I'll write this to the group, as it is of general interest.
LDF4-50 is not a particularly sensible choice of coax at 10GHz: I think
the manufacturer rates it to about 8GHz. Counter-intuitively to many
people's ideas, it's diameter is too large to use use reliably at that
frequency. It can work very well, but it can also work as a very good
attenuator, with an attenuation figure of 20+dB worse than might be
expected. This is due to higher order wave propagation, where energy
combines with the wanted signal propagated in the simple transverse mode
either constructively or destructively: an effect which can be quite
temperature dependent. I've been there, and spent a lot of time
scratching my head before I realised what was happening
It's better to use a smaller diameter cable. I have a 5m length of
LMR240 which works well at 12GHz carrying the LO signal to my 24GHz
transverter, but it is probably not up to use as a low-loss antenna feed.