I'll let others delve into the real depths here...

It is possible (some would say likely) that if you dont separate the
radios by some antenna line filtering (eg diplexer) one transmitter
could damage the others receiver. This will happen even if they are on
differing frequencies because it is likely the first amplifying device
in line will be made to work across a broad range of frequencies. There
is a good chance that you may not notice the damage if the signals you
normally listen to are strong, only to find that the blown front device
is needed when they get weak.

If you wanted to consider a diplexer you have another big problem in
that your operating frequencies overlap and are very wide. Most of these
filters tend to work on a fairly narrow band or widely separated wider
bands. If you segment the transceivers to specific operating frequencies
you can bet that one day you'll want to use the "wrong" transceiver...

You also want to retain a good antenna/transceiver line match. Adding a
T piece and more equipment tends to upset that somewhat. The (extra)
coax length itself will be a load that best case will reduce the
effective power output.

The input impedance of the receiver wont vary as a fucntion of the
receiver operating frequency. Unless of course there is some front end
filtering which is switched in and out. (Thats unusual nowadays - not
having filters keeps the cost down)

I'd suggest that a cheaper path is to use 2 antennas as a
diplexer/filter system will be quite expensive. The other way to do it
might be via coaxial relays that switched the unused receiver antenna to
an attenuator or "open" whilst the other radio is transmitting. This
would take some figuring because you want the relays to changeover
before the radio starts transmitting.

Hope this is of some help.

Cheers Bob


Big Nose wrote:

Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30 -
512 MHz. They would both be tuned into seperate frequencies and need
to share a common broadband, omni directional, vertical monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation and
harmonics would be considered? Would these factors not mean that the
input impedance to the set tuned into the different frequency would
remain high thus not effect matching?