A modulated scatterer for measuring microwave fields

Any attempt to measure a microwave field pattern inevitably results in some perturbation of that pattern. The conventional method, using a probe antenna connected via a length of waveguide to a remote receiver, perturbs the fields considerably because the probe antenna and the waveguide connected to it are electrically conducting and large compared with the wavelength of the radiation. Distortion of the original fields by the probe antenna is not troublesome if the probe antenna is far from radiating or reflecting objects, but it prevents accurate measurement of the fields close to source antennas or reflectors. Hajnal (1987) used a plastic optical fibre to bring the light from a modulated light-emitting diode to a photosensitive load at the centre of a short dipole scatterer; this was then used to make bistatic measurements in the far-fields of antennas at a single frequency. In the work described here, a scatterer similar to Hajnal´s has been used monostatically for the first time to make measurements in the reactive near field of an antenna. The spot-frequency homodyne detector used by Hajnal has been replaced by a broadband instrument, constructed using coaxial components and designed to work over the frequency range 2-18 GHz