Foundations of Amateur Radio

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Milking Software Defined Radio Jun 16, 2019

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Foundations of Amateur Radio

One of the unsung hero components of a Software Defined Radio is the A/D or Analogue to Digital converter. Its job is to convert the analogue signal that's coming in via the antenna into a digital signal that is processed by software.

I've talked about the difference between analogue and digital before and many explanations talk about converting things into zero and one. There are a few steps before that.

Imagine a row of identical glasses, let's say eight. Grab a jug of milk and pour it into the first glass. Keep pouring until it's full. Now do the same to the second glass, rinse and repeat until you either run out of milk, or run out of glasses.

You now have either a row of glasses full with milk and some spilled all over your desk, or you have some full glasses and some empty ones.

Now if you were to mark a one on your logging paper for every full glass and a zero for every empty glass, you'll end up with a row of zeros and ones. Essentially you've converted an analogue signal into a digital one and in effect, this is how an A/D converter works. Each glass represents effectively what's known in computing as a bit. Grab eight of them and you have a byte.

I will point out that this is just one example of an A/D converter, there are many others.

You may have noticed I've skipped over some interesting things here.

For example, what happens when you spill your milk all over the desk? Or what happens if you don't completely fill a glass?

This is the bit where the action is.

So, let's look at that.

If you've ever over driven a microphone or a speaker, heard of clipping, or distortion, those are all equivalent to spilling milk all over your desk. The take-away is that there is an indeterminate amount of milk and no place to store it, so your row of glasses says all full, but you and I both know that there is some spillage. This is lost information, we don't know if there is a droplet spilled or a whole ocean spilled.

So, one consideration in picking an A/D converter is how to deal with high signal levels. You may need to either increase the number of glasses, or bits, or you may need to decrease the signal before measuring it.

Another interesting thing is what happens at the boundary between full glasses and empty glasses. If you pour one glass full and run out of milk, you're good to go, but the reality says that you'll start pouring the next glass and you'll run out, having a glass that's only half-full. Or is it? Is it half-empty? How do you know? Is this glass represented as a one or as a zero?

You could create a row that's twice as long with glasses that are half the size, and at the boundary you'd have a more accurate idea, but you'd still have the same issue, is the glass that you filled only partly, half-full, or half-empty?

There's more to come, but the basic idea of converting an infinitely variable signal coming from your antenna into something used by software is a big part of a Software Defined Radio.

I'm Onno VK6FLAB

When digging gives you more understanding, the magic of software. Jun 09, 2019

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Foundations of Amateur Radio

Today I'm going to go sideways to move forward. In amateur radio we consider circuits, components such as transistors, inductors, capacitors, crystals and how they're connected to each other. The framework in which that exists is embodied by the field of electronics and how these components can be mixed together to shape a radio that you can build or buy.

In a software defined radio there are electronics and components to be sure, but the bulk of the work is done in the field of software and today I'm going to look at that.

Computers surround us, in our work place, in our home, on the street, in our hospitals, across our society. Each of these devices is running a thing called software, as opposed to hardware - a physical thing, software is intangible, in much the same way as your date of birth is intangible. You cannot hold your birth date in the air and point at it. You could write it down onto a piece of paper and point at the piece of paper that has the date on it, but you'd be pointing at a piece of paper, not your actual birth date.

Computers work in much the same way.

You cannot point at software, nor can you hold it in your hand. You can print it out onto paper, and point at that, but you'd end up with a deforestation problem that far exceeds the stripping of all the trees in the Amazon rain forest. To make matters more complex, there are at least two types of software, human readable and computer readable. You can translate human readable source code into a computer readable executable with a tool called a compiler, but doing it in the other direction is much harder.

Think of the ink on the paper that describes your date of birth. You can put the ink on the paper, but putting it back into the pen is more complex.

All this is leading somewhere, I promise.

A little while ago I started digging into how Software Defined Radios work and if you've been following along on my journey, there'll be parts that you can follow, parts that you sort of get, and bits that seem like black magic. This will be different for each person. My black magic is not going to be the same as yours and the things I understand without thinking might make your head explode. If that's not enough, the goal posts keep moving.

As I said, I started digging, much like peeling an onion, removing layer by layer, I've been exploring and learning and hopefully sharing my excitement along the way.

