Microwave Probes for Electric Fields Near Metal Surfaces

An experimental study was made to isolate the types of probe errors made in measuring electrical boundary conditions and nearfield waves over the surfaces of metal diffracting objects and antennas. Because their patterns could be calculated for comparison with measurements, thin metal disks were employed ranging in diameters from 0.1 to 10 wavelengths. The microwave wavelengths ranged from 2 to 80 cm and the probe sizes ranged from 0.50 to 0.01 wavelength long. The probes consisted of wire dipoles with the microwave circuits confined to the detector capsule at the centers of the dipoles. Three types of probe errors were studied: 1) multiple reradiation between probe and surface of object when the axis of the probe is parallel to the surface, 2) bringing the tip of the probe closer than a hundredth of a wavelength from the surface of the object when the fields between the tip and the surface are shaped like electrostatic fields and found to be grossly affected by changes in the spacing, and 3) lengths of probes large compared to the radius of curvature of the lines of force, a limiting case in the measurement of fields near the edges of metal sheets.