In article .com,
an old friend wrote:

haing gotten many of the peices of a 222 band repater and wanting to
put it on the air how much sepertion is required to avoid a paying the
price of a duplexer at 22 band

That depends to some extent on the design of your receiver. Different
designs have different RF-signal thresholds at which front-end
desensitization will become a problem. A receiver with a
broadly-tuned front end (e.g. some converted Motorola radios, most
mobile ham rigs) is likely to be more prone to desense than a
dedicated single-frequency receiver board having several stages of
narrowly-tuned helical resonator in the RF chain.

It'll also depend on how clean your transmitter is. Broadband noise
and spurs from the transmitter can cause problems for the receiver by
"swamping" the desired signal with noise, independent of whatever
desensitization the main transmit signal causes to the receiver
front-end.

My old copy of ARRL's "FM and Repeaters" book (copyright 1972)
suggests that approximately 58 dB of attenuation is required for a 600
kHz transmit/receive offset. They don't give a suggested figure for
the wider 1.6 MHz offset used on the 222 band.

According to the graphs in this book, achieving 58 dB of attenuation,
on the 222 band, requires approximately 20 feet of vertical separation
between antennas, or approximately 100 feet of horizontal separation,
assuming unity-gain antennas (e.g. halfwave dipoles). You'd need
greater horizontal separation when using antennas with gain over a
halfwave. You might be able to get by with less vertical separation
if using gain antennas, depending on the actual pattern, feedline
radiation, local reflections, and so forth.

You might want to Google around for articles which show a method for
homebrewing a duplexer, using a length of surplus large-diameter
hardline coaxial cable as the resonator. These have apparently been
used for several years on the 6-meter band, and I recently saw a
Heliax notch/pass resonator which was made for a local 2-meter
repeater (it adds one additional section of receive
filtering/isolation to a repeater which uses a single antenna and a
commercially-built two-transmit-can, two-receive-can duplexer). The Q
of such Heliax-based resonators is lower than that of conventional
cans, and the insertion losses are higher.

If you don't want to install a full-fledged duplexer and use a single
antenna, you might try something like a pair of 222 antennas with 20
foot or more of vertical separation, plus one band-reject (or
band-pass band-reject) Heliax resonator on each one.

http://www.dallas.net/~jvpoll/dup6m/dup6m.html would be one place to
start looking.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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