My guess would be a sensitivity drop. The noise figure of the BB60C is pretty high, and unless you have a LNA before it, you can lose a good amount of SNR that way.
We had a user using the BB60C for test, and saw the 20dB issue. Since they weren’t setting the noise figure of the test using an LNA, the noise floor of the BB60C was determining sensitivity, and it was fairly bad. Then when they changed the Ref level setting, which controls the attenuator, they kicked in 20dB of attenuation up front, it naturally increased the noise floor by 20dB, and they came to me with this exact problem that you’re posing here.
We tossed in a cheapo LNA in front before the BB60C, and it all worked out nominally regardless of BB60C setup.
A good check for monitoring ANY signal is to take your end measurement device, and kick in 10dB or more of attenuation. With spectrum analyzers you can do that by either manually setting the attenuation, or adjust the ref level to make it change the attenuator. Other devices, you put a 10dB pad in front. If the SNR measured changes at all, then you have a bad setup and need to stick more amplification in somewhere to compensate for test losses.
edit: btw, it’s not an anechoic chamber without foam. It’s a faraday cage. It’s the anechoic foam that makes it an anechoic chamber. An anechoic chamber without absorber on the walls (aka a Faraday cage) is typically worse than just doing it in the open unless you’re time-gating to ensure you’re not getting any reflections. I don’t doubt that small changes in the geometry of anything in the chamber can result in 20dB swings due to standing wave interference patterns. You’re building a mode stirring chamber without stirring the modes or the fancy math to back out performance parameters. I understand that you’re trying to shield yourself from external interfering signals, but you need some absorber in there otherwise you’re just in a big echo chamber.
You’re in essentially the worst environment for testing anything that’s not a pulsed, low duty cycle waveform. Get some absorber, mount it to some plywood and build a half-box around your antenna. It’s cheap, easy, and will dramatically improve test repeatability.
