitted in Australia, we'd gain 6 dB and end up at -73 dBm, or S9. The maximum power output permitted in the United States, 62 dBm, is only 6 dB higher and not even enough to get you "10 over 9" at the other end.
At this point I could say, see, "QRP, when you care to send the very least", and be done with it. While it's true in my not so humble opinion, that's not where I'm going with this.
That 129 dB of loss is made up of a bunch of things. For example, there's the coax loss at either end, the antenna gain at either end and a big one, the path loss between the two antennas.
Let's assume for a moment that coax loss and antenna gain cancel each other out. You might think that's nuts, but consider that 100 m of RG58 coax on the 10m band has a loss of around 8 dB and a dipole has an isotropic gain of 2.15 dBi. In case you're not sure what that means, a dipole has a gain of 2.15 dB over the ideal radiator, a theoretical isotropic antenna. Now it's unlikely that you are going to connect a dipole to 100 m of RG58, so let's say a quarter, or 25 m instead. The coax loss is also quartered, or about 2 dB, which pretty much means that your dipole gain and your coax loss essentially cancel each other out.
So, as a working number, assuming both stations are similar and ignoring SWR mismatch, pre-amplifiers, filters, and all manner of other tweaks in the signal path, 129 dB loss is a good starting point to work with. If you use a free space path loss calculator, that's the equivalent of the loss for a 2,500 km contact on HF on the 10 m band.
Now, if you were to replace the RG58 with something like RG213 coax, the loss drops from around 2 dB to 0.9 dB, so your signal just increased in strength by 1.1 dB, or not enough to make any difference in this example.
Of course there's a benefit in using lower loss coax, I mean, 1.1 dB gain isn't nothing, but it really only matters when the conditions are marginal. If you're going to run your coax to the other side of a paddock, you might discover that your signal changes by a whole S-point, but realistically, most of the time you're not going to notice.
Similarly, and perhaps more importantly, in the scheme of things, your antenna is also just fiddling around the edges when compared to the path loss of 129 dB. For example, if you double your antenna gain, you're only seeing an improvement of half an S-point and most likely you won't actually notice.
Before you grab the nearest chicken to pluck feathers to come after me with, I'd like to point out that each element on their own has a minimal impact on the total system, but that doesn't mean that improving your station is useless, far from it. If you use quality coax, have an antenna that is performing well, is a good match to your transmitter and coax, use appropriate filters and pre-amplification, you're likely to make more contacts more often, but the bottom line is that you actually need to be on air to make noise and ultimately that's going to represent the biggest improvement in your station performance.
Case in point, the other day my WSPR or Weak Signal Reporter beacon, with 10 dBm output, was reported 7,808 km away by DP0GVN, the club station of the German Antarctic Research Station "Neumayer III" in Dronning Maud Land, Antarctica, a first for me. WSPR reported that as a signal of -26 dB.
Previously I proved that when WSPR reports -31 dB, about 75% of decodes are successful. In other words, we can think of my report as being 5 dB above the minimum decode level. This is interesting for several reasons, least of which is that a report of -26 dB doesn't appear to have a relationship to anything else, something which I've observed before.
Looking further, if we use our notional 129 dB loss figure and start at the beacon power of 10 dBm, we end up at -119 dBm, which is between S1 and S2. In reality, the path loss for that contact is more likely to be in the order of 10 dB worse, making the signal at the receiver -129 dBm or around S0. In those kinds of marginal conditions, where there's 5 dB between being heard and not, finding an extra dB or two in better coax or antenna is absolutely worth the investment, but if you're in a contest making points, you're not going to care. Being on the right band, pointing in the right direction and being on-air making contacts is going to be much more important.
That said, I'll leave you with a question. Given our obsession with antennas, what might the impact be of adding an 18 dBi Yagi to your station?
I'm Onno VK6FLAB
The other day I went looking for a software defined radio or SDR for HF. This happened because all such devices on my desk are rated at higher frequencies and I've still not managed to fix the broken SMA board connector on the transverter I purchased over a year and a half ago.
In case you're wondering, the design has two SMA connectors attached at either end of a printed circuit board, also known as a PCB. The board slides into a metal case and both connectors are tightened to either side of the case, which causes the problem when the circuit board is slightly shorter than the case and the nuts pull the connector apart, causing the device to fail.
Replacing the SMA board connectors would be relatively simple, but they appear hard to come by and the micro SMA connectors that a friend purchased to help, changed the task into finding adaptors, which I've not managed to solve yet.
I'm detailing this all for a purpose, trust me.
