I've been working on an interference problem on a 6 meter repeater and would like to pass it by the "brain trust" for some input.
The repeater is on 53.85Mhz with the input on 52.85Mhz. When the repeater is keyed up, NOAA weather radio comes through the repeater output loud and clear. (Decode PL turned off) I identified the problem as intermodulation in a Yaesu VX-2500V transceiver at the site used for telemetery on a simplex frequency of 173.3375Mhz. The mix is: 4(53.85)-162.55=52.85 (repeater input) The VHF transceiver frequency is not involved in the mix, but the PA stage of the Yaesu is where the mix is being created. I proved this by disconnecting the coax to the Yaesu and the IM goes away. Also, when the Yaesu keys up on 173.3375, the interference goes away on the repeater. The IM is only being caused when the Yaesu transceiver is in the receive mode. No cavity is on the Yaesu, it goes directly to the antenna. The site is on a water tank, so there is only about 10 feet of horizontal separation between the telemetry antenna and the 6 meter repeater antenna. The NOAA station is running 500 watts 1.6 mile away, line of site. I added a VHF cavity tuned to 173.3375 to the Yaesu telemetry radio, but it did not fix the problem. (The can had about 25dB rejection at 162.55Mhz and about 40dB rejection at 53.85Mhz.) Prior testing showed that reducing the 6 meter repeater output from 25 watts to 2 watts solved the problem. My next thought is to put a highpass filter and the VHF cavity in series with the telemetry radio antenna. I am thinking of using a 6/2 meter diplexer, terminate the 6 meter port with 50 ohms, and connect the telemetry radio to the 2 meter port. The diplexer should give good rejection to the 6 meter signal going into the telemetry radio (along with the additional isolation of the VHF cavity) and the VHF cavity would give rejection of the NOAA radio signal. If this works, I will contact TX/RX and see what they can provide to make the installation professional. We are guests at the site and need to provide something professional to the water company. Any ideas? We already thought of changing frequency on the 6 meter repeater, but that would be difficult to coordinate. 73, Joe, K1ike
