Comstar Experiment: The Crawford Hill 7-Meter Millimeter Wave Antenna

A 7-meter offset Cassegrainian antenna with a precise surface has been built and tested. Measurements using a terrestrial source were made and compared with calculations for 19,28.5, and 99.5 GHz. Low sidelobe level (≲ −40 dB) at one degree off the main beam and low cross polarization (≤ −40 dB) throughout the main beam are achieved using a quasi-optical 19/28.5-GHz feed system that also demonstrates very low multiplexing loss (∼0.1 dB). The prime-focus gain measurement at 99.5 GHz found the difference between the measured and calculated gains to be (0.79 ± 0.45) dB, which is consistent with the expected rms surface error (∼0.1 mm). Multiple-beam operation accommodates both propagation experiments with the COMSTAR beacons at 19 and 28.5 GHz and millimeter wave radio astronomy observations without physical disturbance of equipment.