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<DIV><SPAN class=690035404-06092005>I found it much cheaper to use two VHF
antennas on my mobile. One for APRS and one for Voice.</SPAN></DIV>
<DIV><SPAN class=690035404-06092005><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690035404-06092005>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?</SPAN></DIV>
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<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left><FONT
face=Tahoma size=2>-----Original Message-----<BR><B>From:</B>
aprssig-bounces@lists.tapr.org [mailto:aprssig-bounces@lists.tapr.org] <B>On
Behalf Of </B>Stephen H. Smith<BR><B>Sent:</B> Monday, September 05, 2005 3:45
PM<BR><B>To:</B> TAPR APRS Mailing List;
kkotch@earthlink.net<BR><B>Subject:</B> Re: [aprssig] Duplexer or
Diplexer<BR><BR></FONT></DIV><A class=moz-txt-link-abbreviated
href="mailto:kkotch@earthlink.net">kkotch@earthlink.net</A> wrote:
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<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">Scenario: <ST1:PERSONNAME
w:st="on">D</ST1:PERSONNAME>ual-band mobile and separate 2m mobile with
tnc. <O:P></O:P></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><O:P></O:P></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">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?<O:P></O:P></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><O:P></O:P></SPAN></FONT></P></DIV></BLOCKQUOTE>Unless
you implement a "kludge" of mechanical coax relays to connect/disconnect the
various radios, NO !<BR><BR>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. <BR><BR>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<BR><BR>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. <BR><BR>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.
<BR><BR><BR><BR><BR>Stephen H. Smith wa8lmf (at) aol.com
<BR>EchoLink Node: 14400
[Think bottom of the 2M band]<BR>Home
Page: <A
class=moz-txt-link-freetext
href="http://wa8lmf.com">http://wa8lmf.com</A><BR><BR>"APRS 101"
Explanation of APRS Path Selection & Digipeating <BR> <A
class=moz-txt-link-freetext
href="http://webs.lanset.com/wa8lmf/DigiPaths">http://webs.lanset.com/wa8lmf/DigiPaths</A>
<BR><BR>Updated APRS Symbol Chart <BR> <A class=moz-txt-link-freetext
href="http://webs.lanset.com/wa8lmf/miscinfo/APRS_Symbol_Chart.pdf">http://webs.lanset.com/wa8lmf/miscinfo/APRS_Symbol_Chart.pdf</A>
<BR><BR>New/Updated "Rev G" APRS <A
class=moz-txt-link-freetext
href="http://webs.lanset.com/wa8lmf/aprs">http://webs.lanset.com/wa8lmf/aprs</A><BR>Symbols
Set for UI-View, <BR>UIpoint and
APRSplus:<BR><BR><BR><BR><BR><BR><BR><BR><BR></BLOCKQUOTE></BODY></HTML>