Hi Bill. The fundamental LO leakage is fairly large for HMC1048LC3B when used as up-converter. This is not uncommon for passive mixers. The conversion loss is typically ~10dB. With a -10dBm RF input signal applied we would expect the output IF signal to be around -20dBm (10dB of conversion loss). Whereas the 1xLO leakage term is 10.3dBc above, we would anticipate LO leakage to be ~-9.7dBm for this up-conversion frequency plan.
I hope this helps to clarify.
Eric N