5.7 GHz Receiver

October 2023

Inspired by OH2FTG's Youtube Video I bought a RX5805 from Ebay for £5.99. It would have undoubtedly been possible to buy cheaper from AliExpress, but I knew this would have no hidden costs.

The RX5808 is a 5.8GHz video receiver for a drones from 5705 to 5945 MHz [Datasheet]. This fits beautifully (or some might say not!) over the higher two segments of the 6cm Amateur Band, 5650-5680 MHz, 5755-5764 MHz, 5820-5840 MHz.

After a quite easy de-soldering of the case, I found that the receiver comprised an RTC6715 chip [Datasheet]. Pin 43 was IF out. Soldering some coax onto the pad near the pin was a little tricky, but not too bad. I used a cable tie as a low-tech solution to stop it moving about.

The RX5805's frequency switches correspond directly to the RTC6715's, as the chip is in the "easy channel selection mode". All three switches open (not earthed) put the receiver onto 5945 MHz. The chip's IF is 479 MHz. Since I wanted to listen on 5760 MHz; 5760-5945 MHz puts the beacon section of 6cms -185 MHz below the IF at 294 MHz. By changing the three switches on the RX5805, I could change the frequency it thinks it is on, hence changing my IF. 294 MHz is a quiet part of the radio spectrum here, so I don't need to worry whilst I'm testing about screening from local nuisances.

Sure enough, listening on 294.91 MHz on my SDR with the antenna on the bench, I found the GB3ZME beacon in Telford some 25 miles away. The RX is obviously a bit drifty at the start, and I can think of a few ways to help it out.

The next part of the project was to add a bias-T so that the whole receiver can be mast-mounted, and the 249 MHz signal should not get attenuated too much down my UHF feeder. As usual I put a LM7805 in the bias-T for the inevitable day I put 12V+ up the antenna!

The complete prototype system works on the bench. The antenna is just a coffee can "horn", which would benefit from optimization!

I'm not going to win any DX awards with it. It is a pleasant couple of evening's work. The only tricky bit is getting a wire from the IF pad of the chip.

I again highlights the importance of the UK's amateur beacon network in providing a confident reference point for testing receivers.