Photoconductive probe for measuring electromagnetic fields

Probes in which the usual connecting leads to the detector have been eliminated have been constructed for measuring the intensity of an electromagnetic field. The new devices are modulated reradiating or scattering antennas that consist of either a small dipole (for measuring the electric field) or a small shielded loop (for measuring the magnetic field) centre-loaded with a photocell illuminated by a chopped beam of light. The modulated scattered signal from the probe is proportional either to the component of the E-field which is parallel to the axis of the dipole, or to the component of the H-field which is perpendicular to the plane of the shielded loop. The scattered signal is received by an additional antenna or by the same antenna as is used for transmission. The received signal is amplified by a lock-in amplifier. The same principle has also been applied to the measurement of the distribution of current along an antenna of arbitrary shape. This was accomplished by sliding a small shielded-loop probe, centre-loaded with a photocell, along the antenna. A very small and constant gap was maintained between the probe and the antenna. The modulation factor of the scattered signal was determined from the measured instantaneous impedance of the photocell. Both the near-field pattern of a ¿/2 dipole antenna and the distribution of current along the antenna as measured with these optical probes are in general agreement with theory. The properties of several other types of probes based on the same principle were also explored.