  We outline calibrated measurements of the microwave reflection coefficient
from the tunnel junction of an ultra-high vacuum low temperature scanning
tunneling microscope. The microwave circuit design is described in detail,
including an interferometer for enhanced signal-to-noise and a demodulation
scheme for lock-in detection. A quantitative, in-situ procedure for impedance
calibration based on the numerical 3-error-term model is presented. Our
procedure exploits the response of the microwave reflection signal due to the
change of the tunneling conductance caused by sub-nm variation of the tunneling
distance. Experimental calibration is achieved by a least-squares numerical fit
of simultaneously measured conductance and microwave reflection retraction
curves at finite conductance. Our method paves the way for nanoscale microscopy
and spectroscopy of dielectric surface properties at GHz frequencies and
cryogenic temperatures. This opens a promising pathway even for dielectric
fingerprinting at the single molecule limit.
