Digital HF Transmission Methods vs. IMC

Posted: 09/17/2015 in Communications

What follows is a belated response to a recent comment regarding Morse burst devices.

Most folks don’t realize that international Morse Code (IMC) is a very early form of binary communications; Dits and Dahs or ones and zeros.

All Amateur (Ham) license holders before 2007 and every Special Forces 18E (Communications NCO) before 2002 were required to learn IMC. Why? Because prior to Satellite communications and the advent of some of the newer software based data transmission methods now available, it was the preferred method of making reliable long haul HF Communications. IMC is more reliable by magnitudes than voice communications across the HF spectrum. It still remains, in my opinion, THE method requiring the least amount of fragile (breakable) equipment. Since the code requirements have been dropped on the civilian side, very few ham operators are willing to invest the time to learn a new language (IMC). In my day, most 18Es could send and receive Morse at 18-5 letter groups-per-minute (GPM) . I knew some that could regularly tap out 24 GPM with a leg key. But comms guys, being the techno geeks they are, were soon experimenting with ways to speed up the method of delivery. More speed meant less chance of intercept and DF. But more importantly, it meant less time someone had to crank that damned G-76 generator.

And in Army SF, why bother with a mode of communication that requires the apparent use of special, secret incantations muttered by the senior comms guy hovering over the AN/PRC-74. He was usually seen conducting this ancient rite while the junior comms guy twirled the appropriate analog knobs and flipped some switches that were labeled on the same heavy green radio in some archaic hieroglyphic script known only our secret sect. It is much easier and faster to throw out that TACSAT antenna, point it at the bird, punch in the freqs, load the fill and talk. So easy in fact, that even an 18B like Mosby could do it….. I digress…. Back to IMC.

One of the first high-speed, low drag code burst devices I had the misfortune of working with was the AN/GRC-81 Coder Burst (took some real imagination to come up with that name, huh). Basically, by doctrine, the Team Commander (in actuality, one of the Echos) encrypted the message on a one-time-pad (OTP), recorded the message onto a tape using what looked like a modern Dyno embossing machine, hooked the device to the transceiver and sent it. The receiving station recorded the message to a tape, played it back and broke it out with the other half of the OTP. It generally sped transmission up to 300 WPM. If you are interested, here is a link:

Next came the Racal DMDG designated the OA-8990/P (or KY-879/P). The DMDG weighed in at about 9 lbs and had a send/receive rate of about 266.6 baud or 27 characters a second when used with HF.  The message was still encrypted using a one-time-pad (OTP), then was typed into the DMDG using the top mounted keypad and displayed on a then state-of-the-art LCD display. The DMDG was connected to the radio and the message was sent. The receiving station used the same device to record and display the message. Again, it was manually decrypted using the other half of the OTP. A typical 120 group message took about 38 seconds to send or receive. It could also be used with the early AN/PSC-3 Satellite Communications Transceiver. Manual link here:

Finally, we were issued the tiny (by 1980 standards) TRW KL-43. The message was typed into the device using the front mounted keyboard. In most cases we used the eraser end of a lead pencil, since the keys were easy to fat finger. The key was pre-loaded into the device (goodbye OTP!) using the same keypad and could be updated remotely. The KL could be used to send/receive over multiple devices including telephone, satcom and just about any radio transceiver. Transmission rate was about 30 characters-per-second on HF.

Nowadays, it seems that the majority of SF comms guys could care less about HF comms. Not nearly as sexy as that TACSAT or world-wide computer network. That being the case, you will now find that almost all HF data transmission innovation comes from the civilian amateur radio arena. And I have to say, most of them work as well, and a lot faster than IMC. So let’s look at what’s available.

Rather than list every available program out there, I’ll list a link that provides a fairly detailed description of the most popular:   A side note: most of these programs require a modem between the computer and the transceiver. Some of these modems are specialized and can be very costly. I have limited my discussion to programs that I have used and that can be interfaced with the Tigertronics Signalink USB. The Signalink can be had at their website for about $120.00 to include the cable for your type of radio.

Which ones do I prefer? Depends on what I’m trying to accomplish.

DigiPan is a pretty good place to start. Easy to install, run and won’t overwhelm a new operator. Don’t expect your messages to have any anonymity if that’s what your after. Everyone on the freq with the program can read what you send. PSK has it’s limitations, but it can be a lot of fun for the beginner. Free.

