Bare with me this is a bit long. Our repeater is on 146.76TX / 146.16RX. In the next town the repeater having the problem is on 147.36TX / 147.96RX. Both repeaters are Motorola Micors. Our repeater is a 100 watt PA running about 60watts with a single band pass cavity between TX and a 4 cavity BPBR duplexer connected to 250 feet of 7/8 hard line and a DB224 antenna. The distance between repeaters is about 18 miles line of sight with no obstructions. The other repeater is running a smaller PA driver with an output of 25 watts. I am not sure of the full power capability of this driver. This PA then drives a ¼ KW tube type final set at about 90 watts. This is connected to a 4 cavity BPBR duplexer, 200 feet of ½ hard line, and a DB224 antenna.
The problem is when the our repeater 146.76 is in transmit and someone keys the 147.36 repeater a loop condition is set up locking on the 147.36 on until the 146.76 drops. Also audio feedback is heard on the 146.36 repeater. This is typical receiver transmitter intermod. Here is what I have done so for. I checked the 146.76 repeater and there is no signal on it on 147.96 or any other frequency other than 146.76. I then drove to the 146.36 repeater location and from my car in front of the repeater building duplicated the problem by transmitting on 146.76. As we used to say in the phone business, the trouble is leaving here OK. Next I put a calibrated receiver on the TX port of their BPBR duplexer and measured about -50DB when the 146.76 repeater was on. Also I could not squelch out the .76 signal by holding my hands over the antenna of my handheld radio in the building. There is lots of.76 signal down there so I suspect transmitter mixing of the 146.76 with 147.36 in the .36 PA. But where? I disconnected the ¼ KW PA and hooked the driver up directly to the duplexer and the mixing signal on 147.96 is worse. I then put one band pass 147.36, band reject 147. 76 cavity between the PA driver and the duplexer. To my surprise the mixing signal on 147.96 is even higher! I also tried a single band pass cavity with about the same results. Having run out of cavity's I came back home. I realize their PA driver most likely needs to be checked. If the driver PA is ok they will need 30 to 50 DB of isolation between the 147.36 final and the duplexer which will be at least two cavities. I was able to duplicate this mixing with a couple of 2 meter radios, a lossy T connector and a signal generator. I took two high pass cavities out of a duplexer tuning then to pass 147.36 and reject 146.76. I ended up with around 70DB of rejection and 2DB in the pass. I recorded the amount of signal needed to cause the mixing. I then put the cavities between the signal generator and transmitter. To my surprise the amount of signal to start mixing was about the same. The cavities are the type with a T on one port and a compression capacitor for the reject tuning. Some questions I have for the group are: 1. What is the best type of cavity filter to use in this situation and where should it be placed? 2. Has anyone else seen that, been there, done that, and what was your solution? Any information or ideas will be appreciated.
