A DIY Radio Data Terminal On The Cheap

What if I told you you could build your own radio data terminal for under $100? And what if I told you its actually a lot simpler than you might think at first glance? Crazy? Nah. It’s actually pretty simple. 

As the students in the RTO Course learn, breaking information down into one of several report formats is once way transmit information quickly and efficiently- that way you don’t forget anything. And during the course we practice sending them via voice as a way to make sure everyone gets over any sort of mic shyness while also getting the proper procedure down pat.

But that said, voice has some issues. First and foremost, in keeping with the basic principles of clandestine radio communications for guerrilla groups, we’ve got to keep our time on the mic short and transmitting power low, and if at all possible, using a directional antenna such as my small UHF Yagi on a simple camera tripod. Transmitting a long message can take some serious time, is susceptible to interference and/or jamming, and also might not be understood on the other end. But most importantly, the largest drawback of using your own voice over the radio is that if a sophisticated opponent is monitoring you, they now have a voice to record and exploit. Students in my Signals Intelligence course have learned exactly how dangerous that can be, creating a massive amount of confusion in a short period of time. After all, exploitation is the primary goal of intelligence- how can I use what I’ve collected against an enemy?

So that points us in the direction of digital communications. For most, one of two things will come to mind here. Either a digital handheld radio, normally a DMR, D-Star or Fusion, or one of the many modes found in a free program called FLDigi. FLDigi is normally thought of only for HF radio, but it has a lot of uses on VHF and UHF as well. And because it gets very little use in these bands, most folks won’t know what it is if/when they hear it. Not only that, with some of the wider band modes available, its transmission time is incredibly fast. A long message, such as a detailed CYRIL report, can be sent in just over four seconds. So if you’re following the other points of a solid communications plan, you’re going to be a hard target to catch.

Let’s talk about how to set the system up. What you’ll need is a Baofeng, Quansheng or similar type handheld with the standard kenwood two prong microphone plug. The next thing you’ll need is an audio interface cable, like the Baofeng APRS K-1 cable. What this does is send the transmitted audio from FLdigi to the radio instead of using the microphone. The most common use for this cable, as the name suggests, is for Automatic Position Reporting System (APRS) which is useful for hams aiding in search and rescue (SAR), but obviously for our purposes we won’t be transmitting position data. The cable plugs into any audio jack found on laptops and mobile devices, and is nearly dummy-proof. Together the total cost is ~$40. Not too shabby.

Light and tight digital data terminal. Concealable, deniable, disposable.

Next you’ll need a laptop or mobile device with FLDigi. The install is pretty straightforward, and there’s a distro of FLDigi for the Android OS. I don’t advise using a phone for this- even if service has long since been disconnected, the phone itself will still ping the closest towers. Unless you disassemble it and remove the transmitter, which might be more trouble than its worth, using it is a liability. But what you can do is either build your own mobile device using a Raspberry Pi, or pick up an Android Tablet specifically marked for wifi-only. But if you’re like me you’ve probably got a few extra devices and older laptops laying around, and there’s no reason not to have one set up for field commo duty. Throw in the cost of the mobile device if you’re buying on the cheap and you’re still under $100.

One thing to note is that the mobile version of FLDigi is a bit different than the full version- its missing several specific modes, including CW (morse code). So its something to keep in mind during your own planning phase. There’s a learning curve to this, and don’t expect it to work exactly right off the bat. It takes a bit of patience and experimentation, but the reward is certainly worth it.

Having a dirt-cheap data terminal is a must-have for clandestine communications. Typing out a message (and even encoding it further via a brevity matrix or One Time Pad) and sending it within a few seconds greatly mitigates the exploitative value of our message. We’ve used less power to get greater range, ensured a solid copy of the message by the robustness of the modes over voice, and the best part, you’re doing it with a tiny amount of money and common-off the shelf (COTS) equipment. Electronic Guerrilla Warfare.

And if you want to learn how to do this, come out to a class. 

9 thoughts on “A DIY Radio Data Terminal On The Cheap

  1. The NBEMS features of FLDigi are definitely an under-used and unappreciated application for VHF/UHF digital mode comms. I’ve used it in the field for ACP-127 formatted messages to QRU stations for passage up to national by M110A HF. Among the choices, MT-63 works incredibly well for NB FM. As you once suggested, I got an old Toughbook for my mobile radio comms and it’s far and away the most versatile platform, hardware/software wise especially if people are technology-fatigued (like myself anyway) and don’t want buy another platform but make existing ones more robust and with multiple uses. With a CF-30 I can run the full FLDigi with FLAmp, FLMsg, and winkey program that works well with the DigiMaster Pro3. Having the same program manage a message that I can then hook up to an HF radio and turn it around for NVIS range or further transmission. The new VARA modem software has FM and peer-to-peer in both HF and VHF and it’s unlocked version uses a quadrature phase modulation. Blows away other HF packet modes without buying PACTOR 4, etc. I do like the idea/concept of a single-board computer for size, simplicity, but for the same price or less having a full PC (air-gapped now of course with Win 7) ruggedized and with more programs available (at least at the moment).

  2. Tator

    Sweet. I’ve been keeping my eyes out for some sort of capable windows tablet that wont break the bank, for something not so bulky as a laptop. There is a lot of things that there is no substitute for on android, unfortunately. Like being able to reprogram radios and scanners for instance for AO considerations. SDR has some functionality, but nowhere near the windows / linux functionality, especially when it comes to some of the more advanced boards / devices that have transmit capability too. And certainly the multi boot capabilities is a big no fuss plus for a windows device as well.
    One of the things I found interesting in one of the earlier articles was the mention of a patriot / militia skunkworks. Something I have been suggesting for a long time now, and that an android platform be developed something similar to the pinephone mentioned earlier also. Looks like they have come a long ways in those goals, and prices seem fairly reasonable, but it looks like they are fighting an up hill battle when it comes to hardware capabilities (if only they had broke the 2 gig mem spec).
    Sony does seem to keep up with the modernity of their devices somewhat better in their open devices project. https://developer.sony.com/develop/open-devices/get-started/supported-devices-and-functionality/

    1. Jefferson Thomas

      I like the older 7 or 8″ tablets with Win 8/8.1 for the portable FLDigi/FLMsg/FLRig. If I could find a good Linux Tablet I’d use that. Putting Linux on an older tablet is do-able, but always ends up crippled.

  3. Hunter

    So what may seem like an obvious point but have to ask. You need the same setup on both ends of this comm link, correct?

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