The other day a mate of mine came by with a new toy. A QRP or low power HF radio. The device itself is entirely driven by software, that is, it's a Software Defined Radio. It has some knobs and buttons, a display, a power socket, a plug for a microphone, an antenna, a speaker port and some other bits and pieces, but underneath all that is software.

What's special about this radio is that the software is Open Source, that is, you can peek inside and see what the code looks like before it becomes ink on the page, the human readable source code, rather than the computer readable executable.

I've touched on Open Source before and perhaps I should spend some time on that soon, but for now, think of it as a set of rules that dictate how you are allowed to use source code.

As any self-respecting IT geek, I went to the website where the software is available and downloaded it.

What struck me was that it was much simpler than I had expected. Don't get me wrong, this is a complex piece of software, not something I'm expecting to pick up in an hour or even a week, but it's simple - as in digestible. I can point at different bits and understand what they do. This part does Morse Code, that does FM, over here is RTTY and look, over here is FreeDV.

If you're wondering, I'm describing the UHSDR, or Universal Ham Software Defined Radio project. Built originally by Chris M0NKA and Clint KA7OEI and sporting an impressive list of contributors, this software offers insight into receiving and transmitting using an SDR across a variety of amateur radio modes, including SSB, AM, FM, Synchronous AM, FreeDV, RTTY, CW as well as CAT or Computer Aided Transceiver, sometimes referred to as rig control or remote control, a way of using an external computer to control a radio.

The beauty of this software lies in its simplicity. Unlike many other projects, there is no code dealing with Windows, or with Mac OS, there is no mouse, touch screen, or any other complex user interface. There is a limited set of buttons, a few dials and a screen for output. The end result is that the level of complexity is much lower than you'd find if you were to start digging into something like PowerSDR or some other code-base.

The point is that the UHSDR project is a really accessible way to start digging into the software behind a software defined radio and another path into this magical hobby of amateur radio.

I'm Onno VK6FLAB

Software Defined Radio offers another path to the same information. Jun 02, 2019

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Foundations of Amateur Radio

Over the past few weeks I've been describing how some of the fundamental concepts of software defined radio work and how some of these operate and interact with each other.

You might think of some of these ideas and technologies as unrelated to the hobby of amateur radio, or not relevant to traditionally built radios.

Nothing could be further from the truth.

Before I go on, I've been trying to find more elegant terms to distinguish between radios built with mostly software and those built with mostly hardware, and I use the word mostly, because a traditional radio like my Yaesu FT857D has software on board and similarly the software defined radio Flex 6600 has traditional components inside the box.

For convenience, until I find a better distinction I'm going to refer to these as hardware radios and software radios. When I refer to a hardware radio, think Yaesu FT857D, when I say software radio, think Flex 6600.

Back to the topic at hand. The techniques used in both disciplines, hardware and software, apply to each other. So for example, a band filter might be useful in both and use similar if not identical circuitry, but a noise filter, or an audio filter like a Collins SSB or CW filter might only exist in a hardware radio and the software radio deals with the issue using little programs.

Similarly, the local oscillator in both determine the accuracy of frequency representation, but a hardware radio might require a screwdriver to adjust, where a software radio offers the adjustment functionality with a single menu option.

With my time spent on software defined radio, I'm finding that my understanding of how spectrum relates to the signal and how filters, offsets and various adjustments all relate to each other.

Don't get me wrong. The things I'm learning are perfectly able to be represented within hardware radios and the education that goes with that, what I'm saying is that they arrived for free for me with understanding the software defined radio.

Let me give you an example.

If you listen to a Morse code beacon signal on the 10m band, lets say the NCDXF beacon on 28.2 MHz. If your radio is set up correctly, you'll hear a tone as each beacon does its thing.

If you tune slightly off frequency, the tone changes, up or down, depending on how you've got your radio configured.

If you've got a Collins CW filter, you might find the whole thing vanishes if you stray too far off frequency.

Now here's the thing.

The tone you're hearing is actually dependent entirely on how far off frequency you are, so much so, that if you're entirely on frequency, you won't hear anything at all.

But you just heard me say that if your radio is set up correctly, you'll hear Morse if you're on frequency, and that's because, setting up the radio includes an offset, in a hardware radio it might be called the CW pitch, and what it does is essentially take your radio off frequency by the pitch amount.

You can test this by setting the pitch to 0 and taking the radio off frequency by say 500 Hz.

