Friday, July 23, 2021

The best digital mode for weak signal keyboard to keyboard QSOs?

Since moving from Sydney to Melbourne I've wanted to be able to chat with my Sydney friends but have wondered about the best mode to use for this from the huge list in fldigi.

Stephen, VK2BLQ, and I have been running WSPR and watching the signal to noise ratio for band openings on 40m. Today looked like this:

When WSPR shows an SNR of better than -10db we try for a contact. Until today I had thought that the best mode would be one of the wide modes such as Olivia 16/500 and we tried that for a while. We could hear each other but could not decode.

On a whim I suggested switching to good old BPSK31 and suddenly we were talking!

I find this rather puzzling and would welcome an explanation. Some commentators compare digital modes and rate BPSK31 very poorly compared to other modes. Local hero David Rowe has been doing some work on data modes for HF radio and I intend to try out what he's been working on.

K8JTK has quite a nice writeup of different modes here and we plan to test QPSK31 over BPSK31 soon.

The noise level here is distressingly high. A solid S9 of broadband noise. I've tried a loop antenna but it's similar.

Update: Good contact on Saturday

I received VK2BLQ at -10 on WSPR so we had a good contact using BPSK31 just now (local time 10:30).

For a while we had perfect copy (some errors are Stephen's typing) but then signals faded and I have a storm coming in.

We experimented with sending PSK carrier and measuring S/N and IMD. Stephen receives me with S/N 25db and IMD of -22dB which seems pretty good. I don't get him as well but we did determine that him reducing power a bit gave rise to a better IMD.

Noise levels for both of us are down today so that might be the explanation rather than just improved conditions on 40m.

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