I got my Amateur Radio license in 1985, primarily to experiment with data communications over VHF/UHF Amateur Radio. At that time, Packet Radio in Amateur Radio was becoming a very big deal; for a time it kind of dominated Amateur Radio.
In very simplistic terms, data communications over Amateur Radio is actually a fluid exchange of bits between four distinct elements:
- The radio - the piece of equipment which takes a baseband input (usually audio) and translating that signal into a radio frequency signal. Typically (but not an absolute given) the radio usually includes the reverse process - receiving a radio frequency signal and translating it into a baseband signal.
- The CODEC or MODEM - as we know them in 2021, radios are usually set up for analog (audio) input and output on the baseband (non-RF) side so you need a device that takes digital data that you want to transmit and codes and decodes that data. That function is sometimes called a CODEC (Coder / Decoder) or a Modem (Modulator / Demodulator).
- The Protocol Engine - The modem just handles the analog to digital to analog chore. The protocol engine is in charge of encoding the bits so they'll survive the rough journey over the air, where there is interference, fading, and other hazards. The protocol engine arranges the bits "around" the actual data. One example is adding a callsign identifier.
- The Applications - I'm vastly simplifying this element but basically once the protocol engine does its job, it hands the data (stripped of all the protocol engine bits) over to the application. One example of an application is email.
I said "fluid exchange" because the needed functionality can drift back and forth between the various elements, depending on who is actually making the elements. For example, the venerable Kenwood TM-D710GA is a pretty capable radio with a built-in modem (1200 and 9600).
Those of us that just want to play with data, like send email over Amateur Radio, would love for all of the above to be packaged up into an inexpensive appliance. Kind of like a laptop with Wi-Fi, only it works over the long(er) distances that we've come to expect with Amateur Radio VHF/UHF communications. And unlike the black box that is Wi-Fi embedded into a laptop, we do want to play at least a little bit.
In 2021, we're not there yet. By mid-decade, we might be if certain technology trends keep going the right way, like the way we keep getting better and better Raspberry Pi computers for the same $35 over the years.
Given the desire for the "data communications over Amateur Radio appliance" described above, I keep hoping. We got close, briefly, a few years ago in the Northwest Digital Radio UDRX-440 project. I told some of the story about why the UDRX-440 didn't quite make it to becoming a finished product in a previous article - Amateur Radio Digital Communications (aka ampr.org / 44net).
The bigger problem than that what the stillbirth of the UDRX-440 represented, is that Amateur Radio is just a maddeningly small market for a manufacturer to actually create, ship, and support a viable product. (Not to mention Amateur Radio being a market that is maddeningly price sensitive.) What we all want to do with our "inexpensive Amateur Radio data communications appliance" varies widely.
Thus, when I've (unwisely) voiced my quest for said appliance over the years to several people that are way smarter than me, they've given me an answer that, at first blush, was irritatingly simplistic:
GNU Radio is the answer... what was the question?
Irritating to me because GNU Radio is simultaneously an incredibly capable software package, and a near-infinite time sink when one (like me) just wants to do data communications (appliance style) and spend most of your time actually communicating data over the air.
Doubly irritating because, in the end, I've come to understand (and admit) that they're right. The fantasy Amateur Radio data communications appliance (such as the UDRX-440) simply isn't a viable product in a market as small and as price sensitive as Amateur Radio. So they're way ahead of me in recommending GNU Radio to me because anything one wants to experiment with in Amateur Radio data communications is probably already in GNU Radio.
So, if GNU Radio is the answer, how do you get started? Fortunately the beauty of GNU Radio, aside from it being open source and constantly improved, is that it's so widely used that it's already been ported to a huge ecosystem of radio hardware.
At the low end, GNU Radio supports the RTL-SDR types of software defined receivers. Advice - don't go any lower-end than buying a genuine RTL-SDR Blog unit for around $25.00 US. Those specific products pack an incredible amount of engineering and customization beyond the very generic, inexpensive RTL-SDR units; less expensive units (though they "work") just aren't worth the headaches, in my opinion.
At the high end "Tesla" class of hardware, GNU Radio supports the Ettus Research (division of National Instruments) Universal Software Radio Peripheral products. They're pricey, but Amateur Radio operators just wanting to experiment with data communications aren't their target customers.
In between those two extremes there are many software defined transceivers, but most of them suffer from various issues such as availability, cost, and manufacturer / community support. Thus I was grateful when a very knowledgeable Amateur Radio friend tipped me off that the Analog Devices ADALM-PLUTO is about the best "starter" Software Defined Transceiver for experimenting with software defined radio (as in the radio frequency hardware) for doing advanced data communications in Amateur Radio.
The ADALM-PLUTO has much to recommend it:
- Capable - RF coverage from 325 MHz to 3.8 GHz, and up to 20 MHz of instantaneous bandwidth, and has separate receive and transmit antenna ports. Regrettably, this precludes the ADADM-PLUTO from being used on the US Amateur Radio 50-54 MHz (6 meters), 144-148 MHz (2 meters), and 219 / 222-225 MHz (1.25 meters) bands.
- Continuously available by a major manufacturer (Analog Devices) and stocked by a major distributor (Digi-Key Electronics)
- Reasonably low cost ($150 at Digi-Key Electronics)
- Software support includes GNU Radio (and by extension, GNU Radio Companion - the graphical front end for GNU Radio)
- It's easy to experiment with in a lab / experiment setting - it's powered and communicated with by USB
The ADALM-PLUTO is likely to remain in production until superseded by a better unit. The ADALM-PLUTO's "mission" is to be an "RF lab in a shirt pocket" for students such as those in electrical engineering programs, and of course it's in Analog Devices' interest to get engineering students familiar with its products and thus the ADALM-PLUTO showcases several of Analog Devices' chips. Thus it's optimized for cost and functionality, not for usability by Amateur Radio operators.
One benefit that the ADALM-PLUTO has over competing software defined transceivers is that for it to be usable by students, it has to be well-documented, and that seems to be the case - see the ADALM-PLUTO Overview page. Upon first connection to a computer, the ADALM-PLUTO enumerates on USB as a drive, with considerable documentation onboard.
The major downside to the ADALM-PLUTO is that the transmit power is very low - 5 dBm which is approximately 3 mW (3 milliwatts, 3/1000 watt). (Remember, it's powered entirely by a USB connection over a micro USB connector.) Given the intended audience and price point of this product, that's to be expected. To get a more reasonable power level, you would have to implement a power amplifier, and it's tough to make a good power amplifier that works over a wide frequency range as the ADALM-PLUTO does.
Thus I'm creating a new reference page - Analog Devices ADALM-PLUTO - Reference to collect all the bits of info that I'll be gathering up about the Analog Devices ADALM-PLUTO. It's a bit sparse as I write this, but it will definitely grow as my learning curve on this looks like a steep cliff at the moment.
One more bit on the quest for an Amateur Radio Data Communication Appliance - while the ADALM-PLUTO will definitely use GNU Radio... it doesn't have to use GNU Radio. IE, you don't have to constantly be in "experimentation" mode with it. Once you have it doing what you want it to be doing, it will keep doing that (IE, appliance mode)... until you decide to change it. Thus, like a lot of other projects, once you have a successful "recipe" for it, you can clone that setup, your friends can benefit from your work, and you can spend your time communicating.
Thanks for reading!
Steve Stroh N8GNJ
Bellingham, Washington, USA
2021-04-15
Copyright © 2021 Steven K. Stroh