The point is that how this works and my understanding of it, came entirely from the software radio side. My training didn't include the intricacies of hardware radios, but my curiosity got me there by another path.

Another way to consider the Morse code signal is to think of it as a tone of 0 Hz. You can't hear the tone, since it's 0, but if you were to move the dial, the 0 changes into something you can hear.

While I'm at it, if you've ever wondered what's the difference is between the two CW modes on your radio, when they don't appear to do much at all?

Consider two signals, side by side, say 500 Hz apart. From a listing perspective, the station you care about is at 500 Hz and the one you don't care about is at 1000 Hz. Change to the other CW mode and the one you care about is still at 500 Hz, but the other station has moved from a 1000 Hz to 0 and it vanished.

I've spoken to many different amateurs over the years and each one learns their craft differently.

I hope that my weekly podcast adds little puzzle pieces to your mind that will over time collide with each other and end up with that elusive Ah-Ha moment that makes you smile with a tinkle in your eye and a better understanding of this amazing hobby.

I'm Onno VK6FLAB

Digital Origami in Software Defined Radio May 26, 2019

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Foundations of Amateur Radio

As a quick recap of what I've discussed before, a Software Defined Radio is a tool that essentially measures the voltage at the base of an antenna system and sends that to a computer for processing.

The faster you measure, or sample, the better the representation of what's coming in via the antenna. The traditional view is that you need to sample at least twice as high as the highest frequency you want to represent.

You may also recall that an antenna system doesn't just receive a single frequency, the one your radio is tuned to, but all frequencies.

So, if you need to build a software defined radio from scratch, your first question might be: What do I want to listen to?, followed by: Which sample rate do I need?

If we were to answer the first question with HF, say up to 50 MHz, the answer would be something like a sample rate of 100 MHz, so you can capture any signal up to 50 MHz.

So, twice the highest frequency as the one you care about, that's the short way of waving your hands about and ignoring any little inconvenient side effects.

Like, what happens to signals above 50 MHz?

First of all, your antenna system will still receive those signals to more or lesser degree, they don't just vanish because your sampling tool isn't interested in anything over 50 MHz.

The second thing that happens is that the signals between 50 and 100 MHz will turn up backwards between 0 and 50, so you'll effectively hear 51 MHz at 49, 55 MHz at 45 and so on.

As a neat little side-effect, for those reversed signals, an upper side band signal will turn into a lower side band signal and vice-versa, but I'll leave that for another time. In case you're wondering, yes, this can be a desired effect.

The signals between 100 and 150 MHz will also turn up where they're not welcome, 105 MHz becomes 5, 110 MHz becomes 10 and so on.

A different way to picture that is to think of a tri-fold birthday card. Lay it flat on the table, put a 0 in the top left, 50 at the first fold, 100 at the second fold and 150 in the top right. You're looking at 0 to 150 MHz.

Now fold it up.

You'll notice that 0 and 100 are in the same place and 50 and 150 are also in the same place. If you need more detail, put some in-between numbers, 25 MHz, 75 MHz and 125 MHz and you should see what's happening.

I've seen it described as digital origami and it is. The technical term is called aliasing and it's also referred to as folding. It happens in day-to-day life as well. If you've ever seen a wheel running backwards on television or on a film, that's an example of folding.

If that's not enough, this phenomenon repeats itself, 150 to 200 MHz is overlapped in reverse, 200 to 250 MHz overlaps normally and so on.

You might come to the conclusion that your magic SDR isn't so magic any more, now you have all this other stuff turning up that you don't want to hear. So what do you do?

One approach is to increase the sample rate, but as I've explained, it doesn't make the problem go away.

But here's the thing: If you were to sample at say 200 MHz, you'd be perfectly fine with any signal up to 100 MHz.

Now here's the kicker.

If you filter out anything above 50 MHz, and as long as there's nothing left by the time you get to 100 MHz, you're good to go, no more unwanted information, no more aliasing or folding.

Essentially what you've just done is created a thing called a bandwidth limited system. You've essentially removed anything above 100 MHz and now your sampling is working as planned and all of the stuff I've said about sampling at least twice the maximum frequency applies.

Yes, there's more, but I'll get to that another time, but to give you a taste, what happens if you want to use the same SDR to listen to the 2m band?

