Abstract: During installation or maintenance of a phased array antenna system in which the transmission line to each element includes an adjustable phase shift device, it is required that the effective electrical lengths of the lines be adjusted to be equal, or alternatively that the phase errors be determined and stored as correction data for use during normal operation. One way of doing this is to phase modulate by .+-.90.degree. the phase shift device of one element at a time, while applying a test signal to all elements. The signal is radiated by the array, received via a test antenna, and supplied to an envelope detector. If the element is out of tune, the envelope has different amplitudes at +90.degree. and -90.degree. of the modulation. The element is tuned by adjusting the length of its transmission line or the adjustment of its phase shift device to obtain a null at the amplitude detector.In an improved technique, to reduce certain errors, sum tuning is used, in which the .+-.90.degree.