Radio Airbnb: Hacking The Airwaves With RTL-SDR

Updated: May 30

One of the (ahem) 'benefits' of the worldwide COVID19 lockdown is having the opportunity to revisit some old hobbies, and see how they've changed over time.

I got my amateur radio license in my early thirties, and used to be active; primarily SWL rather than broadcasting, and my interests were mainly around HF utility stations. WEFAX, NAVTEX and the like. I also did some satellite reception, decoding NOAA APT signals to get real-time weather maps from cameras right above my head.

It occurred to me that even if I can't physically travel, I could do the next best thing by listening in to the sounds and signals from Far, Far Away 😀

Of course, that's easy to do if you have your own place, a loft or external antenna mounts, and a spare room to put all your gadgets. As we're travelling through Latin America right now - or hope to be, when lockdown ends - weight and space are at a premium.

We're staying in Airbnbs, so anything I do needs to be

  • minimally intrusive

  • temporary in nature

  • small and light enough to pack, or cheap enough to throw away

How do you go about setting up a radio shack with a few bits of kit like that? 🤔

The Rise Of Software-Defined Radio

One of the biggest developments in radio over the last decade or so has been the explosion of software defined radios - SDRs. Radios used to be big, smart, and heavy, with dials and LEDs and plugs and switches.

In other words - they were beautiful. SDRs, on the other hand, are functional.

They concentrate on what radio hardware needs to do: receive radio signals. All the controls, filtering, LEDs and switches are implemented in software. All the SDR really has to do is pull in the signal and talk to the computer.

About 10 years back, a bright spark called Eric Fry discovered that a chip commonly used in modern TVs - the Rafael Micro R820T2 - would be a great choice for an SDR. Not only that, but it was manufactured in such quantities that the cost was minimal. You can now get an entry-level radio based on this chipset for less than US$20.

They don't decode the 1.8MHz - 30MHz HF bands that I used to listen to, but they will tune all the way from about 25MHz to 1.7GHz (or higher in some cases). That's a pretty astonishing result for something so small and cheap.

I was scanning through and my fingers were twitching. What would I actually use it for...?

The RTL-SDR Radio Setup

First of all the hardware. I ordered the NESDR Smart from Nooelec. They're a US-based company who only do SDRs and accessories. The radio is a delight - solid, well-constructed, tiny, with an SMA antenna input. They're thoughtfully designed so as not to block adjacent USB ports - I can easily fit two of them side-by-side on my USB hub:

Solid construction with good metal shielding. Great quality build for the price
Standard SMA connector

The bundle version I got comes supplied with a magnetic antenna mount, and three antennas - a 600mm HF/VHF whip, 270mm loaded whip for 433MHz, and a 120mm loaded whip for UHF:

Supplied with 3 antennas and a base - great for VHF through to UHF
The antenna mount is solid with a heavy magnet. It comes with 2m of goood quality cable. I'll talk about this particular antenna in the next article!

It comes to the princely sum of US$38 plus shipping direct from the manufacturers. I paid a little more because I ordered it from a Mexican agent on Amazon.

OK, now we've got the radio setup sorted, let's turn to the laptop.

The Mac Setup

(Technical Note: I don't propose to give installation instructions for the software or operating systems I discuss in these articles. There are any number of good sources for them (start with the homepage) and I'd only be repeating information.)

I use a Macbook Pro for teaching. It comes everywhere with me. So there's no added weight, assuming I use this as the machine to handle the radio. I love Raspberry Pi and have played with it before, but it's just another bit of weight (and cost). If I was at home I'd definitely look at using them - but not for Radio Airbnb  😀

If you spend any time at all searching online for amateur radio software, you quickly realise there is a lot out there. It varies in quality from bare-bones single purpose softwre, to feature-packed apps hacked together by enthusiastic amateurs, to enterprise grade software for professionals. But there isn't a lot of "Mac quality" software available - there isn't all that much at all, in fact.

As an itinerant English teacher (ie - I'm broke), I wanted to spend as little as possible. So I gravitated towards open source and/or free software. But most older applications were for Windows; there are quite a lot for Linux, but that can involve a lot of messing about under the hood.

There just isn't the variety of software out there to run things natively on the Mac, so that wasn't an option. I loathe Windows with a passion, and there was no way it was getting installed.

That left Linux.

I opted for Linux Mint. It's been around for a long time, has a friendly UI, is fairly compact and runs pretty efficiently. It's also similar to Ubuntu which means few problems with compatibility. I love it.

Linux Mint: super-slick under Parallels on a modern Macbook Pro

I tried getting VirtualBox (the free virtualisation package from Oracle) working, but one technical problem after another led me back, inevitably, to renewing my relationship with Parallels Desktop. Another super-mature software package, I download it every few years and am always impressed with its intuitive setup. It genuinely feels like the Linux virtual machine and OSX are just running in two tabbable windows.

The Basics: Talking to the SDR

Before I started digging into Linux, I wanted to do some basic checks on the Mac. I looked at a few native applications, and decided I liked CubicSDR. Nice user interface, easy to use but still powerful.

Apps like CubicSDR, GQRX and others essentially act as the "front panel" to your radio. They don't do much other than tune the thing. There are other ways of accessing it, but for now this would let me check out the SDR and get used to its quirks.

OSX CubicSDR tuned to a local FM broadcast station. The lower waterfall display shows all signals within a band of about 2.5MHz. You can select one and the upper display shows the zoomed in signal, as well as a scope (here with 2 x stereo channels)

Just quickly scanning around, it was clear right away that the unit wouldn't tune down to HF (but see more about tuning to HF in a future post). I knew this was the case, but had a vague hope that it might somehow grab something. The FM broadcast band stations jumped right out though.

There was some hint of activity around the airband VHF frequencies, packet or other signals going on around 2m, and an array of signals at other frequencies I didn't recognise.

I fired up the Linux VM, installed CubicSDR (it's one of those rare, truly cross-platform applications) and ran it up. It looks and behaves virtually identical to the OSX version:

CubicSDR running on Linux.

Great! I've created a truly portable listening station weighing less than 500g, for VHF to UHF. Now to figure out what to do with it...

Next article: Decoding ADS-B with the NESDR Smart!

Notes On The NESDR Smart

The first NESDR I ordered was DOA. I tried it with 3 different Apple USB-C to USB adapters, on both port of my Macbook Pro, and on my Mrs Wench's Mac. Zilch, so it went back. 8 days later another arrived, which was also DOA. Exactly the same problem.

Kudos to the Nooelec support team for fixing it in about 15 minutes. They've had some minor problems with build tolerances on a batch of these units. The solution is to take off the SMA washer and swap it inside the case. As soon as I did this, the unit fired up.

It runs hot. The manufacturers are aware of this, and insist that it's perfectly normal. Certainly I've seen no signs of instability yet, either in frequency lock or general performance.

I'll add any notes here as and when I come across anything worth reporting. For now I'm more interested in playing with it and finding out what I can hear.

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