I am suprised 800 will show passing thru in highband cavities, I would
think moving down one rack is the smartest thing to try, as you noted it
will be a low cost trial.
> "Richard Sharp, KQ4KX" 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
>
>
--
73...Clark Beckman N8PZD
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