Richard,
Have you tried installing a low pass filter between your 2 Mtr duplexer
and the antenna feed line?
I am talking about the large low pass filters that were found in the old
VHF Motorola base stations that were about 1 inch in diameter and about
14 inches long.
I have a similar situation where my 2 mtr antenna is also at the same
elevation as numerous other 800 Mhz repeater antennas and the addition
of a low pass filter in my 2 mtr repeater feed line cured the problem
like you describe.
The low pass filter reduces the 800 mhz energy an additional 60 db or so
to help isolate your 2 mtr repeater from the 800 mhz repeaters.
I also have a dual isolator on the output of my transmitter before it
goes into my 6 cavity celwave BpBr 2 mtr duplexer.
You will probably need both the low pass filter in your feed line and
dual isolator on your transmitter to solve this kind of problem.
Your other solution will be to move your antenna out of the other
antennas main RF field.
John, K7JL
Message: 17
> Date: Tue, 4 May 2004 20:03:48 -0400
> From: "Richard Sharp, KQ4KX" <[EMAIL PROTECTED]>
> Subject: 2m repeater IMD issue
>
> 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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