Anyway, the hunt for an SDR for HF lead me to a project called "Radioberry". It's a design by Johan PA3GSB which is designed to be a so-called "hat" for a Raspberry Pi. Think of it as an expansion card to create functionality, in this case a radio capable of transmitting and receiving on HF, covering 0 to 30 MHz, perfect for my current needs.
The design uses a Raspberry Pi computer to power and control the board, including programming the on-board FPGA, accessing the actual data and sharing that with the user, either via a touch screen, or using USB, Ethernet, Bluetooth or Wi-Fi. The board itself has two external connectors, one for transmit, one for receive and when you combine it with the Pi, fits neatly into a box which you could 3D print. Amplifier and band filters are left as an exercise to the enterprising amateur, though there is an amplifier design on the github repository. If you're curious, it's based on the work by the Hermes Lite 2 group.
Johan specifically doesn't sell this device, instead you can choose to buy it from other enterprising individuals, or better still, build your own. Over the last few years I've started noticing several people in the so-called maker community, people, who a lot like radio amateurs, build stuff for fun, using online printed circuit board services.
If you're unfamiliar with the concept, you can design a schematic, layout a PCB, have it manufactured and optionally even built and sent to you. To get an idea of what this might look like, I picked a random online supplier, uploaded the specifications for a Radioberry and costed the whole thing. Suffice to say that the biggest charge is the $50 set-up fee.
Any enterprising engineer would have punched the "Buy Now" button and be done with it, but in some things I'm pretty cautious, so I haven't, yet. I don't know enough about the design or schematic to know how it works, to troubleshoot it, to fix any potential issues, or even to know what kinds of issues there might be, even if they're obvious to anyone with electronics experience.
To make it clear, my electronics experience is rudimentary at best. I'm comfortable with block diagrams, understand the basic principles behind most passive elements, but if you're going to get into trace length and signal timing, I'm not anywhere even remotely qualified to troubleshoot, let alone spot problems. That's not to say that I am stopping before I start, the opposite is true.
I'm using this as an experience to gently get my feet wet.
Back to the apparently too detailed explanation of the transverter. Joining the dots you can probably guess where I'm going with this. Given the access to countless documented transverter designs, I feel comfortable enough to work on a design, construct a PCB and have it manufactured. At the rate I'm going, that should get a solution before I can find a PCB edge-mounted SMA connector, well, at least that's my excuse. I'm also eyeing off this same process to build a logging volt meter, since the Internet seems to believe that I should pay hundreds of dollars for a volt meter and an I/O port, even if the chip inside costs all of $6.
Oh, the transverter I purchased a year and a half ago costs three times as much as having five of them built on demand, so there's that.
For all my life I've been a firm believer in software. I've also been on a computer driven manufacturing journey for a couple of years, still in the process of commissioning my new toys, much to the merriment of some of my fellow amateurs and the idea that I can have a circuit design built and shipped to my door just makes me tingle with anticipation.
If you're already ahead of me on this journey, please don't hesitate to point at any potholes on the road and if you're following along, if you break it, you get to keep both parts.
I'm Onno VK6FLAB
If you've been following my amateur radio journey, you'll have likely noticed that I've been straying from the fold. The words I use for power have been changing. I've reduced references to Watt and increased use of the term decibel.
Initially this was incidental, recently it's been more of a deliberate decision and I'd like to explain how this came to be. It starts with representing really big and really small numbers.
Let's start big.
On 14 September, 2015 the first direct observation of gravitational waves was made when a pair of black holes with a combined estimated weight of 65 solar masses merged. The signal was named GW150914, combining "Gravitational Wave" and the observation date to immortalise the event.
Following the collision, it was estimated that the radiated energy from the resulting gravitational waves was 50 times the combined power output of all the light from all the stars in the observable universe. As a number in Watts, that's 36 followed by 48 zeros. If you're curious, there's even a word for that, 36 Quindecillion Watts.
Now let's look at small. The typical signal strength received from a GPS satellite, like say by your phone, is about 178 attowatts, or in Watts, 0.000 and so on, in all, 13 zeros between the decimal point and then 178.
What if I told you that the energy associated with the collision of those two black holes could be expressed in comparison with a milliwatt. Remember, this collision emitted more energy than all the output of light from all the stars in the observable universe. The expression for all that power is 526 dBm.
Similarly, the tiny received GPS signal can be expressed as -127.5 dBm.
Just let that sink in. All the power in the observable universe through to the minuscule power received by the GPS in your phone, all expressed between 526 dBm and -127.5 dBm, and not a zero in sight.
As I mentioned, the unit dBm relates to a milliwatt. As a starting point, let me tell you that 1 Watt is 1,000 milliwatts and is represented by 30 dBm.
The decibel scale doesn't work quite the same as other number ranges you might be used to. Adding the value 3 doubles its size and adding the value 10 increases its size by a factor 10.
