Jeff Liebermann wrote:
Take two radios, two j-poles and a lot of coax and make a repeater.

Retch.


Why? Don't you make experimental stuff? I did not say it was a permanent
repeater, it's actually a test bed for some repeater/internet linking
systems we are trying out.

Budget is zero, but I can swing a roll of rg-6 and maybe a $60 duplexer
if I don't buy anything else (radio/computer related) that month.

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.

OK, thanks.


Hint1: You'll be close with vertical antenna isolation.

Explain, do you mean just the distance if they are both vertical, or
the distance AND one is vertical and the other horizontal?

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.

Also almost irrelevant. The radios wll be what I have, HTX-202s or what
I can scrounge (TBD). I'd love to get my hands on 2 channel Maxtracs
or similar radios out of taxis, but the people sitting on piles of them are
much more interesting in refurbishing and enhancing them than selling them
cheaply or giving them away.

I'd be very happy with ones that receive only or have blown finals that I
can get 2w or so out of.


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.

I know, but I don't have a "pair" yet. I'm not even sure there are any
available or a test one. So why be specific when I don't have that information
yet, and why wait for it to ask a technical question, which will have
the same answer no matter which frequencies I use?

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.

And that's the answer I need.

Thanks,

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.