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[aprssig] Re: Duplexer or Diplexer

Dave Baxter dave at emv.co.uk
Tue Sep 6 10:45:58 UTC 2005


Hi.

On the works car, where I am not permitted to "perforate" anything!  I
use an Standard dual band handie and booster on low power (5W) + dual
band physicaly short antenna with mag mount for voice, and a TH-D7e +
seperate 2m 1/4 wave mag mount for APRS when traveling.  Each antenna is
about 3 feet from the other.

Yes, the D7 de-senses the Standard, but only for a second every 2 or
three minutes, not a problem.  Also of course, the Voice TX will desense
the D7 RX, so no incoming packets will appear, not a problem.  Messages
do get through eventualy, and I even get ACK packet back about 50% of
the time if messages are sent.

On the "weekend plaything" (a hybrid Land Rover) I have a dual band
fixed antenna and IC706 on one side of the roof, and a 2m 5/8ths whip on
the other side, about 4 feet apart.  Directly mounted and well grounded
to the alloy roof.  (More old antenna holes in that than I care to think
of!)

Again, the D7 when it tx's temporeraly kills the 706's 2m RX, and as yet
even running the full 20W on 2m from the 706, I havent damaged the D7's
RX.  I tend not to do much true voice "mobile" work, as the vehicle is
too noisy to hear the RX properly, especialy "at speed"!...

Using 70cm's for voice no problem, 100W HF of course (parked up using a
"Tank whip") no problem.  25W at 4m, causes some desence in the D7, but
no fried bits, yet.  Havent used 6m /m yet, but dont expect problems if
I had to.

I haven't (yet) used the D7 for 9600bd UHF packet, something to play
with perhaps.  High speed APRS anyone?  (In the UK, we are advised not
to use 9k6 on 2m, but there is nothing in the licence that actualy
prohibits it <G> )

"Kludges" with relays will work, but you would idealy need to also
switch the PTT to disable the "switched out" radio from transmitting,
just in case.  Figuring out the logic to allow uninterupted voice, or
priority APRS use, not dificult, but takes much thought.

It's also worth sequencing things to prevent damaged relay contacts, or
PA stages.  Not a trivial task, but it could be made to work, if you are
desperate to only use one antenna.  VHF cavities used mobile, would
probably not last long, or stay on tune what with the vibration if the
vehicle etc.

Addopt the KISS principle, and try to seperate the two (or more)
antenna's as much as you can. (Feet not inches!)  Certainly do not let
them touch!  That will almost certainly kill one RX, if the other rig is
TX'ing!  And keep the power down.  100W is only 1 "S" point up on 25W
after all.

Above all.  Have Fun!..

Dave G0WBX.




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