For example, to double power from 1 Watt or 30 dBm, add 3 and get 33 dBm, which is the same as 2 Watts. If you want to increase 1 Watt by a factor 10, again, starting with 30 dBm, add 10 and get 40 dBm which is 10 Watts. Similarly, 50 dBm is 100 Watts and 60 dBm is 1,000 Watts.
Going the other way, halving power, remove 3. So taking 3 from 60 dBm is 500 Watts or 57 dBm. Dividing power by a factor 10 works the same, take 10. So 47 dBm is 50 Watts and 37 dBm is 5 Watts.
If you get lost, remember, dBm relates to a milliwatt. 1 Watt is 1,000 milliwatts and is represented by 30 dBm. Divide by a factor 1,000, remove 30 and end up with 0 dBm, which is the same as 1 milliwatt. I'll say that again, 0 dBm is the same as 1 milliwatt.
It takes a little getting used to, but you can do some nifty things. For example, remove 10 to get a tenth of a milliwatt, or -10 dBm.
This same process of adding and subtracting applies in other ways too. Attenuation, or making a signal weaker, and amplification, or making a signal stronger can use the same rules.
For example, if you apply 3 dB of attenuation, you're making the signal 3 dB weaker, or halving it, so you subtract 3 dB from your power output. If your amplifier is rated at 6 dB gain, you're quadrupling the output and you add 6 dB to your power output.
Similarly, if you talk about the gain of an antenna, you add it. If the gain is 20 dBi, you add it to the power output. You can use this for coax loss calculations as well. A 100m length of RG-58 at 28 MHz has a loss of 8 dB. You can directly subtract this from the power output of the transmitter and know precisely how much power is making it to the antenna.
There's more. The radio amateur S9 signal strength on HF, something which we consider to be a strong signal, can be expressed as -73 dBm or a very small fraction of a milliwatt. An S8 signal is 6 dB weaker, or -79 dBm. A 20 over 9 report is -53 dBm. I will point out that this is at 50 Ohm.
As a result, we now have a continuous scale for all the elements in the transmission chain between the transmitter and the receiver.
While I'm here, I've already mentioned that negative dBm readings relate to fractions of a milliwatt, so values between 0 and 1.
This highlights one limitation of this scale. We cannot represent 0 Watts. Mind you, that doesn't happen all that often. The thermal noise floor in space at 1 Hz bandwidth, that's at 4 kelvins, is -192.5 dBm, which practically means the minimum level of power we need to express. It's also a good value to remember because if you're doing funky calculations and you end up with a number less than -192.5 dBm, you can pretty much guarantee that you've probably made a boo-boo.
0 Watts using the dBm scale is represented by negative infinity, or essentially a division by zero error, really not defined, so there's that.
I'm Onno VK6FLAB
Between decibels and milliwatts ...
As you might recall, I've been working towards using a cheap $20 RTL-SDR dongle to measure the second and third harmonic of a handheld radio in an attempt to discover how realistic that is as a solution when compared to using professional equipment like a Hewlett Packard 8920A RF Communications Test Set.
I spent quite some time discussing how to protect the receiver against the transmitter output and described a methodology to calculate just how much attenuation might be needed and what level of power handling. With that information in-hand, for reference, I used two 30 dB attenuators, one capable of handling 10 Watts and one capable of handling 2 Watts. In case you're wondering, it's not the dummy load with variable attenuation that I was discussing recently.
I ended up using a simple command-line tool, rtl-power, something which I've discussed before. You can use it to measure power output between a set of frequencies. In my case I measured for 5 seconds each, at the base frequency on the 2m band, on the second and on the third harmonic and to be precise, I measured 100 kHz around the frequencies we're looking at.
This generated a chunk of data, specifically I created just over a thousand power readings every second for 15 seconds. I then put those numbers into a spreadsheet, averaged these and then charted the result. The outcome was a chart with three lines, one for each test frequency range. As you'd expect, the line for the 2m frequency range showed a lovely peak at the centre frequency, similarly, there was a peak for the other two related frequencies.
The measurement data showed that the power measurement for 146.5 MHz was nearly 7 dB, for 293 MHz it was -44 dB and for 439.5 MHz it was -31 dB. If you've been paying attention, you'll notice that I used dB, not dBm or dBW in those numbers, more on that shortly.
From a measurement perspective we learnt that the second harmonic is 51 dB below the primary power output and the third harmonic was about 38 dB below the primary power output.
First observation to make is that these numbers are less than shown on the HP Test Set where those numbers were 60 dB and 62 dB respectively.
Second observation, potentially more significant, is that pesky dB thing I skipped over earlier.
