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[aprssig] Duplexer or Diplexer

Stephen H. Smith wa8lmf2 at aol.com
Mon Sep 5 20:45:10 UTC 2005

kkotch at earthlink.net wrote:

> Scenario:  Dual-band mobile and separate 2m mobile with tnc. 
> Can a duplexer/diplexer be built (or is one available) that would 
> allow both radio systems to be hooked up to one dual-band antenna?  
> The dual-band radios with one antenna lead has a built-in duplexer.  
> Is that where the problem would occur trying to coordinate another 
> duplexer?
Unless you implement a "kludge" of mechanical coax relays to 
connect/disconnect the various radios, NO !

The small boxes  (so-called "duplexers" that are more correctly called 
Diplexers) sold to connect separate VHF and UHF radios to a common 
antenna  (or built inside dual-band transceivers like a D700) are merely 
a low pass filter and a high pass filter in the same box. The low pass 
filter (2M port) passes EVERYTHING  below about 300 MHz.  The high-pass 
filter (UHF port) passes EVERYTHING above about 300 MHz.   They have NO 
SELECTIVITY or ability to discriminate against a specific frequency 
WITHIN a band whatsoever.  

By contrast, true duplexers, used by voice repeaters to transmit and 
receive simultaneously in the same band, pass or reject a specific 
SINGLE FREQUENCY (plus or minus 15-30 KHz or so).   Such devices are 
typically bulky assembies of multiple resonant "cavities" (4-to-6-inch 
diameter cylinders 30-something inches long at VHF connected together 
with a coax cable harness ) -- a bit much to put in the trunk of your 
car.    If you need to transmit on a different frequency, the device has 
to be painstakingly  retuned, ideally with a sweep generator and 
spectrum analyzer.    I

If you are willing to leave the two VHF radios fixed on single channels 
and never QSY (presumably at least the APRS radio would meet this 
requirement),  and you are willing to put up with the bulky cavities, 
then you MIGHT pull this off. However since such duplexers can't 
adequately isolate frequencies less than about 500 to 600 KHz apart on 
VHF, the 2M repeater "sub-band" at 144.5 to 145.5 would be off limits.  
The single, fixed, voice channel would HAVE to be above 145.5 or so.    
Furthermore this would have to be a SIMPLEX channel since a voice 
repeater "channel" is actually two frequencies (one for TX, one for RX) 
600 KHz apart.  

I am actually doing this at home.  I use an old 4-cavity VHF repeater 
duplexer retuned  to allow a 144.39 APRS radio and a radio on 146.52 
simplex to share a single high-gain Comet colinear base station antenna.

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

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