Airlink Express is DigiPan on steroids. Lots of new features and easy to use if you are familiar with DigiPan. Free-donations accepted if you are happy with it.

RMS Express is probably my favorite. Replicates email on HF/VHF/UHF radio with Peer-to-Peer (P2P) and Bulletin Board Services (BBS).  You can set up your own private network or join several existing networks. Fairly secure comms set up in a nearly bullet proof format. Free, but donations are accepted. Set up with a NVIS antenna and you’ve got your local-to-mid distance network.

The next 2 programs only require a computer with a soundcard and the radio.

IZ8BLY Hellshreiber is an easy to use, very reliable system that works well in noisy conditions. Free, will take donations.

TruTTY – lots of modes, somewhat complicated. $39.00

MultiPSK – The Mother of All Communications softwares. Pretty complicated for the newbie.

Bottom line: IMC will usually get through when voice comm fails due to HFs inherent noisiness. For those who don’t care to learn IMC, a digital program will probably do just as well if not better. However, it’s pretty hard to beat the equipment simplicity of an IMC key.

  1. Catfish says:

    The digital modes software link appears to point to /dev/null. After poking around a bit on I found this link which seems to work better:

    Also, what about fldigi? It encodes & decodes many digital modes for free.

    • danmorgan76 says:

      Link fixed, thanks. I left out flidigi because I have never used it. The link you provided is pretty exhaustive. However, the author states he doesn’t update it and so some of the newer items like WINMOR and RMS Express have been left off. He gives a newer link at the top of the page.

  2. […] You will want to know both, especially if poor propagation conditions continue. […]

  3. Enob says:

    I’m just getting started with this and have what most of the ppl on this blog would consider zero knowledge on this subject, but I have a question. All of these are PC based programs, does anyone make a mobile version of this? Could a smart phone even be used to do this?
    I would think being out “in the field” with limited electricity and the ease a mobile device can be charged, a modern smart phone would be ideal in stead of a bulky power hungry laptop. Or have I completely missed something?

    • Enob says:

      it would seem that there are mobile apps that, if I’m reading correctly do what the mentioned software does plus more.

      Wolphi Solutions has quite a few mobile device apps.

    • danmorgan76 says:

      You are definitely missing the point. Your mobile apps and devices are dependent on someone else’s huge network requiring vast amounts of infrastructure. I power my HF base station (transceiver, laptop, modem, and antenna tuner on my home solar system. I can move all of it to any of my vehicles and power it from the vehicles 12 volt DC system. In the field I run my 8″ Wintab and Elecraft KX3 from small AA rechargeable batteries which I can recharge using a small, fold able PV panel. Since my system has been replicated by a few close friends in the area, we have a nearly bullet-proof comms system that allows private comms that aren’t stored in some facility in Utah and again, (very important) doesn’t rely on someone else’s infrastructure.

  4. idahobob says:

    There are digital coms tutorials at the AMRRON site.


    • Catfish says:

      Good point Bob, AMRRON is a great resource with more info on fldigi and some audio files you can practice decoding.

    • danmorgan76 says:

      Yep, AmRRON is definitely the place to go. Should have included it in the post.

  5. joe says:

    Have any of ya’ll tried this on a tablet device? Low power drain, etc, what is not to like?

    • danmorgan76 says:

      I use RMS on my 8″Wintab with my Elecraft KX3 in the field quite often. Once RMS is downloaded the tablet stays air-gapped at all times. I don’t use it for anything else.

      • matt says:

        So you also have to carry the modem in your ruck as well? If so how is that powered and is t it rather bulky?

      • danmorgan76 says:

        Yes, I carry it in my ruck or one of my guys will carry it in his if it is cross-leveled. It ways just a few ounces and is a little bigger than a pack of playing cards.

  6. Alfred E. Neuman says:

    Reblogged this on The Dixie Traveler.

  7. Gary says:

    “So easy an 18B like mosby…” Lol….
    Thanks Dan!

