[G0HZU_JMR]

I think it's worth mentioning that the source impedance of the single-ended IF amplifier in the AD831 will be close to zero due to the feedback resistors. It won't be up at 120R as the feedback will make it mimic a voltage source. This definitely means there needs to be a series resistor at its output.

I think the two 82p caps should be something like 470pF or 560pF as this value suits a 10.8MHz IF. This forms a lowpass filter and the aim is to filter away the image frequency and prevent it from reaching the final IF amplifier stage within the AD831.

If the Marconi 2023 is healthy and the 50R coax cable is healthy then the source impedance at the far end of the cable should be 50R regardless of its length. This assumes you have set the 2023 to a reasonably small signal level. The output attenuator of the 2023 should then ensure the source impedance of the 2023 is close to 50R and the 50R cable will also look like a 50R source at the far (AD831) end of the cable. If adjusting the cable length does make a significant difference, then I think something will be wrong somewhere in the hardware setup.