Feedback loops are used within a Multiport Amplifier (MPA) of a communications satellite to maintain phase and amplitude tracking and hence isolation and combining performance. at Ku and Ka-bands, for which there is increasing interest in MPA applications, and where wavelengths are short and maintenance of phase/amplitude tracking becomes highly challenging. Feedback loops are located at strategic points within the MPA Output Network (ONET) to detect tracking errors and provide compensation. Errors are detected through power measurements at “null points”, with zero power corresponding to accurate tracking. The feedback loops adjust the MPA phase/gains such that the levels at these points are maintained at zero. The scheme operates with a pilot signal for measurement of nulls, injected at one of the MPA inputs.