A novel scanning-probe setup is reported that maps the complex transmi
ssion of microwaves. Response to topographic features as small as 15 n
m was observed. The sharpened coaxial probe tip serves as a microwave
antenna and, at the same time, as a tunnel tip to warrant precise dist
ance control by STM (scanning tunneling microscope) feedback. The inst
rument can be applied to map the microwave conductivity of, e.g. thin
films or low-dimensional semiconductors. Consequences for the developm
ent of an infrared microscope are outlined. (C) 1997 Elsevier Science
Ltd.