[amsat-bb] Receiving with an rtl-sdr, desense on transmit

Koos van den Hout koos at kzdoos.xs4all.nl
Sun Apr 30 19:57:23 UTC 2017


I am trying to go full duplex 'on the cheap' by using an rtl-sdr on the
receiving side, keeping an ear on the downlink while transmitting.

Software wise this all seems to work. I use a linux setup, with gpredict to
track the satellites and calculate doppler shift. The gqrx sdr software can
act like a radio that gpredict can control so the receiving frequency is
correct. It controls both gqrx as receiver and my ft-857 as transmitter.

Hardware wise I bought an rtl-sdr with a small preamp and built the pre-amp
into a metal case.

This works up to the level where I received AO-73 and received a weak
Fox-1A and recorded the pass.

But when I pressed the transmit button on the FT-857 (tuned to the 70cm
uplink frequency) the receiver (tuned to the 2m downlink frequency) went
deaf.

Some testing with the arrow antenna and transmitting on 70cm shows the
preamp turns deaf on 2 meter. Even without the preamp but with the internal
amplifier of the rtl-sdr this happens. Setting the gain to 'automatic'
causes a huge drop in signal and takes time to recover.

Complete details, experiences, pictures of my setup and details I left out
start at
https://idefix.net/~koos/newsitem.cgi/1490542545

- has anybody done anything with an rtl-sdr as the receiving side, how do
  you setup the gain? Is there a solution to fix this?

- specific to fox-1a: I am reading somewhat different reports of the
  downlink and uplink frequencies when I search with google. I hope
http://www.amsat.org/?page_id=4690 is right. I did notice the signal is
weak even with an amplifier.

Thanks for any insights and tips,

                                       Koos PE4KH

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