Repeater Desensed?

[rescuecomm]

My squad has a VHF repeater on the local city utility's water tank. It was working very well until about 4 weeks ago when sleet and snow iced up everything. The repeater got very deaf and I went to the site expecting the antenna SWR to be up there. The antenna was good, but the signal level on the receiver was up there. Opening the squelch on monitor, you could barely hear something. When the transmitter keyed up, there was a tone screech. I put the unit on a dummy load and everything was fine. I tried another antenna to the same effect. When the melt out came the following day, the repeater returned almost to its normal operation. But it still does not have the sensitivity as before the ice up. Our WT's are noisy in areas that were full quieting. We share the tower with a cellular provider and what appears to be a 900 mhz paging service. These units are in their own enclosures while we are in the main building.

A fellow radio tech suggested trying a preselector on the receiver. The repeater is a Vertex VXR-7000 on a Sinclair duplexer. It has worked incredibly well for 5 years until last February. We have a UHF repeater at the same site and the original Motorola UHF repeater was unusable due to on channel noise. The Vertex UHF that replaced it had no problems. I did turn it off just to see if it was causing the trouble (nope). The VHF is our primary tactical freq and we have 25 radios (WT's and mobiles) on it.

Any ideas are appreciated.

Bob

[N_Jay]

Originally Posted by rescuecomm

Any ideas are appreciated.

Bob

I would guess a bad antenna, but hard to tell.

If you have checked everything then it is time to call a technician out.

[DaveNF2G]

You have water somewhere. I would suggest ringing out all of the coax segments to see if one has a high-resistance short to ground.

[ve3nsv]

There is a modification for the VXR-7000 (VHF) repeater that fixes the known desense issue when using an external duplexer.

You have to change out a handfull of chip resistors in the TX section of the repeater. I did this mod for a local ski club that had 660khz TX / RX seperation and they had close to 20dB self desense before the mod and no desense after repair.

If you had no problems before though then I would be looking at the antenna and feedline for problems. Use a service monitor with an isolation T and check for self desense and site noise compared to a dummy load. Then you should be able to tell if your issue is with the repeater, duplexer, antenna or feedline by process of elimination.

[stevelton]

I will second the above post about water on the line somewhere.

What type of antenna are you using on the VHF system?

DB224???

Or something fiberglass?

I have had the fiberglass antenna on my uhf repeater fill up with water before.
SWR was perfect, but it would not transmit more than a block.
I drained the water from the antenna, and back to normal.

If water in the feedline, then the whole line would need replaced.

Check the resistence.

Steven

[rescuecomm]

One antenna is a Stationmaster. The antenna/feedline we are using is fairly new that was put up by another agency before their move to a newer/better sited structure. The input/output split is 3.120 Mhz and is the best I could get during the licensing process.

The guys were using it today with no problems. Very interesting.

Bob

[DaveNF2G]

The water probably left the system. This is temporary; the next major rain/snow/ice event will probably bring the troubles back. You will need to figure out where the water is entering the system and correct the leak. Any coax that has had water inside of it should be replaced.