  8. Shocktroop0351 says:

    Thanks for the write up Dan. I just wanted to give some words of encouragement to your other readers. I’ve just started cutting my teeth on hf comms the last few months. I actually only got my tech license and general license at the same time this may. For those of you on the fence or doubting yourselves it is actually very easy to get your licenses with a little bit of studying. And if I can do it with a two year old, a one year old and a full time job, I think most other people should be able to. There are some really useful, free apps on smart phones to help you study, and there are also websites dedicated to helping you take your test as well. After you get your license join AMRRON and start following their guides and join in on the nets. My next step is to learn Morse and get a little cw transceiver. I know it may not be as interesting as playing with rifles and gear but the first time I threw a random wire antenna tied to a half empty water bottle over my house and started listening to conversations between people all over the country I realized just how awesome of a tool my radio will be come shtf.

  9. Badger says:

    Wonderful (and timely) post Dan! Winlink for long-winded stuff & traditional IMC still works for me as tested during the recent “sky is falling” solar event. I will reflect from when I got into digital that the Signallink gear is about as foolproof (and they make it so easy to configure) that it should be near the top of most folks’ list. (Old much-loved J37 key here but the little paddle goes in the bag.)

    AN/GRC-81; one of those memories that can make one laugh & shake their head at the same time. Got to use KL-43’s to off-line prepare debriefs from all the criminals, er uh, refugees Castro sent us via Carter’s hospitality. That little box was also the one that a certain LTC North managed to proliferate (and lose) down in some CenAm jungles.

    The worse the bands get folks, the narrower your bandwidth needs to be.
    Good stuff. 😉

  10. Mike Bishop says:

    Throw up a Yagi, get a bearing to the other operator, and you will start having fun with Digital Modes…

    • Jeff Alan says:

      Could you do a write up specifically on the RMS express capabilities and how you use it with your friends?

      • danmorgan76 says:

        Hopefully I will be able to write something up within the next couple of weeks. Lots of training going on now that has sucked up all of my spare time.

      • Badger says:

        Not to hijack Dan’s forthcoming effort but I will mention something specific to RMS Express, and some other software-aided modes in general. If you are testing in good times & don’t have a circle of friends to do that with yet & using RMS relays, it will pay to know one’s antenna capabilities (real, not theory) and the antenna’s holes. Sometimes the “best” suggested relay may not be the best for your capabilities and your knowledge will trump their suggestion. When going point to point to another individual HF station this importance of knowing your real capability is ten-fold. Other than that, it is Looking forward to the write-up.

    • danmorgan76 says:

      The Yagi works great when using line-of-sight comms on some of the higher frequency ham bands such as 2 meters up to 23 cm. Maybe even 6 meters if you don’t mind using a Yagi with 1/4 wave length elements up to 4′ 8″. Here in the mountains line of sight is about 1/2 mile. My HF set up allows pretty reliable comms at present from 0 out to about 350 miles. It allows me to fill the gaps in the surrounding valleys (where most folks live in the mountains). I can change the distances by raising or lowering my antenna taking advantage of NVIS. Hard to do with a Yagi.

  11. Reblogged this on scavenger xray and commented:
    Just did a test with fldigi and my phone, generated a 50wpm cw message to a wav file, tested the wav file by importing to fldigi, then transfered to my phone and played next from there into the built in microphone on my laptop. Picked it up just fine. So maybe 50wpm cw bursts are doable, and doesn’t appear to use much more bandwidth. Something to add to the commo book.

  12. TeufelsbergSGT says:

    Ha! AN/GRA-71, coder burst keyer group; guy-rohn-teeeeeed to not crack, chip or peal for the duration of your mission… Oh brother do I feel your pain. You had to have a feel for how fast/slow to draw that little handle back while putting the ‘blips’ on tape – The case even came with a tiny camel hair brush , made like a lipstick (for !LTs on date night) to clean out the clock work innards.
    The only problem with Morse is that you have a training and retention burden. However, since the human body has a higher reliability in rough conditions than even the coolest tech devices, I’d say it’s best to know both, so that you can communicate with minimal tech-burden. These days, if the little tablet you’re using as a burster (FLdigi in a fast FEC mode ) decides to take a dump (BT&DT) during your PRIMARY sked, you can still make your ALTERNATE, CONTINGENCY or EMERGENCY skeds and let Mama know in Morse where you want the pizza air dropped.

    Teufelsberg SGT

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