GNURadio AM television receiver with very narrow bandwidth

(2019-07-27) PE1ITR

I participate in amateur television contests and the intention is to transfer a "secret" 4-digit code via the image to the other station. Use is made of the old modulation method and norm, amplitude modulation with residual sideband suppression (C3F). I prefer the 70 cm band, but this radio band is getting busier and it is more difficult to receive the broadband television radio signals.
However, for a contest, the quality of the image is not that important, so both the sender side and the receiver side can reduce the bandwidth considerably.

In an earlier post I wrote how to limit the bandwidth on the sender side with a filter. This is also called SATV.

On the receiver side, I used SDRsharp with TV plugin first. This worked to satisfaction, but the problem of inband interference signals remained. I tried to solve this with analogue filters. The signal was first converted from 434 MHz to 28 MHz. At 28 MHz a filter was placed between the converter and the SDR. This has already led to improvement, but in practice a flexible solution was necessary. Narrow filters and also filters for which you could vary the band pass where needed.
So I thought we should fix this with GNU radio.

The first setup in GNU Radio proved to be a success. I could immediately detect very weak signals without too much interference. This allowed us to immediately win the contest at 70 cm :-). Mission accomplished.

I gave a presentation about this on the VERON VHF day 2019-03-23 Apeldoorn. You can download this presentation here on this website. The presentation also includes videos in which the operation is demonstrated. I have put this on youtube and can been seen below. In practice, 400 KHz medium frequency bandwidth appears to be the best value. Sometimes you look narrower, sometimes you look wider. I used a HackRF with a sampling rate of 10 MHz.


This is a video with an ultimate experiment. When the ssb transponder of OSCAR-100 was initially released, I came up with the idea to put an analog SATV signal of 20watt at 13cm in the uplink. The transponder is 250 KHz wide and so this signal with large letters on screen should also be detectable in the downlink. Below a video of the downlink signal with the GNU radio receiver described above. My call letters were read with some difficulty.

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