I'm Onno VK6FLAB

SDR: How many colours inside a Software Defined Radio? May 19, 2019

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Foundations of Amateur Radio

If you were asked to make an image of the Sydney harbour bridge and only use four dots, the viewer might struggle to determine what was the bridge, the sky, the water and the Sydney Opera House. Regardless of the number of colours available to you, the number of dots would not be enough information for most people. You might have a nice piece of art on your hands, but it might be ineligible for the Archibald prize. Even if you were allowed many colours, and just four dots, figuring out if the blue dot was water, sky, or the background of the Australian flag on top of the bridge might be just as complicated.

If you were asked to make the image with one hundred dots, and only use black and white, from the perspective of the viewer you'd have a result that was easier to understand. Use a thousand dots, even easier, even if you only used black and white.

Now, if you were to use a hundred dots, with ten colours, your image might be just as easy to understand as if it was a thousand dots in black and white.

The point is, there are two things going on here. The number of dots and the information contained in each dot.

More dots or more colours, or both, will help your image.

Similarly, in Software Defined Radio, more dots, that is, more samples, will help and as I've previously mentioned, you need at least twice the number of samples as the highest frequency that you're measuring. But what of the colours in relation to an SDR?

Measuring voltage as a human with a piece of paper is pretty straightforward. Provided you've got a Volt meter, a piece of paper and a scribble stick, you're good to go. If you measure your voltage as 1 Volt, you write 1, if it's -1 Volt, you write -1. Similarly, if it's 100 Volts, you'd write 100, 13.8 Volts and you'd write 13.8. We'll get back to colours in a moment.

Provided your paper is big enough, you can record as many values as you need and as accurately as you desire. 13.8 or 13.8853, makes no difference to a piece of paper.

Computers represent numbers internally using powers of two, called bits. A single bit can represent two values, 0 and 1. Two bits can represent four values, 8 bits represent 256 values and 16 bits represent 65536 different values.

The takeaway is that there are a specific number of values that you can represent inside a computer, depending on how many bits you use.

Consider the values I've mentioned, 1, -1, 100 and 13.8. That's four different values. If it's not immediately obvious, what ever solution you come up with, tracking positive and negative, tracking small and large, whole and fractions should all be part of the mix. In case you're wondering, we're essentially describing here how many colours or values we are going to allow, or in terms of a computer, how many bits.

Let's consider all the values you might measure and represent inside a computer. How many different voltages do you want to be able to record between 1 Volt and 100 Volt?

If you allow for ten values, you can record 10 Volt, 20 Volt and so-on, but you can't record 15 Volt.

If you allow for a hundred values, you can record 1 Volt, 2 Volt and up, but you won't be able to record 1.5 Volt.

If you account for a thousand values then you can record 1.1 Volt, 1.2 Volt and so-on, but you can't record -10 Volt.

Remember, our computer representation can only manage a specific list of values and the size of the list is determined by the number of bits you're using.

The rabbit hole goes even deeper.

Radio signals vary massively in their strength, which is why we use a decibel scale to represent the signal strength, instead of saying station A is a thousand times stronger than station B, we say it has a signal level that's 30 dBm higher. That's comparing a 1 Watt station to a 1 kilowatt station, and in terms of voltage, that's between 20 Volt and 632 Volt.

If you're designing a mechanism to store your measurements inside a computer, you might decide to use dBm to record your measurement. Let's say 30 values from 30 to 60 dBm. Sounds great, where do I sign up?

Not so fast. What happens if our station is running less than 1 Watt, or if it's running 100 kilowatt, like when you happen to receive a nearby FM broadcast station?

Not only do you need to contend with a whole range, called a Dynamic Range of measurements, you also need to deal with what happens to the overall picture.

Let me say that in another way.

Your voltage measurements at the base of your antenna are a representation of the RF information that your antenna is receiving, or transmitting for that matter. Representing that inside a computer means that the values you're using, and how fast your gathering them, determine how well the RF signal is represented.

One thing to note is that the largest values represented by what ever you choose is only part of the problem.

A signal that is stronger than the largest value you can record is not going to be recorded correctly. Similarly, a signal that is so small that it doesn't register as a change, also has an incorrect recording.

Picking the right combination of dots and colours, sample size and bit-depth, doesn't end there, because there's even more to this, but I'll leave that for next time.

