Connecting SDR to transmitting RF path to observe received signals


We’re searching for a better solution for connecting an SDR to the GAS Ground Station, currently used to communicate with the GASPACS CubeSat. Essentially, we have a UHF radio from Endurosat (1W output) connected to an amplifier (10W output), which is able to communicate with GASPACS (both TX and RX). The ground station has a rotator and yagi antenna. We would like to connect an SDR to observed the signals we receive from satellites, so that we can monitor both GASPACS transmissions and add the ground station to the satnogs network.

Here is a diagram of our current setup (left) and desired setup with SDR added (right):

We tried using this SDR switch from MFJ, which does have the correct features for what we need, but is very poor quality and adds a lot of loss and noise (as I’m sure many of you could guess). We either need a way to disconnect the SDR from the RF path when transmitting (to not fry the SDR), a setup with circulators that @phasewrap has suggested, a RF power limiter between the Endurosat TRX and SDR, or something else. GASPACS transmits down “pings” after a command approximately 75ms after tx, so if we use a system that disconnects the SDR from the circuit, it needs to be rapidly switchable. We are using a Raspberry Pi Zero W to send the transmission commands to the Endurosat TRX, so we can control a relay/switch from the GPIO pins if needed.

Our budget is preferably under $500 USD, with a maximum of $1000 (we could potentially exceed that with a very good reason).

We have a pretty good setup with an SSB SP-70, and we’re upgrading our rotator to a SPID RAS, but we don’t have a way to let this station be a part of SATNOGS.

Any suggestions would be greatly appreciated!

-Jack KJ7RSW