Compound modulated scatterer measuring system

An apparatus for measuring the amplitudes and phases of two components of the electric vector E in free-space microwave fields at X-band frequencies is described. The apparatus uses a small compound scatterer consisting of two crossed dipoles loaded by phototransistors. These allow the scattering cross-sections of the dipoles to be modulated by optical signals guided to the scatterer along optical fibres. The scattered radiation is picked up by a remote receiver antenna and coherently detected to determine its amplitude and phase. By modulating each dipole in turn and coherently detecting for a second time at the modulation frequency, two different components of E are measured. This enables precise information about polarisation to be obtained with minimum disturbance to the measured fields. The procedure used to calibrate the compound modulated scatterer measuring system is discussed and some sample field measurements presented.