The obvious first task would be to run the frequencies
through one of the IMD calc programs and see what mix
hits you end up with for the frequencies in use.
> "vintageaudio2004" <[EMAIL PROTECTED]> wrote:
> Bellow please find the copy of said message.
> So this morning I completely disconnected the combiner
> from the repeaters, and hooked one tower antenna directly
> into CH6 TX, and the other into CH5 TX. Then I connected
> the small mobile antenna (just outside the window) as RX
> to the RX multicoupler input. Then I programmed a conventional
> portable radio so I could transmit on any of
> the RX frequencies of the repeaters, so as to try to
> simulate the same condition as when a MPT call is placed,
> the control channel is also on the air, and the radio that
> initiated the call is keyed, which is the
> condition where the interference comes up.
I'm not sure if the above test is worth the time... you might
first want to check the actual desense using the proper
rx antenna after you're sure you haven't shot yourself in
the foot with an IMD Mix problem.
Normal the system back up... put the receiver back on the
top rx antenna. Bypass the rx antenna system preamp. Tx
channel 5 and 6 at the same time, but put tx 6 into the
lower temp antenna (only if it has good swr/reflected power).
Still get the pooped up receivers?
> Now I can manually key up both CH6 and CH5 (with their
> mikes), and when I transmit on the portable the interference
> now DOES come up as when the MPT call is placed. So this of
> course rules out any problems with the Fylde equipment,
> and I guess also the combiner, as they both are completely
> out of the loop.
You need to keep the tx combiner in the path for both
transmitters. The circulators and bp filters really are a
must use. The above test might be generating the same
crap by a different method. As an example your first
problem might be a 3rd order overload (distortion) in your
receive preamplifier. Without the tx circulators and
band-pass filters, you might be generating a mix in one
(or more) of the RF power amplifiers. You might have moved
the problem to a different location.
> Remember that I am now receiving with the small mobile
> antenna, as both tower antennas are used as TX for CH6
> and CH5 repeaters, so the natural way the signal from
> the portable can get into the equipment
> seems to be trough the mobile RX antenna?
If ... a big "if"... the tx5 & tx6 frequencies don't
generate a mix problem, you would need to get a handle
on what's comming in on the rx antenna and how to prevent
the unwanted items from hosing things up.
A mix of tx5 & tx6 main or sideband energy? a mix with
something else? The rx preamp might be hosing up...
Desense and effective sensitivity are measurements you
should make along with antenna 1 and 2 isolation.
Lots of wild cards in the system...
> So in view of the aparent preamp lead I once again
> I started inserting the 6db pad at several places
> in the RX multicoupler chain, and saw some slight
> variation on how many, and which channels where
> affected.
The preamp can be a non linear mixer.. bypass it from
the system until so you don't chase your tail.
> Then I decided to bypass the preamp altogheter (and
> turn OFF the multicoupler power, but I still get some
> interference (much less) on some channels. I guess
> this would definitively rule out the preamp as
> a cause.
Not really... could be a multi tier problem. Leave the
preamp in bypass mode (out of the circuit) for now.
> This way the repeater rx is very hard, as the squelch
> starts to open up at arround 1uV input signal.
Yuck, who makes the repeaters?
>
> So what else in your opinion could we try? Basically
> we are now down to 2 repeaters conected directy to two
> TX antennas on the tower, and the mobile antena outside
> the window used as RX connected to the multicoupler. All
> connections between the multicoupler are now made
> with RG142, and all the 9913 jumpers are out of the loop.
> Between the TX ports of the repeaters, and the 7/8 line
> that goes to the antenna, we used two 1/2-inch superflex jumpers.
>
> Any further suggestions and or help would be greatly apreciated.
> Thanks for all your help so far.
>
> ====================================
Put the system back up as normal, less the preamp in the
receiver distribution side.
Measure the effective site and receiver sensitivity and
desense as described on Eric Homa's web pages (and possibly
the RB pages).
I'd need a bit more information about the receiver pre-selector.
You might read my other post to Bruce about how I construct
a receiver distribution system.
The problematic repeater tx combinations might need different
tx antennas as described in my first post back to you.
No fun isn't it... now take what you're doing and move it
to a busy mountain top with a few radio and tv stations
around. What joy...
your turn
cheers,
skipp
skipp025 at yahoo.com
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