The repeater is a Tait TB8100 and operates on TX: 155.2500 and RX: 155.6850 and is getting 3rd order intermodulation interference on 154.8150.
OK, we'll you have to first rule out a receiver mix. This is done by measuring the signal level of the mix by metering RSSI or limiter voltage. Then insert an attenuator, say 2 or 3 dB in the antenna port of the receiver and measure the level again. Use a signal generator to replicate those RSSI or limiter voltages. If the attenuated value drops exactly the same dB value of the attenuator, the IM product is generated externally from the receiver. If it drops nonlinear lyrics, say 6 or 9 dB, the mix is inside the receiver.
In the case you are describing, you could have IM generated within the transmitter itself. If so, measuring the product would require a spectrum analyzer and a notch filter set at the transmitter frequency to gain enough dynamic range to see it.
Receiver IM will call for a notch filter at the undesired frequency.
Transmitter IM will call for an isolation panel (circulators) and a harmonic filter. A notch filter at the undesired frequency may be required as well.
The IM product could also be generated in the antenna, the coaxial connectors and even in the coax cable itself if it is constructed of anything other than copper conductors. Low passive inter modulation Low PIM, antenna systems are best solution.
You might even have antenna support structure corrosion creating PIM conditions.
Is this a rooftop or tower? What kind of duplexer etc?
A third order mix is the worst to deal with.
Oh and the TX RX spread of 435 KHZ? Is your duplexer isolation up to this?
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