Weak Signal Modes on HF?
Have you ever tuned around 20M when the band is totally dead and not a station can be heard… but you hear this odd sound, like a kid playing songs on a cheap synthesizer? You might think, “Hey, what’s that kid doing on the ham bands!?!”
Well, it’s no kid and no kidding around. It’s actually a digital mode for extremely weak signals, and it’s quite interesting because you can use it to communicate even with signals that are 10db below your human audio perception level!
The program that does this is called WSJT, and it was created by Nobel Prize winning Princeton professor, Joe Taylor, K1JT. Its purpose is to send and receive various weak signal modes for meteor and ionospheric scatter as well as EME (moonbounce) on VHF and UHF bands. And, it’s both free and well supported. (WSJT Summary Page)
But, did you know WSJT is great for HF skywave propagation too? You can sometimes hear it on 20M around 14.090 +/- 10 Khz, and it sounds like a poorly written song.
Songs on HF?
Why a song? Well, here’s my 2 cent theory on it: When you hear a song, you may not now precisely the next note, but you sure do know what notes you’re not going to hear. In other words, you correlate what’s going to be heard next. You don’t know the exact note, but you statistically know what the “scale is” so you’ve actually gained some information. In fact, you’ve added true gain to your actual signal detection, just like you would with a better antenna. Cool, eh?
I first heard of this method called auto-correlation back in the early 1980’s when building an ionosonde receiver for NSF. Back then this mode was used for microwave-band tropospheric backscatter detection (using a 3 megawatt ERP beam). But now, any ham can use similar techniques just by connecting the audio channels of your computer to your HF, VHF, or UHF radios, and you can do it using just QRP, not megawatts.
I discovered how to use WSJT by accident. A couple weeks ago I was tuning around the 20M band during a solar flare and meteor shower trying to decode whatever digital mode I could find. I came across a signal that sounded almost identical to MFSK (multiple frequency shift keying, the name for using a fixed set of tones at different frequencies, and when sent via SSB, it’s actually a very crude low bandwidth digital form of audio-based FM.)
Anyway, I spent two hours trying to decode the MFSK signals but could not. That’s when I remembered Len, WA6KLK, telling me about WSJT and weak signal EME modes. On a whim, I tried the program, and it worked! (You know you’ve got an interesting program when part of its window is showing you the azimuth and elevation of the moon updating in real-time.)
The first station I decoded was a UA0LOQ in Vladivostok, Russia, running 60W. But, here’s the cool part, the band was otherwise dead… and his signal here in Ukiah was 19 db below the noise floor!
It’s not a difficult program to download and run. You can get it operational for reception in just a few minutes, assuming that your computer’s input audio is connected to your rig. Just spot a signal on HF in SSB mode (around 14.090 +- 10Khz) select from the mode menu JT65A (F8 key), hit the Monitor button, and wait a few minutes for messages to appear. Each message is very short, just a few characters, so hams tend to use a lot of abbreviations, similar to CW.
If you can’t hear a signal, go to the WSPRnet Map to check propagation. WSPR stands for weak signal propagation report. It’s a ham network of QRP stations probing the skies for current conditions. The WSPR Database page will show you activity spots.
Of course, the WSJT program has many other features and modes, most of which are still a mystery to me, but the success of using it on HF motivates me to learn more.
So, I encourage you to check it out. It costs nothing to try, and it offers some amazing “gain” for weak signals… and I’m hoping that some of us in Mendocino county might get together and do a moonbounce project of some kind. Fun stuff.