Monday, March 23, 2015

Receiving JT65 with RTL-SDR

A couple of weeks ago I bought a RTL-SDR (software defined radio receiver) dongle with the 820T2 tuner chip. You just cant beat the fun/price ratio on this one. For 11$ (with free shipping) it is a bargain, and it is a great way to learn about radios (at least the reception part).

A bargain on AliExpress

Initially I used Gqrx on my Mac to communicate with the RTL-SDR, but eventually I had to resort to SDR# on Windows 7, as it has a bit more features. I have the impression that most radio (and ham-stuff) is made for Windows rather than Mac or Linux.

Apparently not much activity on 10m, but JT65 lurks there in the noise
Even with the standard antenna I had no problem receiving FM broadcast between 88-108 MHz, DAB around 200 MHz, some public transport transmitting FM on 151 MHz and some sporadic narrow FM around 140 MHz. I could also see that there was a lot of activity on 434 MHz and 868MHz bands from car keys, weather transmitters, baby calls and so on.

A few JT65 signals in the WSJT-X waterfall
However, what I really wanted to receive was amateur radio digital modes JT65 and PSK31 on the HF bands (preferably on 10m and 20m). I made a half-wave dipole for the 10m band (28MHz), and soldered the coax directly on the board. The RTL-SDR seem very noisy on HF and all I got initially was a weak RTTY signal on 10 m (which I was unable to decode), and a brief SSB-reception. I guess that one reason for the bad performance is that the strong FM broadcast signals on 88-108MHz interferes with the lower frequency bands.

Receiving JT65 in WSJT-X

However, after a bit of tweaking with the audio levels in SDR# and WSJT-X, I was able to receive some JT65 messages. It was incredibly fun to receive signals from Spain, Russia, South Africa and Brazil (among others) on this 11$ stick and my home-made dipole antenna. (Now I finally understand what the amateur radio people are so intrigued about.)

Received JT65 on 10 m from ZS6KMD in South Africa (9715km)
JT65 on 10m from PY2RED in Brazil (10550km)

Obviously, there are some improvements that can be done with the RTL-SDR to help HF reception. A FM-broadcast bandpass filter would eliminate the interference from the strong FM broadcast transmiters. As I am mainly interested in HF, and do not care about the performance above 30MHz, a simple low-pass filter at 30MHz will probably also do the trick.  Better shielding and filtering on USB can also help. Many folks are also using an upconverter, for example the Ham-it-up, which is undoubtedly the way to go to improve HF sensitivity.

Although I might try to improve HF reception on the RTL-SDR later, I will rather go building a Softrock RXTX SDR Tranceiver. The said kit is already on my workbench, and the soldering iron is hot. More to come...

No comments:

Post a Comment