To blow your mind, the Dynamic Range, bit-depth and sample size I've talked about in relation to Software Defined Radio, also applies to many other things, like taking a photo with your digital camera, or sampling digital audio, so understanding this in one area will likely help you in other places as well.

The final takeaway is that a computer records a range of values that can represent a measurement in the real world. Picking the correct range of values determines how well your computer represents what your measuring.

I'm Onno VK6FLAB

SDR Sample Rates: How fast is fast enough? May 12, 2019

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Foundations of Amateur Radio

If you measure the voltage at the base of an antenna and record the readings, you end up with a collection of numbers that represent the voltage over time. These numbers, or samples, can be used to represent the antenna signal inside a computer.

An antenna system voltage is an example of an analogue signal, continuous over time, the recorded readings, the samples are an example of digital, discrete and intermittent.

It's possible to reconstruct an analogue signal from digital samples and that's exactly what Software Defined Radio or SDR is all about.

The process of sampling essentially converts a continuous signal into an intermittent one. As recording separate samples implies, there is loss of information in this conversion.

For example, if you sample once a minute, you'd represent a continuous signal as 60 samples per hour, probably enough to reconstruct where you've driven in your car along the highway, but hardly enough to reconstruct the route through the middle of the city, let alone represent an antenna signal that varies millions of times per second.

So, how often do you need to record a sample?

Turns out that if you sample at least twice as fast as the highest frequency you're representing, you're good to go.

So, for sound, the human ear can hear about 20 kHz, so more than twice that, explains some of why a CD is sampled at 44 kHz.

If you want to represent the 20m band, up to 14.350 MHz, you need at least a sample rate that's double that, or 28.7 MHz.

As an aside, there are other ways to look at this problem. If you managed to move the 20m band down to 0, then you'd only need at least a sample rate of 700 kHz to do this.

Let me say that in a different way.

The width of the 20m band is 350 kHz. So sampling it would require at least twice that, or at least 700 kHz. Moving frequencies around is something that we've been doing in traditional radios for a long time. The technique uses one or more frequency mixers. This means that combining some traditional radio tools with an SDR gives you even more options.

Truth be told however, this idea of moving the band with one or more mixers is becoming less important as technology improves and there are plenty of reasons not to use this. I'll talk about that at another time.

So, the first takeaway is that to sample a continuous signal and be able to represent that signal accurately requires a sample rate that's at least twice as high as the highest frequency in the continuous signal.

Without going into the actual proof of this, consider a sine wave that oscillates at 1 Hz. If you sample it at anything less than 2 Hz, you'll end up with some cycles being sampled only once, which isn't enough to represent the sine wave. If you sample it at exactly 2 Hz, you'll have two samples on every cycle, but if you happen to sample when your signal is 0, all you'll ever measure is 0. By sampling at a rate greater than 2 Hz, you overcome that limitation.

I'll make brief mention of another phenomenon, that of over sampling. An interesting thing happens if you sample twice, three times or more than the minimum sample rate. In short, the higher sample rate improves the dynamic range, noise performance and filtering, all very useful when you are processing radio signals. Cheaper and cheaper hardware are making this very attractive and it explains some of the reasons why SDR manufacturers are using sample rates that far exceed double the highest frequency being sampled, for example, the Flex-6600 samples at 245.76 Mega Samples Per Second, or Msps, even though the maximum receive frequency is between 30 kHz and 54 MHz.

In case you're wondering, yes, I'm leaving out a lot of detail here, one thing at a time.

The opposite, under sampling, has its uses as well, but I'll also leave those for another time.

The second takeaway is that higher sample rates are used to reduce cost, increase performance and reduce component count.

Some of what I've talked about can be explored with the popular RTL-SDR USB dongle which is actually a mass produced commodity digital television receiver, made in the millions and accessed directly thanks to the combined efforts of many different people. If you'd like to start to play, $25 should get you a dongle and most of the software you can start to experiment with is free. Check out rtl-sdr.com to get started.

If you'd like to get in touch, please do, cq@vk6flab.com.

I'm Onno VK6FLAB

Software Defined Radio is fundamentally a different way of looking at radio spectrum May 05, 2019

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Foundations of Amateur Radio

We think of radio as operating on a specific frequency. We select an antenna resonant on a single band. We configure the radio for that same band and then turn the dial or the VFO, or Variable Frequency Oscillator to a particular frequency within that band.

All of our language is geared towards this concept of tuning, of picking out, selecting one special tuned, resonant frequency and listening to it.

