Radio Diary for March 2022

This month I’ve delved further into the local amateur radio community, seeing what repeaters I can reliably pick up and when they tend to be active, as well as more spontaneous conversations and simplex conversations whether scheduled or not.

(Simplex is when two – or more – people are conversing on the same frequency. Kind of like a walkie-talkie. They both need to be in range of one another. A repeater is an automatic device that takes an input on one frequency and spits it out on a slightly different one. You set your radio accordingly, and you can speak to people outside your normal range, as long as the repeater is in the middle of the Venn diagram of both your ranges.)

My little Baofeng does an admirable job at allowing me to listen in to a few local repeaters, and it’s an annoyingly nice device to use given how cheap it is. It even just feels nice in the hand.

Sure, it’s clunky in adjusting the settings and programming channels (I do not have a USB programming cable), but once a set of channels has been programmed in, it’s easy to just switch it on and set it scanning through those channels for any signals, or going to a particular memory channel when you think there’s going to be activity. (It can monitor two channels at once, which is nice.)

The scan function can be set to either stop when it finds a signal and stay on that channel until the signal cuts, or to do so briefly but move on after a few seconds. The latter is preferable as the Baofeng is occasionally susceptible to interference which makes a channel sound active and thus the radio stops on it, playing a burst of static and noise. In time mode, this only lasts a few seconds before continuing the scan in silence; in signal mode the radio will simply stay locked to that burst of static and noise until the user presses a button.

I now routinely just switch the radio on at random times, or around times I know local groups have scheduled meetings/conversations. And although it’s a bit weird just listening in to strangers talking, it is oddly not too weird as a) they know it is a public medium, albeit a very niche one, and b) the conversation matter is usually at least partly to do with radio and related technologies, and usually relating to local clubs, repeaters, atmospheric conditions etc. So there is usually something for me to find interesting or learn from. And the conversations have certain protocols which make them simple enough to follow: the speakers must identify themselves by callsign every now and then, and it’s relatively simple to ‘feel’ the flow of the conversation based on these and other standard greetings.

It’s also oddly calming as – certainly with repeaters – only one can speak at a time. The ‘overs’ (each person’s part of the conversation) can last anything from a brief word to a couple of minutes, and in a flowing conversation, they become a little list of responses to what was said by the previous speaker. In this way it’s a little like letter-writing, or web forum posts, and it makes it easier to follow.

It does, I suppose, get a little harder to follow when the voices all sound similar, the names are generic (and often identical!), and the call signs are said a bit too briefly to catch. But I’ve spoken briefly about amateur radio’s issues with diversity, and that’s a bigger issue for another day.

I have also found that the Baofeng can be set to frequencies set aside for PMR or personal mobile radio. My understanding is that this is a set of frequencies made available to use by unlicensed users, and there is a range of radios available to buy that can only be used on these specific channels and users can talk simplex to one another, like walkie talkies, or CB radio. They seem to be mainly for business use, but I have found people using them for casual conversation (which I think is permitted), and so I’ve added the channels to my scanning frequencies.

If this makes you think of the cliched police scanners which were around years ago, those days are mostly over. Most emergency services use encrypted digital radios now, which can’t be easily monitored. But I have found that the marine world uses analogue channels which are in the range of my Baofeng, and I’ve added some to the memory for scanning when I’m near the coast. I’ve heard a mixture of the Dover coast guard instructing a fishing vessel, as well as two fishing vessels agreeing who should go on which side of who while passing – spoken variously in English, French and Dutch(?).

Anyway. My point is there seems to be a lot of activity in my local area, and I’m finding a lot to learn and enjoy about it all.

Before we moved, I had been studying for the RSGB’s foundation amateur radio licence, and I was pleased that it covered not just the manners and protocols used by amateur radio users, but the technology and science behind it. I held off on taking the exam to get my licence as I wanted to see if I would be able to make use of it once I’d moved – to see if there is an active amateur radio community in this neck of the woods. It seems that there is – so watch this space.

I’ve had an SDR (software defined radio) USB dongle for years now, and it lets me do lots of things.

It’s primarily designed to let the user watch digital terrestrial TV (DVB-T), but it has a wider frequency range than that, and it can be used to scan the radio bands, from broadcast to amateur radio and beyond. It’s a great little toy with many uses.

As with all these things, there are basic ones for about a tenner and amazing ones for a few hundred quid. Mine is the former – and it still does all these things admirably.

One use for these dongles is to decode a very specific frequency which is transmitted by aircraft to relay their altitude, speed, bearing, etc. It’s called ADS-B, and it seems to be broadcast by most aircraft over a certain size.

If you’ve ever used Flight Radar 24 or a similar website to see the details of a flight going overhead, those websites basically use a global array of receivers which grab the ADS-B data local to them, plot it on the map, compare it with others nearby, and then FR24 and others collate it all to make a global map of air traffic.

I’ve always found it very cool, and I was pleased to find that amongst things I already own, I can make a small, automated unit that grabs ADS-B data and submits it in real-time to FR24 (and others). I can also view the local data for just my device, giving me a sort of air traffic controller’s RADAR view of nearby aircraft. And by submitting the data to FR24, I get a free premium subscription, which is nice – I have actually paid for FR24 in the past as I find the service to interesting.

The bits I scraped together were: my SDR dongle, the antenna it came with, a Raspberry Pi Zero (kindly given to me by my buddy Troels years ago – tak, brör!), a few Pi accessories like a USB hub (mainly as the Zero is so tiny and doesn’t have on board wifi), and a few cables.

