A reader (they’d like to stay anonymous) of DX-World got in touch about a hidden system issue with regards WSJT-X FT8 Fox/Hound DXpedition mode and we thought it useful for others to read about too:

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There is a hidden system issue with the WSJT-X FT8 Fox/Hound DXpedition mode. It is NOT a software bug.  Instead, it’s a not entirely appreciated condition having to do with receiver performance.

It seems that many radios cut off the low frequency of the audio passband normally used by WSJT-X. This is often something to do with the firmware in the radio, or the settings used on the radio.

Here are plots I made using the internal waterfall graph within WSJT-X. In all the cases, the plots were taken with the receive frequency set to above 28.100 MHz, on a pretty dead band.  All the receiver gains were turned up so that the noise floor shape could be seen.  Receiver AGC was turned off. This effectively shows the receiver passband.

This is with the (anonymous) radio set to DATA mode, as you might naturally decide to use with WSJT-X. The receiver Passband Tuning was set to 4000 Hz bandwidth.

See how the noise rolls off rather dramatically starting at about 400 Hz?  (That 4000 Hz passband bandwidth is obviously an estimate.)

The problem for Foxes is this.

The default Tx Freq for a Fox in DXpedition mode is 300 Hz.  The Hounds call above 1000 Hz.

Imagine that P5JX is using the default Fox Tx Frequency of 300 Hz, and his radio has a passband like shown in the screen shot above.

MM0NDX calls P5JX at 1140 Hz and P5JX puts him into the queue.  P5JX will then send MM0NDX a report.  Let’s say that it’s -01.

MM0NDX’s radio will then tune itself down to 300 Hz to respond to P5JX.  But, MM0NDX isn’t heard because P5JX’s radio rolls off a lot at 300 Hz and MM0NDX is in the noise.  Oops.

P5JX will send a report over three sequences to MM0NDX.  Unless a contact is completed, P5JX will go on to the next caller in the queue.  At the same time, MM0NDX’s radio will retune itself to 300 Hz higher and keep calling.

If P5JX is paying attention and MM0NDX isn’t being interfered with by other stations put into the repechage band, P5JX might put MM0NDX into the log and send RR73 if he sees MM0NDX’s report. If not, well, the QRM piles up and MM0NDX has to find another frequency to call above 1000 Hz. Hopefully, this pattern doesn’t repeat itself. But, very often it does.

How many times have you seen this on the bands?  Not especially efficient.

The good news is that there are a couple solutions for Foxes confronted with this.

1. Buy a new radio without this problem.  (That could be tough if you find yourself on an expedition somewhere.)

2. See if there are adjustments or different operating modes that get around the issue.  For example, the very same radio as shown above, with the very same settings (receiver Passband Tuning still set to 4000 Hz), when used in USB mode instead of DATA mode looks like this:

Now, the radio is flat down to below 200 Hz. That default 300 Hz Fox Tx Frequency looks pretty good now! (4000 Hz is still an estimate..)

Here’s a comparison of the two:

3.  Make a slight setting change in the WSJT-X software, which is very simple to do.

The Fox can change the Tx Frequency to, maybe, 500 Hz.  Some DX stations have already started doing this, to great success.

Look down to page 6 here:  https://physics.princeton.edu/pulsar/k1jt/FT8_DXpedition_Mode.pdf  This is a screen shot of the applicable section:

Simple as that. Now, the Fox will transmit at 500 Hz and up and the Hounds will respond there, where the receiver passband is flat. Many people will be happy. Smiles will flow across the radio bands.

This is a known thing, but it hasn’t been explained very much.