I've said this before, but that's not actually what's happening.

Your radio is receiving all RF frequencies, all of them, all at the same time, all the time. Your antenna is better at hearing some frequencies than others, but that doesn't stop it from hearing everything at once. Your radio is getting all that RF information at the antenna connector. After that, every step along the way is removing unwanted information, first it removes all the bands you're not listening to, then the VFO selects which part of what remains to let through to the decoder and the result finally arrives at the loudspeaker.

Ultimately, all your radio lets you play with is what's left over. Say about 3 kHz bandwidth. Using traditional radio, if you want to listen to two repeaters, you either need to switch back and forth quickly, or you need two receivers.

Now without going into how precisely, imagine an SDR with a bandwidth of 3 MHz, one thousand times larger than your traditional radio. Before you think I'm being fanciful, a $25 gadget can do this. This means that you could process most if not all of the 2m amateur band and then pick out which bits you'd like to decode. You could decode all the local FM repeaters, an overflying satellite, the International Space Station SSTV, a beacon, Morse, Packet, RTTY and simplex contacts, WSPR, APRS, EME, whatever is happening on 2m, all at the same time.

Let me say that again. All of the 2m band, all at the same time.

The point is that all this information is there, all the time. We can opt to decode or ignore the information. In a traditional radio, you can only decode one signal at a time, but on an SDR, you can extract as much or as little as your computer can handle. Some SDR language talks about using multiple receivers, but a better description is multiple decoders.

This means that software defined radio is fundamentally a different way of looking at radio spectrum. Instead of filtering out everything we don't want to decode, we select which decoder to apply to which part of the spectrum.

With an SDR you could represent the 2m band as a 3 MHz slice of spectrum as a series of measurements. There is no loss if you reuse the numbers, so if you process the same data multiple times, you have no loss of signal, no deterioration, no extra noise.

All we do is feed the same data into each decoder, pick out the bit we want to decode and have at it.

There is a misconception that you need serious computing power to do this. That's not strictly accurate. A $5 Raspberry Pi single board computer is more than powerful enough to do this. You can argue that this is serious computing power, compared to what we used to land on the moon it is, compared to your mobile phone, it isn't.

I fully intend to go into the maths behind this, but it's not scary, despite what you might think or have been taught. My week has been about the maths and it's become clear to me that there are lots of explanations around, each trying harder than the next to scare you away.

If you feel the need to run screaming for the hills when you hear the words Nyquist, Shannon and Fourier, then get it out of your system and come back when you're ready.

I'd like to mention that I've been working on how to explain this over much of the week, I've lost count of the number of drafts I've written, but it keeps coming back to the words that are almost as old as I am: My god, it's full of stars.

No doubt you might be convinced that I've lost my marbles and that I'm going well outside the Foundations of Amateur Radio, but I have to confess, this is what radio is today, and I'm thrilled to be here learning more about how this all works. Hopefully you are just as thrilled.

I'm Onno VK6FLAB

How does a Software Defined Radio or SDR work? Apr 28, 2019

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Foundations of Amateur Radio

If you've been around the hobby in the past decade, you may have come across the invention of a Software Defined Radio, or SDR. You might even own one and if you've looked into how it works, read the explanation that essentially describes it as a traditional radio where all the components are implemented in software. To me that's like explaining how a radio works by waiving your hands and saying: here is magic.

How it actually works is something all together more interesting and thought provoking.

If you think of sound, like my voice, coming from a speaker, you can imagine putting a volt meter on the speaker terminals and measuring every second what the voltage is. As my voice gets louder you might measure a large voltage, as I take a breath, it will be smaller. You could chart the different measurements and show a waveform that would represent the loud and soft parts of what I'm saying. The faster you measure, the more accurate the picture represents my voice. For comparison, a CD player does this measurement 44 thousand times per second.

If you were to play back those sound measurements at the same rate into a speaker, you'd end up with my voice, and that's actually more or less, what's happening if you're listening to this podcast. Yes, for the purists, there's more to it, but not relevant at this point.

Similarly, if you were to hook up a volt meter to an antenna and take measurements, you'd end up with a chart that represented the signal strength that your antenna is receiving and the faster you measured, the better the representation. What it exactly represents I'll come to in a moment.

