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<a class="moz-txt-link-abbreviated" href="mailto:kkotch@earthlink.net">kkotch@earthlink.net</a> wrote:
<blockquote cite="midE1ECo7F-0000Gy-RX@smtpauth04.mail.atl.earthlink.net" type="cite">
<pre wrap="">radio 1 is a dual-band, radio 2 is a 2m packet radio all connected to 1
dual-band antenna. Both sides of the dual-band and the packet radio can
receive at the same time. It's only when one of the three transmits,
(actually it would be when one of the two radios transmits) that the others
need to have the rf blocked. When the packet radio transmits, the signal is
blocked from backfeeding through the coax to the other radio. </pre>
</blockquote>
[ I assume the "dual-band" radio means 2M/450; i.e. VHF/UHF ]<br>
<br>
Huh?? All connected to one antenna? If you are actually connecting
two radios to one antenna with a T connector or something similar,
NOTHING is preventing one radio from "backfeeding" into the other and
you are almost certainly going to blow out the front end of the other
radio. <br>
<blockquote cite="midE1ECo7F-0000Gy-RX@smtpauth04.mail.atl.earthlink.net" type="cite">
<pre wrap="">When the
dual-band transmits the internal duplexer there protects one side and the
external duplexer circuit protects the packet radio.
</pre>
</blockquote>
Only if the two radios are on DIFFERENT BANDS, not just different
frequencies. That is, only if the dualband is transmitting on UHF.
You CANNOT isolate two radios on the SAME band with a simple
diplexer. <br>
<br>
The external diplexer is a band-sensitive device (just like the
internal one) and since presumably the separate packet radio is using
the VHF-only port of the diplexer, the dual-bander must be connected to
the UHF-only port of the diplexer. As a result, you will only be able
to transmit and receive on the UHF half of the dualbander since the
external diplexer WILL NOT pass 2M on the UHF port. [You might
transmit on VHF and be heard 50-100 feet away, but not much farther
due to the 40-50 db loss that the diplexer's UHF passband will inflict
on a VHF signal. ]<br>
<br>
<blockquote cite="midE1ECo7F-0000Gy-RX@smtpauth04.mail.atl.earthlink.net" type="cite">
<pre wrap="">If it can be done in any dual-band, dual-receive radio, why can't it be done
outside the radio?
Keith, KF4BXT
</pre>
</blockquote>
Because you are trying to use two frequencies on the SAME BAND. As I
explained in my previous post, these diplexer devices are simple
hi-pass/lo-pass filters with the "UHF" side passing everything above
300 MHz or so, and the "VHF" side passing everything below 300 MHz.
Simple diplexers can only combine or isolate DIFFERENT BANDS. They
CANNOT isolate different frequencies on the SAME band. The only
reason the diplexer inside the dual radio works is because the two
haves of this radio ARE ON DIFFERENT BANDS. <br>
<blockquote cite="midE1ECo7F-0000Gy-RX@smtpauth04.mail.atl.earthlink.net" type="cite">
<pre wrap="">
-----Original Message-----
From: <a class="moz-txt-link-abbreviated" href="mailto:aprssig-bounces@lists.tapr.org">aprssig-bounces@lists.tapr.org</a> [<a class="moz-txt-link-freetext" href="mailto:aprssig-bounces@lists.tapr.org">mailto:aprssig-bounces@lists.tapr.org</a>]
On Behalf Of Jim Lux
Sent: Tuesday, September 06, 2005 8:18 PM
To: <a class="moz-txt-link-abbreviated" href="mailto:aprssig@lists.tapr.org">aprssig@lists.tapr.org</a>
Subject: [aprssig] diplexer/duplexers for APRS
It occurs to me that since APRS is a single frequency kind of operation,
you might be able to get a SAW filter made that just passes 25kHz around
144.39, and another that rejects the 25kHz around that same frequency
(actually, it might be the same filter).
</pre>
</blockquote>
SAW filters are low power devices suitable only for receiving. You
can't place them in the external antenna line where they would be
exposed to transmit power. Inserting these into the D700 would
require extensive internal hacking of the D700 to insert these devices
in low level receive-only front end circuitry inside the radio. In
turn, this would be a major undertaking since the D700 doesn't have the
traditional metal cans interconnected by mini-coax with RCA connectors
for it's receiver frontend, mixer, IF block, etc. EVERYTHING RF, IF,
audio and digital, except for the TX PA, are on a single PC board. This
means hacking PC traces with an Xacto knife and tacking on coax jumpers
running to external filters. <br>
<br>
For some pics of the inside of the D700, visit this page on my website:<br>
<a class="moz-txt-link-freetext" href="http://webs.lanset.com/wa8lmf/ham/index.htm">http://webs.lanset.com/wa8lmf/ham/index.htm</a> <br>
<br>
Scroll about 3/4 of the way down the page and look for the topic<br>
<strong><small>"Interior Views of Kenwood TM-D700 146/440
Dual-Bander with built-in TNC"<br>
<br>
<br>
<br>
</small></strong>
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