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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