The antenna that comes with the SDR dongle is small and basic. But it’s the right gear for receiving digital TV, so it does the job. The dongle has a small connector for antennae, which makes it usable with other types, too. ADS-B operates on a wavelength not too far off DVB-T, and the fundamental thing about antennae is that their length should ideally match the wavelength you’re tuning to – or some fraction of it, too.

What this means in reality is that the DVB-T antenna can basically be snipped off to a certain length to optimise it for ADS-B reception. It’s brutal – but it works.

An aside. We moved house recently, and we’re still in the process of finding things in boxes. One of those things was my dongle’s antenna. I started playing with ADS-B a few months ago, and I was using a homemade antenna which I had made for reception of amateur radio signals from the repeater on the International Space Station – more about that another day. But I found when I set up my FlightRadar24 unit that my stupid little antenna, with silly bits of wire hanging off a choccy block terminal strip… Well, it worked. I was getting signals from aircraft 20 or even 30+ miles away!

But I always knew I could do better, and thankfully I found the dongle’s original antenna (along with my RSGB foundation amateur radio licence study book and my revision notebook – convenient). So I re-fitted that to the dongle, trimmed it down to the correct length, and put it back in place. And the results were great! I’m now routinely getting signals from aircraft more than 60 miles away (it’s actually nautical miles for some sort of aviation-related reason), and I find that amazing.

Pro users can get signals of over 200NM, almost at the bounds of line-of-sight reception owing to the curvature of the earth – but even in my range of 60-70NM, being able to get such good data from a cheap device hacked together and just stuck in the window of an upstairs room is incredibly cool to me. It’s slightly mad to think that although I can only see up to the nearest ridge on the hillside, there’s a little radio next to me that can see an aircraft in the air over north west London and beyond. I love it.

I can tell I’m going to want to improve this antenna further. That’s the thirst of radio play: better, more, further. Gear acquisition sydrome, like with photography. So I’ll try and keep that in check and make do with what I’ve got. In many ways, of course purpose-built hardware will do a better job. Part of the joy here is using absolutely basic equipment and getting somewhat decent results out of it.

And finally, a brief nod to leaky feeders.

I first came across these a few years ago reading about their application in US colleges and summer camps to broadcast radio in a small area. The basic principle is using a wire which runs the length of a building or location as an antenna. Normally with wires and antennae you have the antenna at one end spraying the signal out like a sprinkler, and the wire leading to the device is carefully shielded to keep the precious radio waves in.

Imagine a tap with water pumping out, and a hose carrying it to the sprinkler. Just like you wouldn’t normally get any water randomly coming out along the length of the hose itself, you also wouldn’t get any radio signals along the wire either. But what if you wanted to? Leaky feeder! Cut some small, regularly spaced holes along that length of wire (or hose, for my analogy), and you end up with small, controlled bits of radio (or water) leaking out along its whole length.

Now imagine running that cable around the length of a college campus, or strung up around the various bunkhouses dotted around a summer camp ground. Now all the people in any of those spaces can tune a radio and get the signal – and you’re not just broadcasting it out to the wider world.

A more UK- (okay, London-) centric application for the leaky feeder is the tunnels of the London Underground.

Using radios for communication in underground tunnels is notoriously difficult. Radio signals want to radiate out in all directions, but tunnel walls just block them. But what if you run a leaky feeder down the length of the tunnel? Now that signal can be picked up down its whole length. And if it’s wired up correctly, you can have connections going up through the earth to the ticket office and down to platform level, and it all just works.

Fast forward to a year or two ago and Vodafone installed a 4G leaky feeder into the middle part of the Jubilee Line tunnels, giving passengers full 4G when not just in the stations but while travelling through the tubes as well. Amazing.

(Less amazing was that this meant Vodafone pulled out of the coop wifi in use across most? all? tube stations, and now I can no longer use my phone on the Tube unless I’m on the Jubilee Line, which is rare nowadays.)

The Vodafone 4G installation was done, I believe, in conjunction with a new emergency services radio system which – quite understandably – aims to improve their radio use in Tube tunnels.

(There’s a decent Wired article here which gives a fair bit more detail – while oddly referring to all the measurements in inches throughout…)

It all reminds me that one thing that blew my mind when visiting Paris probably 20 years ago was that I had mobile phone signal underground! Looking back, I don’t actually know if this was simply due to the Paris Metro being more of a cut-and-cover shallow network in comparison to the Tube’s deep level tunnels. But perhaps they had installed leaky feeders for mobile phone access way back then? I should check.

But it got me thinking – of course it did – that it would’ve been fairly straightforward to install leaky feeders on the Tube for FM radio reception. Wouldn’t it? If I understand the technology correctly, this would’ve enabled passengers to listen to the radio while travelling underground.

Let’s argue that this would’ve been done in the 1990s when people were less likely to carry a mobile phone and more likely(?) to carry a pocket FM radio. I don’t know. Probably the answer is either a) no one wanted this, and so it never happened, or b) enough people would’ve wanted it, but it’s simply not technologically feasible. I’m sure there’s an obvious answer. I just don’t know what it is, so here I am rambling about my silly concepts.

Anyway, that in a nutshell is another month in radio. If you made it this far, I can only apologise. And I’ll see you again in four weeks or so.