E&M Signaling, Radio Control, Relay Control

The short answer is to give the relay back and say thank you but no thanks, and then request an antenna diplexer. This will allow you to connect both the transmitter and the receiver to the same antenna without worrying about switching. It will provide you the opportunity for Full Duplex operation.

Refer to R.F. Industries web site

http://www.rfi.com.au/

You don't say what frequency your operating on, RFI are just one of several companies that can help you with diplexing gear.

Duplexer is a great idea. In looking at some of the RFI products, I may have space claim challenges. I do have (require) filters now, I may be able to replace them with these duplexers.

My frequencies are 118.3, 126.325, 121.5, and 128.5 MHz where typical operation is T/R on the same frequency (half duplex) each on its own antenna (air traffic control channel plan lacks the advantages of more advance separate receive and transmit channel plans) where all channels are expected to operate simultaneously.

Part of the equation/requirement is the integration of cavity (notch )filters for each frequency (Rf noisy environment)

Does the Duplexer application require several MHz separation between T/R? Perhaps I can put 118, 121 and 126 RXs on one duplexer (low power smaller combiner?) to one antenna; the same 3 freqs TX on another antenna. and the last, too closely spaced 128 TX and RX on a 3rd antenna. But then, I have the limitation of getting through the cavities tuned to specific frequencies.

Argh!

Ah ha! Now that I have more salient information I can respond to your query with better answers.

The Simplex channels in the "Air Band" do not lend themselves to diplexer use.

You will still need to have Filters in place to avoid other problems and due to the operating frequency they will need to be physically large.

When you mentioned satellite I thought you were operating at UHF/SHF where duplex channels are allocated.

To drive the relay here are some options;

You could drive the relay solenoid with a transistor or FET.

Or you could use a ULN2003 7 element driver, which takes a TTL input and will drive a relay on its output. You might want to drive the ULN2003 with a 75HC14 hex inverter to get the right orientation of the PTT signal to relay operation.

http://focus.ti.com/lit/ds/symlink/uln2003a.pdf

http://www.fairchildsemi.com/ds/2N/2N7000.pdf

Dont forget to put EMF blocking diodes on the relay solenoids.