Dan Andersson wrote in
:

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?


Simple...

Get a two port circulator. That'll solve your problem ( but at a
price. Try E-Bay ). At your low frequencies, try build one yourself.

Regarding the impedance, that is not that problematic. You can
basically transform any impedance to your taste. You will incur signal
loss tho'.

Also, as you unfortunately are looking at the lower end of the RF
spectra, all these parts will be bulky but it's still possible.

Possible, but a bit tricky to set up, since you'll need quite a few db of
isolation between the ports for this to work even reasonably. 60db would
be enough for separations in the range of 10mhz or more, but below that,
you're starting to need 75 or 80db in order to hear something on the
other receiver.

If it's only necessary to use one radio at a time, then maybe 25 or 30db
would do it OK, since all you really need to do then is prevent receiver
damage. But if it's desired to use both radios simultaneously, then the
more isolation, the better.


--
Dave Oldridge+
ICQ 1800667