Tell me about it. I'm just a hobbyist myself, but for $REASONS I splurged on a moderately high-end (albeit used) Keysight logic analyzer a while back. And the probes for that dang thing are approximately more expensive (new) than the analyzer itself was. I still only have half a set for that reason. So yeah, probes for high-end gear are a unique and expensive beast all apart from the rest. :-(

Yeah, even with differential 100x probes, how do you get a controlled measurement from the test unit (antenna/amp) to the 110GHz instrument? I've used 25GHz spectrum analyzers and probes... and they're nuts!

If you're in the US and near a major metro, keep an eye on bidspotter. Every once in a while you can catch a lot of high end test gear and/or accessories at lower prices than ebay.

I generally agree, but I disagree with calling 1 mm IR. That's sort of arguing that the whole THz range is IR which I think is silly (I note that you did not make this up, but it is somewhat commonly used)

I kinda agree 1THz is considered Far IR at 100um, which is a LOT of bandwidth! From working with folks who did Tunnel Junction mixers, I think it's because things start looking particle-like, beyond wave-like at those frequencies.

I guess it's actually called EHF up to 300GHz.

yes and there's these for some models https://github.com/OpenHantek/openhantek