I don't think its quite that simple (circulator) and I'm not sure how the
circulator solves the problem particularly if it has only two ports. In
addition a circulator for 30-512 probably isn't in existence. Perhaps more
explanation is needed on this, Dan.

This depends on the actual frequencies.
A diplexer must have two non-overlapping frequency ranges to "split" in two.
Like one is 30 - 90 and the other is 100-512. Each can be quite wide.
What your post has is OVERLAPPING frequencies and that is a problem. If one
was 30-90 and the other 120-512 that would be easier. There are units that
are quite inexpensive that will do 2-150 and 200-512.

You seem to have the correct idea about the impedance "seen" at one, but at
the frequency of the other, but this applies when using some methods, not
all.

If the two radios will be operated at the same time, then the problem is
more strict in the requirements of the method used. When one transmitter is
transmitting, it puts considerable power into the system at the transmit
frequency, and harmonics as you well know, however, a transmitter also puts
out considerable NOISE all over the place. There is typically more noise
nearer to the transmit frequency. This noise must be reduced to prevent it
from desensing the other receiver and this may require additional filtering.

I know this makes things more difficult, but from your post, it looks that
way.

73, Steve, K,9.D;C'I



"Dan Andersson" wrote in message
...
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.


Cheers


Dan / M0DFI