A Quasioptical Feed System for Radioastronomical Observations at Millimeter Wavelengths

A Quasioptical Feed System for Radioastronomical Observations at Millimeter Wavelengths By P. F. GOLDSMITH (Manuscript received March 1, 1977) We describe a quasioptical feed system for use with a 7-meter Cassegrain antenna at millimeter wavelengths. This system is designed to take full advantage of low noise, broadband mixer receivers and will be used for radioastronomical observations at frequencies between 60 GHz and 140 GHz. Two offset parabolic mirrors couple the radiation from the f/D = 5.7 antenna into the receiver feedhorn. A Fabry-Perot resonator operating at oblique incidence is used to inject the local oscillator energy into the signal path and to suppress response at the image frequency. The loss of the Fabry-Perot diplexer is 0.25 dB for the signal, while the coupling loss between the mixer waveguide flange and the main lobe of the antenna pattern should be 500 MHz. (ii) A well-controlled antenna illumination pattern which should remain unchanged over as large a range of frequencies as possible. (iii) A provision for making accurate absolute calibrations of the receiver gain and atmospheric attenuation--both of these require suppression of the image frequency response in systems incorporating mixers.