Well, things are starting to heat up here in good old Sydney Town, both weather wise and in the shack. I’ve been playing with the 2nd tracker and attempting to get it transmitting some RTTY, and finally tonight after several nights of learning I finally have it working.
First of all I’m still using the FlexTrack code with a Radiometrix TX2EH but have modified the code slightly to work for my purpose. The code appears to use FSK for a 3v model whilst my transmitter is 5volts. For what ever reason I didn’t have any real success with it. I had to tune my radio to USB to hear any tones, and for some reason I was only getting a single tone.
I wanted to have the ability to decode using a handheld and my handheld doesn’t have SSB, so I looked into the AFSK option. Instead of varying the frequency of the transmitter I generate an audio tone and transmit that instead. Tested the receive with my Yaesu FT-857 and the frequency looks a little off from what I would expect, but this may just need some further tweaking of the code.
Consulting a AFSK tone table for RTTY on the KLOTH.NET website, I chose a 425 Hz shift and selected a 2295 Hz high tone and a 1870 Hz low tone. I then exchanged some lines of code that would normally cause the transmitter to frequency shift with the Arduino tone command. After some mucking around, I finally had some success and was able to successfully decode the RTTY
After setting the correct RTTY decode options in dl-fldigi a big smile came across my face as I was decoding the output from the tracker. The example screenshot is with the GPS disconnected, but with the GPS connected, the zero’s quickly filled with the GPS coordinates.
I’ve still got some tweaking to do, as I’m not happy with the transmitter, it seems unstable with frequency drift and you can tell from the waterfall image it is a little noisy. Hopefully I just need to tweak the code to get this right.
But at least I’ve managed to get it working, as I can now look at experimenting with other digital modes. I know RTTY is fairly old these days and other protocols do exist, but RTTY just seems simple and if it works from near space, then I’m happy. Happy to hear from anyone who can suggest any improvements.
Next I want to de-key the transmitter between transmissions so that a receiver can be used to receive commands from the ground. May have to look at a DTMF decoder or something.
Looking at something like thisan Solu DTMF MT8870 Decoder Module for Arduino.