a phase array antenna is monitored by applying to each element of the array in turn a signal known characteristics , detecting the signal output from the selected element as a result of the applied known signal , and comparing the detected signal with the applied signal whereby to monitor changes in the applied signal due to the signal path . the detection is preferably carried out using a single monitoring unit in the near - field of each antenna element . alternatively the detection is carried out using a single source of applied signals a number of individual monitoring units one for each element of the array , and a switching device for selecting the element of the array to be monitored .