Chuck W. wrote:
"I`m wondering what is the best practice for connecting two tranceivers
to a single transmission line / antenna,----?"

A "circulator" is the device to route the right energy to the right
destination among multiple input/output devices. It is composed of line
segments in multiples of 1/4-wavelength of transmission line or
waveguide. I made one of 600-ohm open-wire line to parallel two 50-KW
shortwave broadcast transmitters to one antenna and a dummy load in case
of upset. Both transmitters were excited from the same oscillator. It
worked well and there was no upset. I didn`t need the dummy load. The
circulator is sometimes called a "rat-race". In any case, I swear that
they work.

Microwave repeaters are often equipped with waveguide ferrite isolators
which allow energy to pass through in one direction with almost no loss
but which absorb rf power in the opposite direction. These make clever
use of "Faraday" rotation of the plane of polarization as the microwaves
travel through a ferrite slab lying in a magnetic field. These ferrite
sections allow the transmitter energy to reach the antenna but not reach
the associated receiver input of the repeater, so it won`t be
overloaded, desensesitized, or accept power which would be better spent
in reaching the next repeater.

With ferrite isolators, energy can be routed almost at will. I am still
embarrased when I recall dropping an isolator in Motorola`s Chicago
plant 45 years ago. The ferrite shattered like glass. They cost several
hundred dollars each at the time. The Motorolans graciously said to
think nothing of it, it happens all the time. Well I don`t drop valuable
stuf every day. If I did , no one would let me in! , unless I was going
to spend a few millions, which we did.

I think you can probably use a "rat-race".

Best regards, Richard Harrison, KB5WZI

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

I wrote in a prior posting:
"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."

I still have not come across the repeater books but my 1987 ARRLHandbook
has such a circulator (called a hybrid ring) in Fig. 15 on page14-6, A
short escription is on the previous page. How it works is more
completely described in the repeater books.

Best regards, Richard Harrison, KB5WZI