On Tue, 30 Aug 2011 15:29:03 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:

Allodoxaphobia wrote:

We still don't have a clear description of what the OP is trying to
accomplish.

Take two radios, two j-poles and a lot of coax and make a repeater.

Retch.

I want to place one J-pole (or similar antenna) at one corner of my garden,
and another at the other one (10 meters apart).

Plugging into:
http://awapps.commscope.com/products/bsa/_calculators/qhisolation.asp
I get about 30dB isolation (assuming 2dBi antenna gains).
The synthesizer noise belching from your xmitter is maybe -60dB down
from the +43dBm xmit carrier. That puts the noise level at -17dBm at
the receive antenna. Your receiver sensitivity is probably -106dBm,
which means you need about 90dB of isolation. The antenna spacing
will provide 30dB of that. You cavities are suppose to provide the
remaining 60dB of isolation. That's not going to happen with tiny
mobile duplexer cavities.

Hint1: You'll be close with vertical antenna isolation.
Hint2: Some radios have quite a bit of synthesizer noise, much of
which will be on your repeater receive frequency. This is why some
repeaters still use crystal oscillators instead of synthesizers.

One will be used to receive a signal, the other to relay it. They both
will be somewhere on the 144-146mHz band, with the output being a few watts
with a max of 20.

Somewhere? Duplexers are usually bench tuned to some specific
frequency. It's not a trivial exercise and requires some expenditures
in time and equipment. You can't easily move in frequency.

Besides the distance, I was looking for a cheap way of not having
the transmitted signal block the receiver. The duplexer in question
is rated at 75dB isolation with a 3.5mHz split, I can only have .6 mHz.

What I was wondering is that since at a .6mHz split, the isolation will
be a lot less, can I somehow combine the two sides to make a better
filter?

You can't improve things by simply adding more cavities. All you'll
do is add more loss:
Bigger cavities = higher Q and therefore closer frequency spacing.
More cavities = deeper notch and therefore more isolation.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558