Koos,

I am working on a similar project and started with an RTLSDR device. I have since switched to using an SDRPlay RSP2, due to its better input filter, Digital resolution, and shielding.

One problem with these RTLSDR devices is that they have very broad-banded input filters. Consequently, the transmitter signal, although attenuated, is received by the RTLSDR, causing it to desense. You will need add more filtering to sufficiently attenuate signals outside of the receiver band of interest.

73,

Mac / AE5PH

On 04/30/2017 02:57 PM, Koos van den Hout wrote:

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

Quoting Eduardo PY2RN who wrote on Sun 2017-04-30 at 20:26:

Hi Koos, I suggest not using preamp since the RTL-SDR has way too much gain already, unless your preamp is narrow band (has a built in pass-band filter, ie: 144 -148MHz) it may be useful as a filter, but in any case you need to adjust manually RTL-SDR AGC to a minimum necessary gain. AO-85 uplink freq 435.180 ST 67.0hz, dwlink 145.980.

Thanks for your suggestions.

The preamp is wide band, so the solution is to find a filter.

The 2m/70cm filter that I already have is the diplexer in the arrow antenna. I'll retry my tests using that.

Koos van den Hout PE4KH