I think this was the best purchase from the Hamkarelia2024 hamfest.
HP/Agilent 11970V harmonic mixer for 50-75GHz, with WR-15 flange.
I hope to use it to potentially see things on 47GHz (iffy) and 76GHz (more likely).
100eur. So not free, but a decent price.
There was also a WR-15 noise source, but that was also 100eur and I had no immediate use for one, nor a noise test set.

