Measuring microwave fields with a minimum of perturbation

The use of an optically modulated scatterer to map the amplitude and phase of the electric near fields of microwave antennas is described. The scatterer, which perturbs the measured field very little, is used monostatically with a broadbanded receiver. Multiple paths between the scatterer and the detector are allowed for by reciprocity. An equation that relates the voltage at the homodyne detector due to the presence of the scatterer to the electric field E parallel to the dipole, which has been derived, has been verified experimentally by using the modulated scatterer to measure the amplitude and phase of the electric near field of a radiating open-ended waveguide. The subject of the study was a pyramidal horn antenna with a nominal gain of 20 dB with an X-band rectangular waveguide feed oriented so that the electric field vector in the waveguide feed was vertical. The electric field distribution over a plane parallel to the aperture was measured at several frequencies between 8.2 and 12.4 GHz and at several different distances from the horn aperture.<>