As an LMR tech who has seen more than a few
duplexers screwed up by seemingly competent people I need to weigh in here....
I would never "recommend" seat of the
pats duplexer tuning on site as a superior
solution to a "lab" alignment but the variables
need to be removed..That does not mean the
duplexer was tuned correctly as not all shops,
Motorola or otherwise are qualified and/or
trained to do duplexer tuning... "Motorola" only
owns a couple shops left in the whole USA, the
others are private owned shops that are Motorola
dealers or Reps... this does not make them
specifically qualified by virtue of name only...
That being said it COULD be right, and it COULD be wrong...
Assuming they did it correctly there are
many things which could make the SOP tuning
"look" better in the presence of other
problems... if the duplexer was eating more tx
power before, then less tx power could have been
reaching the antenna where it may or may not be
mixing badly with other things....there are some
tests you need to run.. shut the TX OFF and
connect the repeater directly to the antenna and
repeat the rx test from the portable...establish
a viable rx reference... then connect the rx side
of the duplexer in series.. is it the same or
worse ????... should be same.... now while
incoming signal form portable is incoming, add tx
cables and flip tx on or off.. does it go
away???? this is all process of elimination....
Do not assume anything... test each phase
duplexer in as well as out to verify paths did
not change or antenna radiation angle did not
change due to temporary events... VHF band noise
comes and goes here on a day by day basis... too
many variables you need to specifically test and account for....
Get scientific and rule them out... stop
dwelling on the duplexer may or may not be
tuned... test its function in an apples to apples
test... what happened yesterday has little to do
with today... from a scientific standpoint....
Duplexers are gated filters...they pass... they
reject... you need to figure out what is not
happening.... measure power and power out... it
should be less than 3db no matter whose
duplexer....IF the duplexer is part of the
problem as in things get worse when it was in
series of the rx and or tx makes rx worse....
take it back to the shop and have them verify as
something is wrong... if it passes muster you now
have an antenna system problem which may or may
not involve other local antennas or
transmitters.... we once had a 222.38 rx and a
444.75 tx on same site... never did the math
until we finally noticed the 222.38 desense was
tied to the 444.75 tx cycles....just happened that way....
The notch portions of a duplexer are the most
important in protecting your rx from your tx....
they are not normally something you can fine tune
in the field.... I would think you would mostly make things worse...
YMMV
Doug
KD8B
At 11:37 PM 7/29/2008, you wrote:
>Rbers, I posted a note very early this week
>about my looking for a someplace to get a 220
>duplexer tuned in the TAMPA area. Having not
>much luck I contacted a local MOTOROLA shop and
>paid $95 for the service. The receipt returned
>with the cans indicates that the specifications
>published by WACOM are very close. Having tuned
>these merely to incoming signals before, peaking
>them while the repeater is still in a testing
>mode, seemed to return decent results but the
>tune-up was thought to be a better idea. Not
>so
. Todays tune-up hardly was worth the wait
>or the price based on the results. While a 5
>watt HT 10 miles away could work the repeater,
>now 25 watts from a roof top antenna is now just
>about full quieting. Fifteen watts does not
>make the repeater through the same roof top
>ground plane. Does logic dictate that we go
>back to seat of the pants tuning and cast fate to the wind? - Mike
>
