Abstract: In an antenna array of large dimensions, such as might be used for high frequency radar, the antennas 1a, 1b, will be connected by short feeders 2a, 2b, to receivers 3a, 3b, which will consequently be distributed over a considerable distance. Calibration of such an antenna array to compensate for variations in the transfer functions of the receivers will necessitate the same test signal being fed into each element in turn to measure the receiver output, and this could be time consuming and hence reduce the time available for use of the array. To overcome this disadvantage, a loop 5 is connected at various tappings to the feeders 2a, 2b, the respective antennas are disconnected, and sinusoidal tones are injected into the left hand and right hand ends of the loop. The outputs of the receivers are measured, and provide a measure of the transfer functions of the receivers and hence enables discrepancies between them to be corrected.