Near-field probing the beam profile of superconducting receivers in the 200 GHz frequency range

At frequencies above about 100 GHz, the most sensitive heterodyne receivers in operation incorporate superconductor-insulator-superconductor (SIS) tunnel junctions. In order to reap the maximum benefit from their ultra-low noise performance, it is important that the beam emerging from the receiver matches the optical design of the entire receiving system. This is particularly so for receivers employing planar feed antennas since their radiation patterns are less well documented than waveguide horns and their open structure nature render them more susceptible to influence from mounting structures. We have implemented a near field measurement system that measures the two-dimensional beam profile of receivers operating in the 200 GHz frequency range in situ. The system is basically an extension of well established near field techniques employed at microwave frequencies and is appropriate for probing paraxial beams where probe correction is not important.<>