By connecting the RF generator directly after the CONV-B UNIT, sensitivity went up to -123dBm so the problem was in that module. After the second relay, there is a 20dB attenuator and then a 10dB one. Signal from the antenna is sent to the down-converter for frequencies above 1GHz, or bypassed by the two relays otherwise. The converter unit (CONV-B UNIT) in the VNA This was not right, I had to find why that was. The specifications say better than -113dB which is not very far, but the much simpler/cheaper receiver was almost 10dB better at -118dB.
A trip to the lab and I found the sensitivity SINAD was only -110dBm. Moderate signals were readable on both but I could hear clear voices on the other than I could not even notice on the 8500. I was happy with the IC-R8500 until one day I used a splitter to connect another receiver along this one to the same antenna at the same time, and I found it was performing a worse than the other.
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