Noise temperature contribution of a dichroic plate in a beam-waveguide antenna system

A 34-m-diameter antenna, built for NASA/JPL, utilizes a beam-waveguide (BWG) design. The BWG antenna performances at three different focal points were previously reported by Bathker et al. (1992). The ellipsoid in the subterranean room has since been made rotatable so that numerous feed systems can be installed in a circular perimeter around the ellipsoid and operated from their own individual f3 positions. One of the recently installed front-end systems at a new f3 position is the S/X system, which permits simultaneous operation of S-and X-band downlink. Measurements of the X-band system showed that the dichroic plate noise temperature contribution was 18 K compared to a predicted value of 1.5 K. It was later discovered that if a dichroic plate is installed in a BWG antenna environment, high noise temperatures can result if the receive system passband is greater than the passband for which the plate was designed. A method is presented for accurately calculating dichroic plate noise temperature for these conditions.<>