Which SDR Switch?
When choosing your SDR Switch there are 3 main parameters to consider. These will set the performance level of your station, both on TX and RX, for your desired frequency range.
Insertion Loss.
This is how much signal you will lose by inserting your SDR Switch inline for any given frequency. This will always be a negetive number measured in dBs. The closer to 0dB the better your SDR RX will perform.
Return Loss/VSWR
This one is the easy one to understand. With modern transceivers we all aim to have a VSWR of 2:1 or less. Ideally far less, as this effects the performance of both your TX and RX. Return Loss, is quoted as a negative number measured in dBs and is just another way of measuring your VSWR. Return Loss is shown on a VNA sweep as S21 Gain when measuring the VSWR. It will always be a negative number. Ideally you are looking for a VSWR under 1.5:1 measured on a VNA or into a dummy load.
Isolation.
Quite simply this indicates how much RF power is sneaking past your relays and into the front end of your SDR RX when you transmit. Although RF power is measured in dBm a VNA sweep will show you a dB reading. You can convert this dB reading to dBm to find out how many mW are sneaking by if needed. Too much RF power getting past your relays can cause damage to your SDR RX or LNA. On a VNA Isolation sweep, this will show as an S21 Gain measured in negative dB. The further from 0dB the better your SDR Switch is performing. A -49dB reading is better than a -20dB reading. An isolation VNA sweep is easily recognizable as the VSWR will be an astronomical number, such as 5000:1 rather than the usual 1.5:1 you are used to seeing.
The 3 measurements above are the ones to use when choosing your SDR Switch. The manufacturer should always make these extremely clear in their sales literature as these will determine how well your choice of SDR Switch will work on your chosen frequency range.
How to choose your SDR Switch.