Calibration of gain and phase for a phased array system installed on a radio telescope

We report two methods used to calibrate a phased array system installed in an asymmetric parabolic cylinder antenna operated at UHF frequency for radio astronomy. In the loop method, the relative gain and phase for each channel are derived by making a loop of transmission lines linked through each array element. The use of transmission lines of known length enables us to remove the 180° ambiguity in the phase measurement. The calibration system with the loop method can keep gain errors to less than 0.1 dB rms and phase errors to less than 3.2° rms for all channels. Although the loop method is useful for determining the gain and phase following each preamplifier unit, one cannot measure the gain and phase in the path from the reflector to the pre-amplifier unit. Therefore, we used the rotating electric-field vector method (REV method) to deal with all paths including this path. We compare the results of the REV method with those of the loop method and show that the surface accuracy of the main reflector affects the gain and phase primarily. This effect was confirmed to be less by matching the radiation pattern observed from a known radio source using the loop method with the calculated value including the difference between the loop and REV methods.