I've seen some of the other posts. You should be running
a dual port circulator in addition to the proper duplexer.
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.
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).
You can measure the system desense of the terminated
feed, which will provide additional clues.
You should try for min 1/4 wave spacing (at vhf) from
the 800Mhz antennas, where possible.
cheers
skipp
www.radiowrench.com
> "Richard Sharp, KQ4KX" <[EMAIL PROTECTED]> wrote:
> Hi gang,
>
> Our club was allowed space (no cost) on top of a 400' tower using an
> existing 7/8" line. The existing UHF antenna was replaced (by the
club)
> with a PD220-2 (142~150 range) antenna. The coverage is very good.
> However, on occasion when users with HTs or during a squelch tail
the IMD is
> audible. I have added cavity filters to both the rx & tx and the
IMD is
> still there. I've looked at it with a spectrum analyzer (connected
to the
> rx port of the duplexer) and I do see IMD when the 2m repeater's tx
is on.
> Although, since I have the cavity filters inline the IMD is only
noticeable
> within the passband of the rx cavity. The IMD levels are around
-90dBm give
> or take a couple.
>
> Ok, here's the source of the IMD. At the top of this tower is also
six
> other transmit antennas that are for 800MHz trunk systems. A total
of 30
> channels. The top platform is rather large but the closest 800
antenna to
> the 2m antenna is about 5 feet. The farthest is about 12 feet. Of
course,
> these antennas are in the same horizontal plane with the 2m antenna.
>
> Signal levels in the 800 tx band that I see at the TX or RX port of
the 2m
> duplexer without the bandpass cavity is about -40dBm. With the
cavity
> about -75dBm.
>
> I'm thinking of just moving the antenna about 50' down the tower
(where
> there's no other antennas mounted) to eliminate the problem. My
thinking is
> that with the extreme RF levels present on the top platform that I'd
have to
> spend a fortune in filters on the 2m equipment & perhaps the 800
stuff that
> it'd just be cheaper to move the antenna from the top. Any
thoughts?
>
> I was looking into a solution using an isolator but after further
research I
> discovered that a VHF isolator will ALLOW RF into the 2m transmitter
that is
> in the 800MHz range. With the bandpass cavity inline there's no VHF
> (150~160) or UHF (450~470) signals that show up on the spectrum
analyzer.
> Only the co-site 800MHz stuff is getting through the cavity.
>
>
> Richard
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