Digital Modes

FT8 – How (Not) To Cause QRM

This is a complete beginner post, but I was inspired to write it after seeing a question on Facebook by someone new to FT8, he had completed a QSO with someone, then got a message QSYQRM followed by AHOLE. He was questioning how you QSY (change frequency) on FT8 and if he’s done anything wrong, the operator in question had never altered the settings as he didn’t want to mess anything up.

The response here is clearly rude, totally unesscesary and possibly against license conditions depending on one feels about the phrase used (and the specific license conditions)…however the operator was it seems (unwittingly) causing QRM, so how?

Not wanting to mess things up the op didn’t adjust any settings, so he was just TXing on the frequency that happened to be in the Tx box or whatever frequency he might have set when clicking on the waterfall or selecting a call to respond to.

So how to avoid this?..first of all read the WSJT-X documentation, that should be obvious, but it really is a good document so make use of it, specifically take a look at the FT8 Basic Operating Tutorial…that being said there are a few steps it’s worth pointing out:

  1. Just Listen First – When you load up WSJT-X (or whatever software branched from WSJT-X you use) just RX for a couple of mins, watch the waterfall and…
  2. Select A Clear Frequency – Yes, you need to choose the frequency you’ll be TX-ing on, don’t just leave it where it is, you need to be actively involved to avoid causing QRM.
  3. Hold Tx Freq – This isn’t actually a must-do for using FT8, but the problem with hopping to the frequency of whoever you are responding to (when you double click on the decoded text) is that it might not be clear of activity for the opposite time slot, holding your Tx frequency ensures you are in control of where you are Tx-ing and avoids landing on top of someone…this is annoying when it happens to you, even though it is generally very innocent.
  4. Listen Again – This is really to avoid you being the victim of QRM, take a break from transmitting to check that your frequency is still clear, someone may have accidentally ended up on your frequency (you might be able to hear them, they might not be able to hear you). Regardless of whoever is at fault you’ll probably want to move to avoid interference in either direction.

This isn’t very in depth but hopefully it highlights the important basics to avoid causing QRM.

JS8 & JS8Call Part 2

So a little more about JS8Call, as I mentioned in the previous post the JS8Call software is derived from WSJT-X which most radio amateurs have used, most likely extensively. On top of the JS8 modulation JS8Call is a “directed calling” protocol to allow freeform messages to be sent and received. The messages are sent in blocks similar to the FT8 15 second send receive cycle however rather than a (semi) automated send receive in even and odd time slots you can send as much text as you like, the remote operator can then respond likewise.

In addition to the freeform messages there are a number of other interesting features within JS8Call which I’ll come on to in a future post.

How fast can JS8Call transmit?

JS8 uses similar transmission cycles to FT8 but has variable encoding, so some characters take longer to transmit than others, and there are currently (as of v2.1) 4 different speeds (this is from the documentation, see the latest documentation on

  • Slow – 30 second frames – 25Hz bandwidth – around 8WPM decoded down to -28dB
  • Normal – 15 second frames – 50Hz bandwidth – around 16WPM decoded down to -24dB
  • Fast – 10 second frames – 80Hz bandwidth – around 24WPM decoded down to -20dB
  • Turbo – 6 second frames – 160Hz bandwidth – around 40WPM decoded down to -18dB

The software will show you the average WPM of any message, I believe that the above WPM are taken from the classic PARIS calculation used in CW.

JS8 & JS8Call Part 1

I recently volunteered to give a talk on the JS8 mode and JS8Call software for Warrington Amateur Radio Club. I don’t have a huge amount of experience with this mode so it was very much a talk that I needed to read up and have a go to prepare, I thought I’d pop an overview here for any who may be interested. Much of this was taken from the JS8Call website and the JS8Call guide.

What is JS8 and what is JS8Call?

So the first question that I had was the exact difference between JS8 and JS8Call, my understanding was that JS8 was the mode and that was correct. JS8 is a custom form of the FT8 modulation (Jordan Sherer designed 8-FSK modulation), I’m not 100% clear on the differences but I think one of the main ones is that JS8 uses variable encoding, so the characters don’t all encode to the same size, also there aren’t the character restrictions of FT8.

JS8Call is the software (which is largely based on WSJT-X), so JS8Call allows for the the directed calling (JS8 + Directed Calling), message relaying, message storage…

Installing and setting up JS8Call

JS8Call is fairly easy to install, download and follow the instructions… As JS8Call is based so heavily on WSJT-X it is fairly easy to set up if you’ve already got WSJT-X installed, the options are much the same and so setting up your rig and logging software shouldn’t differ much. If you don’t have WSJT-X installed then there are a lot of setup and trouble shooting guides around. The issues won’t always be the same but many won’t differ.

See below the similarities in the settings.