If you recall, when someone says dB, they're referring to a ratio of something. When they refer to dBm, they're referring to a ratio in relation to 1 milliwatt. This means that when I say that the power reading was 7 dB, I'm saying that it's a ratio in relation to something, but I haven't specified the relationship. As I said, that's on purpose.
Let me explain.
When you use an RTL-SDR dongle to read power levels, you're essentially reading numbers from a chip that is converting voltages to numbers. In this case the chip is an Analog to Digital Converter or an ADC. At no point has any one defined what the number 128 means. It could mean 1 Volt, or it could mean 1 mV, or 14.532 mV, or something completely different. In other words, we don't actually know the absolute value that we're measuring. We can only compare values.
In this case we can say that when we're measuring on the 2m band we get a range of numbers that represent the voltage measured along those frequencies. When we then measure around the second harmonic, we're doing the same thing, possibly even using the same scale, so we know that if we get 128 back both times we might assume the voltage is the same in both cases, we just don't actually know how much the voltage is. We could say that there's no difference between the two, or 0 dB, but we cannot say how high or low the voltage is.
This is another way of describing something I've discussed before, calibration.
So, if I had a tool that could output a specific, known RF power level, and fed that into the receiver and measured, I could determine the relationship between my particular receiver and that particular power level. I could then measure at all three frequencies and determine if the numbers were actually the same for these three frequencies, which is what I've been assuming, but we don't actually know for sure right now.
So, at this point we need a known RF signal generator. The list of tools is growing. I've already used a NanoVNA to calibrate my attenuators and I've used a HP RF Communications Test Set to compare notes with.
At this point you might realise that we're not yet able to make any specific observations about using a dongle to make harmonic measurements, but you can make pretty pictures...
There's a good chance that you're becoming frustrated with this process, but I'd like to point out that at the beginning of this journey I can tell you that I had no idea what the outcome might be and obviously, that's the nature of experimentation.
If you have some ideas on how to explore further, feel free to get in touch.
I'm Onno VK6FLAB
Over the weekend a friend of mine convinced me to help plant some trees. Mind you, I was told that this was going to be a blue tree painting day. The Blue Tree Project is now a global awareness campaign that paints dead trees blue to spread the message that "it's OK to not be OK", and help break down the stigma that's still largely attached to mental health.
In the process, I learnt that my physical stamina is not what it once was and my current appetite for bending over and shovelling dirt is, let's call it, muted.
After the digging and the sausage sizzle under the branches of an actual blue tree, there was some opportunity for playing radio, something I haven't done in much too long. I wasn't sure when I last got into the fresh air to actually listen, but I must confess, the coax cable that I picked up out of my shed had been hanging there for several years.
The location where we planned to play was in a rural setting, right next to a dam, which surprisingly actually had water in it. The idea was to set-up a vertical antenna with a couple of ground radials, plug in a radio and have a listen. I have to say, after the digging I was really looking forward to this.
My piece of coax, about 20 meters long, was used to connect the antenna to the radio so we could sit in the shade whilst the antenna stood out in the sun near the dam.
The antenna, a telescopic one, came with a ground spike and about eight radials and needed to be tuned to some extent, as-in, near-enough is close enough, since we had an antenna tuner with our radio. To achieve the tuning we wanted to connect a NanoVNA to the coax which was the first challenge. The BNC connectors on my coax were pretty dull, likely a combination of poor quality, accumulated dust, humidity and lack of use.
As an added bonus the centre pin on one end seemed a little bent.
After working out how to get an SMA adaptor into the connectors we were in business. Connected up between the antenna and the NanoVNA we set out to get things lined up. The SWR on the display, hard to read in the full sun at the best of times, seemed to be a little odd. Not something I could put my finger on, but if you've seen enough SWR plots you know what it's supposed to look like and for some reason it didn't.
We bravely carried on, connected the radio to the coax and started tuning around. Didn't seem to be a lot of activity on the 20m band. We couldn't hear the local NCDXF beacon which was odd. Also no FT8 activity, also odd.
If anything, it seemed like there was nothing happening at all.
Before we continue, I'll point out that this can happen with a big enough burp from the Sun. I hadn't seen any alerts, so I wasn't buying it. We removed my coax, plugged in something much shorter and the bands came alive with all the activity we'd been expecting.
And then it started to rain.
Seriously. Finally got out into the world, got radio activity going, had actual signals to tune to and it starts raining. Glynn VK6PAW and I took one look at each other, shook our heads and dashed for the radio to bring it under shelter. I put on my raincoat, and together we disassembled the antenna and the station and went home.
Clearly, my coax was faulty. Lesson learnt. Test your coax before you go out and you'll have a better outcome.
