Microwave phase comparators for large antenna arrays

Three instruments are described which are useful in equalizing path lengths in large antenna arrays. With each instrument the difference in length between two paths is measured directly, thereby avoiding many of the difficulties in measuring very long paths. A gas discharge tube, used as a modulated reflector, is placed at each of two antenna elements of an array. A null in the detected modulation occurs when the paths leading to the reflectors are equalized in length. The accuracy, limited by instability of the discharge tubes, is about
3\deg
in phase at a wavelength of 10 cm. With the most sensitive instrument, this accuracy is maintained when there is a two-way loss of as much as 100 dB between each reflector and the receiver terminal of the array.