I'm off to EU-011 for IOTA and I know that sometimes it might be good to use an SDR RX to monitor either same or different band for activity. But it is important when TX you connect the SDR input to ground.
The easiest way and an initial scribble (image should follow) suggests the SDR RX RF signal should pass through the antenna relay, this is either switched by the transceiver PTT output to ground, and RF sensed or manually.
It will be important that this is both fast enough to protect the SDR RX and does not cause any relay chatter. Obviously the relay between the SDR RX should be NC and it is switched to OPEN on TRX/PTT.
I suspect 2N7000 or 2N3904 might be used to switch the relay.
The RF sense, the circuit I have from the http://www.norcalqrp.org/nb6mminiboots.htmN6BM miniboots PA worked FB so will probably try to reuse that. Alternate might be from www.radio-kits.co.uk/hunter/RK_Hunter_R1-1.pdf
RF from the transceiver should be rectified with some current limiting. The resulting DC voltage turns on a simple NPN transistor or FET. A switch would have the same effect. The turn on of this device flips a relay so changes the antenna input over to ground. The PTT input should switch in the same way.
I seem to recall it is a good idea (probably stolen for a transverter design) to include some kind of protection to the RX line so a pair of back to back diodes should be placed just after the relay to ground to protect the SDR front end from static or induced current as a result of TX/Operation.
The TX RF will NOT be going through this part so I suspect will need to have a sense circuit added to the TX feeder.