Today, a week later, I'm sitting on the floor of my shack with the offending coax between my legs, surrounded by adaptors, a NanoVNA, a RigExpert, a dummy load, a short and an open terminator. No matter how I test, no matter what I test, everything is as it should be. I can tell you that the Time Domain Reflectometry shows me that the coax is 25.8m long, useful information, but not really any surprise.
There's also no significant return loss, unless you head for 1 GHz, but even then it's perfectly respectable, if anything, better than I expected.
There are no loose connections, nothing rattling, nothing amiss.
The only thing that I can even begin to think might be the case is that one of the centre pins on one end of the coax is slightly shorter. Combined with "close enough is good enough" when I attached the SMA adaptor in the field, might account for a connection that never got made, since the adaptor wasn't seated deep enough.
So, I'm not quite ready to cut off the connectors and re-terminate this coax. I'll be taking it into the field again, but I'll make sure that I bring an alternative, just in case. I'm also leaving the SMA adaptors connected to the coax. Future me will thank me.
Oh, yes, in-case you're wondering, I'm slowly working out how to improve my stamina. That was not fun. If you want to know more about Blue Trees and its message, check out the BlueTreeProject.com.au website and if you ever just want to talk, get in touch.
I'm Onno VK6FLAB
Before we start I should give you fair warning. There are many moving parts in what I'm about to discuss and there's lots of numbers coming. Don't stress too much about the exact numbers. In essence, what I'm attempting is to explore how we can reduce the power output from a transmitter in such a way that it doesn't blow up a receiver whilst making sure that the signal is strong enough that we can actually measure it.
With that in mind, recently I discussed the idea of adding a series of attenuators to a transmitter to reduce the power output by a known amount so you could connect it to a receiver and use that to measure output power at various frequencies. One hurdle to overcome is the need to handle enough power in order to stop magic smoke from escaping.
None of my attenuators are capable of handling more than 1 or 2 Watts of power, so I cannot use any of them as the first in line. As it happens, a good friend of mine, Glynn VK6PAW, dropped off a device that allows you to divert most of the power into a dummy load and a small amount into an external connector. In effect creating an inline attenuator capable of handling 50 Watts.
The label doesn't specify what the attenuation is, so I measured it using a NanoVNA. To make our job a little interesting, it isn't constant. Between 10 kHz and 1 GHz, the attenuation decreases from 70 dB to 10 dB. We want to measure at a base frequency on the 2m band and its second and third harmonic. The attenuation at those frequencies varies by 11 dB, which means we'll need to take that into account.
So, let's subject our currently imaginary test set-up to some sanity checking. Our receiver is capable of reading sensible numbers between a signal strength of -127 dBm and -67 dBm and we'll need to adjust accordingly.
If we transmit an actual 20 Watt carrier, that's 43 dBm. With 110 dB of attenuation, we end up at -67 dBm, which is right at the top end of what we think the receiver will handle. If we're using something like 5 Watts, or 37 dBm, we end up at -73 dBm, which is well above the minimum detectable signal. Our best harmonic measurement was around -30 dBm, which means that with 110 dB of attenuation, we end up at -140 dBm, which is 13 dB below what we think we can detect.
So, at this point you might wonder if this is still worth our while, given that we're playing at the edges and to that I say: "Remind me again why you're here?"
First we need to attenuate our 20 Watts down to something useful so we don't blow stuff up. Starting with 110 dB attenuation, we can measure our base carrier frequency and its harmonics and learn just how much actual power is coming out of the transmitter. Once we know that, we can adjust our attenuation to ensure that we end up at the maximum level for the receiver and see what we are left with.
So, let's look at some actual numbers, mind you, we're just looking at calculated numbers, these aren't coming from an actual dongle, yet. Using Glynn's dummy load as the front-end, at 146.5 MHz, the attenuation is about 30 dB. If we look at a previously measured handheld and rounding the numbers, it produced 37 dBm. That's the maximum power coming into our set-up. With 30 dB of attenuation from Glynn's dummy load, that comes down to 7 dBm. We'll need an additional 74 dB of attenuation to bring that down to -67 dBm, in all we'll need 104 dB of attenuation.
The third harmonic for that radio was measured at -26 dBm. So, with a 104 dB of attenuation that comes out at -130 dBm, which is below the minimum detectable signal supported by our receiver. However, remember that I told you that our dummy load had different attenuation for different frequencies? In our case, the attenuation at 439.5 MHz is only 19 dB, not 30, so in actual fact, we'd expect to see a reading of -119 dBm, which is above the minimum detectable signal level.
