| Feed network, as a passive front end of the radio astronomy receiver, is used to receive, separate circular-polarized signal from radio source and calibrate receiver, and its performance will influence directly the quality of the received signal. Shanghai 65m TianMa Telescope is the largest one in Asia, which is planned to be equipped by eight receivers covering L/S/C/X/Ku/K/Ka/Q. However, in domestic radio astronomy domain Q-band broadband feed network has neither been developed nor been applied to any telescope.In this paper, based on Q-Band (35-50GHz) TianMa Telescope Dual-pixel Cryogenic Receiver, a Q-band feed network, including noise injector,90° differential phase shifter, orthomode transducer and transitions of each part, was proposed. The difficulties and challenges of the Q-band feed network design, which are broken through in this thesis by analyzing with electromagnetic theory and optimizing the structure, are high frequency, broadband, dual-circular polarization, high signal to noise ratio, the stringent tolerance of manufacture.Results of measurement at operating frequency (35-50GHz) are shown as follow. The typical coupling coefficient of the circular waveguide directional coupler is 28 with a variation of 2dB and its cross-polarization isolation of circular-polarized waves is as good as 35dB. The phase shift of 90° differential phase shifter is 82.5°±5°. The output isolation of the turnstile orthomode transducer is better than 32dB and the cross-polarization isolation of linear-polarized waves is better than 32dB. Rectangular waveguide reflection coefficients of two channels of feed network are better than -15dB and circular waveguide reflection coefficients are better than-13dB, measured output isolation of 20dB is good enough and the insertion loss of each channel is 1.2dB or better. The non-gold-plated version of the feed network has been successfully developed with satisfying performance. Few modifications are needed to meet requirements of TianMa Telescope. |
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