ATV Operating (3)
One of my friends recently showed me an interesting YouTube video; a tutorial on receiving digital amateur TV pictures with SDRangel. All I've done is follow the instructions in the video and tested SDR devices which I own. Both RTL dongles worked fine; with the newer metal cased version suffering less drift. My LimeSDR Mini also performed faultlessly; and with the increased frequency range over the RTL devices is a better choice for me. The last test has not worked out well so far and that's using an ADALM Pluto, I understand USB performance may be an issue with this device. I'm still looking at the Pluto setup and I'll add some notes here if I can get it to work. What I'm seeing is an odd shaped constellation with the four points looking like four tear drops.
Below here are a couple of screen grabs showing results at 437MHz and symbol rate 333kS/s.
Link to the SDRangel download which has the DATV plugin included.
Link to the YouTube tutorial video. Below the video you'll find links to ffmpeg and a script which you run to add ffmpeg to your Windows path.
Hope you enjoy playing with this as much as I have. Lastly thanks to YouTuber "Corrosive" KR0SIV.
Following receipt of my 4 metre NOV I dug out my 6 metre kit and hastily got it running at 71MHz. The power amplifier is based on a Mitsubishi brick designed for 50 - 88MHz and so needed no modifications.
Next up; the transmit filter. This is a bandpass type which I originally constructed for 50MHz; luckily it works at 71MHz with a little tweak. It's good to see it put to use again.
My BATC Portsdown is designed to work on 4m and gives enough drive to couple to the PA via the filter. I had wondered if that output connector would ever get used.
With no 4m antenna installed at the QTH I had to rummage around the garden shed and find my old Sirio vertical. The experience reminded me that the shed needs a clear out; large spiders scampered about annoyed by my intrusion.
Raising the antenna just a few feet off the patio I discovered the VSWR wasn't ideal but probably safe enough for today. Given a bit more time I can get it lower using my MFJ analyser.
Putting a call in to Arthur G4CPE we agreed I would transmit to him; my signal looked OK on the analyser. This was going to be a one way contact because my up-converter was in storage.
Here is a screen grab taken by Arthur showing the result; it shows a slightly asymmetric constellation but the spectrum was clean. Power output was 14 Watts.
Tomorrow we'll make it a two way contact
Two way contact now completed.
My receive converter has been swapped out thanks to Arthur. The current one has an amplified 70MHz SAW filter on the input and seems to be much more sensitive.
Arthur G4CPE has modified a PA originally intended for use at a much higher frequency; new coils and capacitors have been substituted and a clean 10 Watts is output at 71MHz.
These are early days and further tests and adjustments will be made. Here is a screen shot showing Arthur's received testcard.
Using equipment seen in my earlier posts a 2 way NB DATV contact was completed on 12-01-14. My QSO with Arthur G4CPE was over a fairly short path; some 4.65 Kilometres.
During our contact powers as low as 200mW were used and signals remained solid throughout. Transmission mode was MPEG-2 at 543kS/s, and the QPSK encoder is a Link Research device with output at 70MHz. My DDS generates 216.500MHz and the two signals are mixed to 146.500Mhz and filtered before amplification by the PA.
Initial transmissions were carried out using collinear antennas. Over the next few day we will be carrying out various tests including some using horizontal polarisation.
Acknowledgements to Arthur G4CPE who supplied some of the components including a complete receive converter and lots of technical guidance. Thanks.