I realise that's a lot of numbers to digest, and they're specific to this particular radio and dummy load, but they tell us that this is possible and that we're potentially going to be able to measure something meaningful using our receiver. I'll also point out that if you're going to do this, it would be a good idea to take notes and prepare what numbers you might expect to see because letting the magic smoke escape might not be one of your desired outcomes.
Speaking of smoke, what happens if you consider changing the attenuation when you're measuring at another frequency, like say the second or third harmonic and you see a reading close to, or perhaps even below the detectable signal level as we've just discussed. You might be tempted to reduce the attenuation to increase the reading, but you need to remember that the transmitter is still actually transmitting at full power into your set-up, even if you're measuring elsewhere. This is why for some radios you'll see a measurement that states that the harmonics are below a certain value because the equipment used doesn't have enough range to provide an actual number.
To simplify my life, using a NanoVNA, I created a spreadsheet with 101 data points for the attenuation levels of Glynn's dummy load between 10 kHz and 1 GHz. I charted it and with the help of the in-built trend-line function determined a formula that matched the data.
I've also skipped over one aspect that needs mentioning and that's determining if the receiver you're using to do this is actually responding in the same way for every frequency. One way you might determine if that's the case is to look at what happens to the signal strength across multiple frequencies using a dummy load as the antenna. One tool, rtl_power might help in that regard.
Is this going to give you the same quality readings as a professional piece of equipment? Well, do the test and tell me what you learn.
I'm Onno VK6FLAB
Recently I had the opportunity to use a piece of professional equipment to measure the so-called unwanted or spurious emissions that a transceiver might produce. In describing this I finished off with the idea that you could use a $20 RTL-SDR dongle to do these measurements in your own shack. I did point out that you should use enough attenuation to prevent the white smoke from escaping from your dongle, but it left a question, how much attenuation is enough?
An RTL-SDR dongle is a USB powered device originally designed to act as a Digital TV and FM radio receiver. It's normally fitted with an antenna plugged into a socket on the side. I'll refer to it more generically as a receiver because much of what we're about to explore is applicable for other devices too.
Using your transceiver, or transmitter, as a signal source isn't the same as tuning to a broadcast station, unless you move it some distance away, as-in meters or even kilometres away, depending on how much power you're using at the time. Ideally we want to connect the transmitter output directly to the receiver input so, at least theoretically, the RF coming from the transmitter stays within the measuring set-up between the two devices.
Assuming you have a way to physically connect your transmitter to your receiver we need to work out what power levels are supported by your receiver.
For an RTL-SDR dongle, this is tricky to discover. I came across several documents that stated that the maximum power level was 10 dBm or 0.01 Watt, but that seemed a little high, since an S9 signal is -73 dBm, so I kept digging and discovered a thoughtful report published in August 2013 by Walter, HB9AJG. It's called "Some Measurements on DVB-T Dongles with E4000 and R820T Tuners".
There's plenty to learn from that report, but for our purposes today, we're interested in essentially two things, the weakest and strongest signals that the receiver can accommodate. We're obviously interested in the maximum signal, because out of the box our transmitter is likely to be much too strong for the receiver. We're going to need to reduce the power by a known amount using one or more connected RF attenuators.
At the other end of the scale, the minimum signal is important because if we add too much attenuation, we might end up below the minimum detectable signal level of the receiver.
Over the entire frequency range of the receivers tested in the report the minimum varies by about 14 dB, so let's pick the highest minimum from the report to get started. That's -127 dBm. What that means is that any signal that's stronger than -127 dBm is probably going to be detectable by the receiver and for some receivers on some frequencies, you might be able to go as low as -141 dBm.
At the other end of the scale the report shows that the receiver range is about 60 dB, which means that the strongest signal that we can use is -67 dBm before various types of distortion start occurring. For comparison, that's four times the strength of an S9 signal.
So, if we have a 10 Watt transmitter, or 40 dBm, we need to bring that signal down to a maximum of -67 dBm. In other words we need at least 107 dB of attenuation and if we have a safety margin of two, we'll need 110 dB of attenuation, remember, double power means adding 3 dB.
So, find 110 dB of attenuation. As it happens, if I connect most of my attenuators together, I could achieve that level of attenuation, but there's one further issue that we'll need to handle and that's power.
As you might recall, an attenuator has several attributes, the most obvious one is how much attenuation it brings to the party. It's specified in dB. My collection of attenuators range from 1 dB to 30 dB. Another attribute is the connector it comes with, I have both N-type and SMA connectors in my collection, so I'll need some adaptors to connect them together. One less obvious and at the cheap end of the scale, often undocumented, aspect of an attenuator is its ability to handle power. Essentially we're turning an RF signal into heat, so an attenuator needs to be able to dissipate that heat to handle what your transmitter is throwing at it.