The waveform that represents my voice is actually a very complex signal. In much the same way as a piece of music is made up of different notes, played in sequence and in concert with each other, my voice is also made up of separate frequencies, played together to form the words that you hear.

If you were to measure those separate frequencies and draw a waveform for each, you'd see how every one contributes a little to the overall effect, and if you were to add them all together, you'd have my voice again.

In the same way, the waveform that represents an antenna signal is made up of all the separate frequencies that go into the overall signal. You might be surprised to learn that an antenna is actually hearing all frequencies at the same time. Some better than others, but typically, all of the RF spectrum at any given time.

Your radio is also essentially hearing all frequencies. When you tune to a local station on 720 kHz, you're actually telling your radio to ignore all the stuff that isn't 720 kHz and to only process that small bit of what it's hearing. The selectivity of a radio is the measurement that represents how good your radio is at being deaf to all the things you don't want to hear.

To help that filtering, a traditional radio and antenna works by pre-selecting part of the RF spectrum, when you press the AM button on your car-radio, you're selecting which chunk to listen to, press the FM button on the same car-radio, you'll select another chunk. On an amateur radio, you select by choosing the 80m band, the 40m band, etc. Similarly, your antenna is pre-disposed to hearing a particular chunk better than others, but that doesn't make it immune to signals across the entire range.

You may have heard described that a Software Defined Radio hears all frequencies at the same time. Essentially it's a volt meter connected to your antenna, spitting out measurements as fast as it can for processing by a computer.

The waveform that comes from those antenna voltage measurements represents all of the RF spectrum and it's just the beginning of what you can do next.

In the same way that my voice is made up of lots of different parts, all played together, the RF spectrum is made up of the local broadcast stations, the local TV stations, mobile phones, garage remotes, Roy on the 7130 DX net, this podcast on your local repeater, all at the same time, all played together, to make the waveform that represents the measurements you make at the base of an antenna.

I'm going to ignore for a moment how exactly we extract the various bits, or how we decode an FM or SSB signal using software, it involves some math, instead we can look at something that is easier to explain.

Unlike with a traditional radio, which has to work hard to filter out undesirable information, a software defined radio can filter out information by just deleting those measurements you're not interested in.

Yes, there is more to it, much more, but that's the beginnings of how an SDR works.

If you'd like to get in touch, please do, cq@vk6flab.com.

I'm Onno VK6FLAB

Antennas out of sight and out of mind ... Apr 21, 2019

Show notes

Foundations of Amateur Radio

Previously I've spoken about the dynamic nature of your station. Even if from day to day use, nothing changes, things around you are always in flux. Propagation changes, power fluctuates and the environment in which your antenna operates is dynamic. Mobile stations even more so.

A few days ago we had a gale come through, strong enough to do some major damage, rip off some roofs, break some trees, cause flooding, cause power outages, plummeting temperatures, the first of the Winter Storms.

Obviously, checking out your antenna after such an event is expected. Better still, stowing your gear before the event is even better.

Such extreme weather events are an obvious trigger to attending to antenna health and well being, not to mention, maintenance and repair.

The thing is, it's not the only time you should check out your antenna. Every day it's subject to change. The sun rises in the East, follows its path along the sky and eventually sets in the West. The temperature and humidity change throughout the day and continue to change through the night and the next day it starts all over again.

Peppered with sun, rain, snow, salt, corrosion, expansion and contraction, your faithful antenna sits there ready for you to get on air and make noise, until one day it isn't.

You could just wait until it falls down, dies, perhaps becomes a hazard to anyone within gravity range, not to mention, destroy your radio when you key it up. Or you could check your antenna regularly and look after it. Inspect and test it regularly, run you analyser across it every couple of months, you know the drill.

Most antennas are out of sight for most of their life, but they should never be out of mind.

During the weekly F-troop net we started discussing this - as well as an in depth conversation about launching wire into trees - and there were several suggestions worth investigating.

One amateur pointed out that the level of complexity in the air dictates the amount of maintenance. A log periodic antenna on a rotator needs more Tender Loving Care than a wire hanging off a tree.

Another suggested that you should regularly check the tower supports - technically the mast supports - a tower is self-standing and a mast is not - the best way to remember is that is the Eiffel Tower doesn't have any guy wires.

Before a storm, if you have warning, you should check the supports, wind down anything that goes up and down and you should think about how you're going to earth the coax. I've previously covered the weirdness that lightning and charge represents, even at distance, so don't wait until it's overhead.

