The CX/Ka-band Receiver system for Geodesy and Astrometry of the Korean VLBI Network

We have developed a new VLBI receiver, which enables to make simultaneous observations in C/X and Ka bands (C: 6.2~7.0 GHz, X:8.0~8.8 GHz and Ka: 28~34 GHz). The primary motivations behind this development were that S-band is mostly excluded in observationdue to severe condition of radio frequency interference (RFI) in Korea and we are planning to participate in the recent activities on multiband Astrometry. The measured noise temperatures of the developed receiver system are around 40K at 6.2~7.0 GHz, 30K at 8.0~8.8GHz, and 30K at 28~34 GHz, respectively. When measuring the receiver noise temperatures, the quasi-optical system was not includedand it is likely possible that the total receiver noise temperatures go higher than the above measured values due to larger beam sizes atlower frequency band compared to mirrors in the quasi-optical system. Contrary to low-frequency VLBI receiver system of which thefeedhorn is located directly at Cassegrain focal point or primary focus, the developed receiver system utilizes a quasi-optics throughwhich beam waist sizes are changed to be appropriate at Cassegrain focal point of the Korean VLBI Network (KVN) radio telescope.Since the KVN antenna was designed for millimeter-wave VLBI observation, the antenna focal length is much longer than typical lowerfrequency VLBI antenna. It means that the feedhorn size for lower frequency observation using the KVN antenna must be extremelylarge. In order to avoid this feedhorn size problem, a quasi-optical system using two ellipsoidal mirrors was designed for the CX/Ka-bandreceiver system. Septum polarizers have been developed for dual-circular polarization observations. Directional couplers for phasecalibration noise injection are combined with the septum polarizer for CX/Ka-band. The developed receiver system with the quasi-opticalsystem will be installed at Pyeongchang site, which is one of 4 sites of the KVN early this year. The second receiver system will beassembled until this year and it will be installed at Yonsei site of the KVN in the next year. These receivers will be used for geodetic andastrometric experimental campaign in collaboration with National Geographic Information Institute (NGII) in the following years. Inaddition, it is also being planned that the developed CX/Ka-band receiver system will be used spacecraft tracking test observations.