I said that from a safety perspective I'd like to be able to handle 20 Watts of power. Fortunately we don't need all our attenuators to be able to handle 20 Watts, just the first one directly connected to the transmitter. If we were to use a 20 Watt, 30 dB attenuator, the signal through the attenuator is reduced to 0.02 Watts and the next attenuator in line only needs to be able to handle that power level and so-on.
To get started, find about 110 dB of attenuation, make sure it can handle 20 Watts and you can start playing.
Before you start keying up your transmitter, how might you handle a range of different transmitters and power levels and can you remove an attenuator when you test on a different frequency?
On that last point, let me say "No", you cannot remove the attenuator when you're measuring a different frequency.
I'm Onno VK6FLAB
At a recent local HAMfest we set-up a table to measure second and third harmonic emissions from any handheld radio that came our way. The process was fun and we learnt lots and in due course we plan to publish a report on our findings.
When we received a handheld, we would disconnect the antenna, and replace it with a short length of coax and connect it to a spectrum analyser. We would then trigger the Push To Talk, or PTT button and measure several things. We'd record the actual frequency and how many Watts that the transmitter was producing and then record the power level in dBm for the base frequency, double that frequency and triple that frequency. In other words, we'd record the base, second and third harmonics.
This resulted in a list of numbers. Frequency and power in Watts are obvious, but the three dBm numbers caused confusion for many visitors. The most perplexing appeared to be that we were producing negative dBm numbers, and truth be told, some positive ones as well, we'll get to those in our report.
How can you have negative power you ask?
As I've discussed before. A negative dBm number isn't a negative value of power, it's a fraction, so, -30 dBm represents 0.000001 Watts and you'd have to admit that -30 dBm rolls off the tongue just a little easier.
What we measured and logged was the overall transmitter output and at specific frequencies. As I've discussed previously, if you transmit using any transceiver, you'll produce power at the intended frequency, but there will also be unintended or unwanted transmissions, known as spurious emissions.
The International Telecommunications Union, or ITU, has standards for such emissions. In Australia the regulator, the ACMA, uses the ITU standard for radio amateurs, but I should point out that this might not be the case where you are. It's entirely possible, and given human diversity, probable even, that there are places where there are more stringent requirements, so bear that in mind.
I'll state the standard and then explain.
For frequencies greater than 30 MHz, the spurious emission must not exceed the lesser of 43 + 10 * log (power) or 70 dB.
That might sound like gobbledegook, so let's explore.
First thing to notice is that this is for transmissions where the transmitter is tuned to a frequency greater than 30 MHz, there's a separate rule for frequencies less than 30 MHz and the ITU also specifies a range of different limits for special purpose transmitters like broadcast radio and television, space services, and others.
Second thing is that the spurious emissions are calculated based on total mean output power. This means that your spurious emissions are considered in relation to how much power you're using to transmit and it implies that for some transmitters you can be in compliance at one power level, but not at another, so keep that in mind.
The phrase "the lesser of", means that from a compliance perspective, there's a point at which power levels no longer determine how much attenuation of spurious emissions is required. You can calculate that point. It's where our formula hits 70 dB, and that is at 500 Watts. In other words, to meet the ITU standard, if you're transmitting with less than 500 Watts, you're subject to the formula and if you're transmitting with more than 500 Watts, you're required to meet the 70 dB standard.
It means that, at least in Australia, spurious emissions for amateurs are dependent on transmitter power because the maximum permitted power is currently 400 Watts for an amateur holding a so-called Advanced License.
Now I'll also point out explicitly that the emission standards that the ITU specifies are for generic "radio equipment", which includes amateur radio, but also includes anything else with a transmitter.
One thing to mention is that spurious emissions aren't limited to the second and third harmonics that we measured, in fact they're not even limited to harmonics. If you're using a particular mode then anything that's transmitted outside the bandwidth of that mode is considered a spurious emission and there are standards for that as well.
As an aside, it was interesting to me that in many cases amateur radio is treated separately from other radio services, but the ITU considers our community just one of several spectrum users and it's good to remember that the entire universe is playing in the same sandbox, even if only some of it is regulated by the ITU and your local regulator.
So, let's imagine that you have a handheld radio that has a total mean power output of 5 Watts. When you calculate using the formula, you end up at 50 dB attenuation. In other words, the spurious emissions may not exceed -13 dBm. So, if your radio measures -20 dBm on the second harmonic, it's compliant for that harmonic, but if it measures -10 dBm, it's not. I should also point out that this is for each spurious emission. About half the radios we tested had a second harmonic that was worse than the third harmonic.
