[aprssig] Duplexer or Diplexer
Stephen H. Smith
wa8lmf2 at aol.com
Mon Sep 5 16:45:10 EDT 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
"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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