Skipp has some good ideas, here's another one. Try
adding an LPF between your duplexer and hard line. A
harmonic filter out of a high band Micor or something
similiar. This shouldn't affect your signal at 2M
much, but it should drop the 800 MHz stuff down 80 dB
or more, depending on filter. If there is no change in
IM, then it is probably occuring at the antenna or
tower.
A cracked solder joint in your antenna, or one of the
800M antennas could be causing the problem. This could
act as a mixing point when your 2M TX is on. Of course
anything in close proximity to your TX antenna could
be causing this as well, if it is acting as a
junction.
Joe
--- "Richard Sharp, KQ4KX" <[EMAIL PROTECTED]> wrote:
> > -----Original Message-----
> > From: skipp025
> >
> > I've seen some of the other posts. You should be
> running
> > a dual port circulator in addition to the proper
> duplexer.
>
> Well, I don't necessarily disagree with using an
> isolator/harmonic filter.
> However, the isolator will not keep 800MHz signals
> from getting to the
> transmitter of the 2m repeater. I'm sure the
> harmonic filter may reduce the
> 800 levels some but the isolator isn't going to stop
> it. Of course, an
> isolator will keep other VHF signals from getting
> back through it but I
> don't have any VHF signals that are a problem. The
> duplexer is a new TX/RX
> 4 can vari-notch. I've added bandpass cavities on
> the rx & tx side between
> the duplexer & repeater.
>
> > You mention the 7/8 line is used. If you remove
> the
> > connector from any end and the line is not shinny,
> clean
> > the oxide off with a brass brush and replace the
> connector
> > back on the cable. If the oxide color is really
> dark, you
> > have moisture leakage in the cable and that's
> really bad
> > news.
>
> I'd don't know exactly how old it is but it is in
> "good shape" according to
> the tower crew. It was previously used by a law
> enforcement agency before
> they switched to the 800 system.
>
> > If the PD220-2 antenna is over 10 years old,
> consider
> > trying a swap out. I have replaced a number of
> aged PD
> > antennas, which (along with station masters) can
> become
> > IMD generators when placed in modest proximity to
> other
> > antennas (and metal objects).
>
> I purchased the PD220-2 antenna new late last year.
> It was drop shipped
> directly from Celwave.
>
> I installed a new polyphaser at the entry port but I
> haven't tried taking
> it out of the circuit to see if it's contributing to
> the problem. Has
> anyone seen a polyphaser do bad things?
>
> > You can measure the system desense of the
> terminated
> > feed, which will provide additional clues.
>
> Good idea.
>
> > You should try for min 1/4 wave spacing (at vhf)
> from
> > the 800Mhz antennas, where possible.
>
> The closest antenna to the PD220 is about 4 to 5
> feet away.
>
> > cheers
> > skipp
> >
> > www.radiowrench.com
>
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