So, what does this mean for your radio? I'd recommend that you start reading and measuring. You'll need to measure the total mean power, and the signal strength at the base frequency and the second and third harmonic. I will mention that surprises might happen. For example, the Yaesu FT-857d radio I use every week to host a net appears to be transmitting with a power level that doesn't match its setting. At 5 Watts, it's only transmitting just over 2 Watts into the antenna, but at the 10 Watt setting, it's pretty much 10 Watts.
You also don't need a fancy tool like we were using. All these measurements are relative to each other and you could even use a $20 RTL-SDR USB dongle, but before you start transmitting into its antenna port, make sure you have enough attenuation connected between the transmitter and your dongle, otherwise you'll quickly discover the escape velocity of the magic smoke inside.
I'm Onno VK6FLAB
There's nothing quite as satisfying as the click of a well designed piece of equipment. It's something that tickles the brain and done well it makes the hairs stand up on the back of your neck.
If time was on my side and I wasn't going somewhere else with this, I'd now regale you with research on the phenomenon, I'd explore the community of people building mechanical keyboards and those who restore equipment to their former glory, instead I'm encouraging you to dig whilst I talk about the second and third harmonics. This is about amateur radio after all.
Over the years there has been a steady stream of commentary around the quality of handheld radios. Some suggest that the cheaper the radio, the worse it is. Given that these kinds of radios are often the very first purchase for an aspiring amateur it would be useful to have a go at exploring this.
When a radio is designed the aim is for it to transmit exactly where it's intended to and only there. Any transmission that's not where you plan is considered a spurious emission. By carefully designing a circuit, by adding shielding, by filtering and other techniques these spurious emissions can be reduced or eliminated, but this costs money, either in the design stage, or in the cost of materials and manufacturing. It's logical to think that the cheaper the radio, the worse it is, but is it really true that a cheap radio has more spurious emissions than an expensive one?
To give you an example of a spurious emission, consider an FM transmitter tuned to the 2m amateur band, let's say 146.5 MHz. If you key the radio and all is well, the radio will only transmit at that frequency, but that's not always the case. It turns out that if you were to listen on 293 MHz, you might discover that your radio is also transmitting there. If you're familiar with the amateur radio band plan, you'll know that 293 MHz is not allocated as an amateur frequency, so we're not allowed to transmit there, in fact, in Australia that frequency is reserved for the Australian Department of Defence, and there's an additional exclusion for the Murchison Radio-astronomy Observatory.
293 MHz isn't a random frequency. It's twice 146.5 MHz and it's called the second harmonic.
There's more. If you multiply the base frequency by three, you end up at 439.5 MHz, the third harmonic. In Australia, that frequency falls into the amateur allocation as a second use, its primary use is again the Department of Defence.
These two transmissions are examples of spurious emissions. To be clear, the transmitter is tuned to 146.5 MHz and these unintended extra signals come out of the radio at the same time.
This is bad for several reasons, legal and otherwise. The first, obvious one, is that you're transmitting out of band, which as an amateur you already have no excuse for, since getting your license requires you to understand that this is strictly not allowed.
The International Telecommunications Union, or ITU, has specific requirements for what's permitted in the way of spurious emissions from an amateur station.
Spurious emissions also mean that there is energy being wasted. Instead of the signal only coming out at the intended frequency, some of it is appearing elsewhere, making the 5 Watts you paid for less effective than you hoped for.
So, what's this got to do with the click I started with?
Well, thanks to Randall, VK6WR, I have on loan a heavy box with a Cathode Ray Tube or Green CRT screen, lots of buttons and knobs and the ability to measure such spurious emissions. It's marked "HP 8920A RF Communications Test Set". Using this equipment is very satisfying. You switch it on and a fan starts whirring. After a moment you hear a beep, then the screen announces itself, almost as-if there's a PC in there somewhere - turns out that there is and the beep is the Power On Self Test, or POST beep. Originally released in 1992, this magic box can replace 22 instruments for transceiver testing. I started downloading user manuals, oh boy, there's lots to learn. Bringing back lots of memories, it even has a programming language, Instrument BASIC, to control it. Where have you been all my life? Turns out that in 1992 this piece of kit cost as much as my car. Anything for the hobby right?
At the next HAMfest I'll be using it to measure as many handhelds as I can get my hands on and taking notes. I have no idea how many I'll be able to test, but I'm looking forward to putting some numbers against the repeated claims of quality and price. I can tell you that a couple of weeks ago I got together with Randall and Glynn VK6PAW and spent an enjoyable afternoon testing several radios and there are some surprising results already.
Perhaps this is something you might attempt at your next community event, gather data, rather than opinions...
I'm Onno VK6FLAB