A method of determining one or more attributes of an antenna beam based on measuring phase and/or amplitude differences at different points in a feed network. Amplitude and/or phase are measured by a probe (13) at an input to a differential variable element (7, 8, 9) and by a probe (14) at fewer than all of the outputs of the differential variable element. By using lookup tables based on actual measurements of antenna beam attributes for phase and/or amplitude differences at different points in a feed network computation may be simplified. The method enables a relatively inexpensive control circuit to be employed while providing accurate measurement of antenna beam attributes.