RE: [Repeater-Builder] 2m repeater IMD issue

5 May 2004 02:25:18 -0000

Hello Richard,
    Have you tried an additional cavity in the rx path? I had a similar situation with a UHF repeater and 800 mhz systems and an additional cavity worked for my problem. Also, do you have a preamp? If so, where in the rx path is it? You should have all the filtering ahead of the preamp.
 
73,
Bryan
-----Original Message-----
From: Richard Sharp, KQ4KX [mailto:[EMAIL PROTECTED]
Sent: Tuesday, May 04, 2004 8:04 PM
To: Repeater-Builder@yahoogroups.com; [EMAIL PROTECTED]
Subject: [Repeater-Builder] 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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