Floyd Sense wrote:
"I wouldn`t trust an RF-sensed switch to do the job (have never seen one
anyway)."

They abound. Plenty of devices are keyed when RF is sensed. Catastrophic
failure can be avoided by secondary protection for more security.

If simultaneous operation of two or more devices with one antenna is
required, switching between them wont work anyway. Some form of combiner
is needed.

A device for automatic switching is the duplexer used with RADAR which
automatically switches the antenna from receive to transmit at the
proper times.

A diplexer, on the other hand, is used to combine the picture and sound
transmitters to the same TV station antenna. This may be a bridge
circuit or something similar. Its purpose is to isolate the transmitters
from each other while feeding the same antenna.

That was also my goal in combining the outputs of two shortwave
broadcast transmitters to the same antenna but they were of the same
frequency and phase.

The circulator I suggested in an earlier posting is a variation on a
device described in a couple of books I used to have on amateur radio
repeaters. In my case I wanted to make a 100-KW transmitter from two
50-KW reansmitters. It worked well.

The medium-wave broadcast plant I worked in, with 950 KHz and 1320 KHz
transmitters, accessed the same tower through frequency pass / reject
filters in the coax lines to each transmitter from the same antenna. It
was no problem, given the frequency separation between the transmitters.
An amateur VHF repeater has a much more serious problem given a much
smaller percentage frequency separation between its receive and transmit
frequencies. The amateur repeater nust often use cavitiesto separate the
frequencies enough..

Best regards, Richard Harrison, KB5WZI