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We report simultaneous multi-frequency observing performance at 22 and 43 GHzof the 21-m shaped-Cassegrain radio telescopes of the Korean VLBI Network(KVN). KVN is the first millimeter-dedicated VLBI network in Korea having amaximum baseline length of 480 km. It currently operates at 22 and 43 GHz andplaned to operate in four frequency bands, 22, 43, 86, and 129 GHz. The uniquequasioptics of KVN enable simultaneous multi-frequency observations based onefficient beam filtering and accuarate antenna-beam alignment at 22 and 43 GHz.We found that the offset of the beams is within <5 arcseconds over all pointingdirections of antenna. The dual polarization, cooled HEMT receivers at 22 and43 GHz result in receiver noise temperatures less than 40 K at 21.25-23.25 GHzand 80 K at 42.11-44.11 GHz. The pointing accuracies have been measured to be 3arcseconds in azimuth and elevation for all antennas. The measured apertureefficiencies are 65%(K)/67%(Q), 62%(K)/59%(Q), and 66%(K)/60%(Q) for the threeKVN antennas, KVNYS, KVNUS, and KVNTN, respectively. The main-beam efficienciesare measured to be 50%(K)/52%(Q), 48%(K)/50%(Q), and 50%(K)/47%(Q) for KVNYS,KVNUS, and KVNTN, respectively. The estimated Moon efficiencies are77%(K)/90%(Q), 74%(K)/79%(Q), and 80%(K)/86%(Q) for KVNYS, KVNUS, KVNTN,respectively. The elevation dependence of the aperture efficiencies is quiteflat for elevations > 20 degrees.

Single dish performance of KVN 21-m radio telescopes:Simultaneous observations at 22 and 43 GHz