There were suggestions of using spark plugs and Mason Jars, but I've got no supporting evidence either way. My geek background is sceptical, but I'm open to learning more.

I've seen installations where a coax switch is used where the antenna is switched to a shorted socket, so the inner and outer braid of the coax are connected to each other.

One amateur suggested that an antenna tuner is cheaper than a radio, and that if you leave it in place during a storm, blowing that up is cheaper than blowing up a radio, but your mileage may vary. Also, if you have spare cash to burn, I'm happy to take your donation and relieve you of that fire hazard.

It's interesting in and of itself that antenna maintenance is often discussed in terms of extremes, lighting, storm, wind, ice, etc. and less so in terms of regular maintenance.

Finally, if you're only using a temporary antenna, you're not exempt from this. You're actually likely to have more failure, since the act of erecting and lowering of the antenna is likely to cause more wear and tear.

The antenna is the final part of the transmission chain and it should be treated with the same respect as the power supply at the other end.

I'm Onno VK6FLAB

Fragility of Communication Apr 14, 2019

Show notes

Foundations of Amateur Radio

Our day to day life is full of communication. We listen, although less and less, to the radio for news and entertainment, sometimes mixed together as food and games for the masses.

We can communicate with family, friends and the rest of the global population using a telephone. With the internet as a transmission medium, we exchange text, sound and vision with impunity to anyone who stumbles across it on a mind boggling collection of outlets, websites, social media, email, streaming services to name a few.

The vast majority of this kind of communication is a commodity, that means that with little or no training most of the population has access to this.

Another aspect of this commodification is that it's reliable. It works most of the time, it's generally good quality, with little or no loss, as in, you speak into your phone and there's an extremely high chance for the other party to hear your voice. While there are occasions that calls drop out, or the audio is chopped up, it's more an exception rather than a regular occurrence.

In stark contrast, amateur radio is none of those things. It's not a commodity, it's not reliable, it's a poor man's version of the ubiquitous mobile phone.

As amateurs we know why it's not the same, for starters, to make contact between say Perth and Bermuda using amateur radio requires exactly two pieces of equipment. Your radio and theirs. Making this contact with a mobile requires that both ends have a phone. They'll also need a way to connect to the phone network, either a local base station or a telephone exchange, those in turn connect via many different ways to each other, including repeaters, relays, perhaps a satellite, a fibre optic cable or three, too many devices to count today. Extreme level of complexity.

I'm mentioning this because it's simple to conclude that amateur radio is obsolete, but its just not true.

With the lack of reliability associated with an amateur radio connection comes something that is unique to society today. Thanks to reliable communication, we have come to expect that all communication is reliable, even our experimental hobby, but if you spend any time on air at all you'll quickly realise that for amateur radio, we need to conduct ourselves with protocol, using specific procedures, phonetics, structured phrases, callsigns and the like to overcome some of the aspects of unreliability.

Talking on the local repeater looks and smells like a mobile phone chat room, but it's not. It relies entirely on the participants collaborating to ensure reliable communication.

Similarly, calling CQ on HF, requires that you understand that the other station isn't on the end of a telephone connection and that parts of what you're saying are going to be missing at the other end. Using phonetics, speaking slower, waiting longer and monitoring, all assist with making contact.

If you're unsure about this, just listen in on a local net for regular confusion, or use an online receiver like WebSDR to hear what you sound like at the other end.

To make things a little more interesting, every amateur band has a different failure mode. On 20m from one breath to the next, the path might close, on 80m you might get overwhelmed by noise, on 40m you might find yourself all of a sudden sharing the frequency with another station, both of you blissfully unaware of the other's existence.

Communication in amateur radio is collaborative and there are common courtesy behaviours. If you're working a rare DX station, that's not a personal friend, don't start a whole conversation about your dogs, your medical issues, or the level of amazingness of your station. You're not alone in attempting to make the contact and they're not there for your personal enjoyment. Hogging the frequency is a sure fired way to acquire the ire of your fellow amateurs, especially in marginal conditions, where band conditions are rapidly changing.

There is nothing like getting your feet wet by actually getting on air and making noise, but when you do, remind yourself that this is not a telephone and it's not perfect. Be mindful of your on-air conduct and you'll find a globe full of friends.

I'm Onno VK6FLAB

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