[aprssig] Duplexer or Diplexer

Tue Sep 6 05:09:10 UTC 2005

I found it much cheaper to use two VHF antennas on my mobile. One for
APRS and one for Voice.

Anybody have a problem using two antennas? I'm not sure but when I had
one single magmount 2 meter on my cars trunk I think I got better
distance than when I switched to two trunk lip mount antennas. Somebody
suggested that it might be a grounding issue so I peeled the paint back
under the antenna mount and ran a separate grounding wire from one of
the antennas trunk lip mount to a ground that I found in the trunk.
Would size of the ground wire make a big difference? Currently I'm just
using two 20 gage wires twisted together for the ground run. What would
be the best position for the antennas?

-----Original Message-----
From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org] On Behalf Of Stephen H. Smith
Sent: Monday, September 05, 2005 3:45 PM
To: TAPR APRS Mailing List; kkotch at earthlink.net
Subject: Re: [aprssig] Duplexer or Diplexer

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

"APRS 101"  Explanation of APRS Path Selection & Digipeating 
  http://webs.lanset.com/wa8lmf/DigiPaths 

Updated APRS Symbol Chart 
  http://webs.lanset.com/wa8lmf/miscinfo/APRS_Symbol_Chart.pdf    

New/Updated "Rev G" APRS     http://webs.lanset.com/wa8lmf/aprs
Symbols Set for UI-View, 
UIpoint and APRSplus:

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