Adjustable waveguide branch, and directional coupler

Abstract

A rectangular waveguide series junction has a layer of photoconductive material in one branch near the junction. The photoconductive material, when dark, is essentially a dielectric, which effectively increases the dimension of the waveguide branch over what it would be if not modified, thereby increasing its impedance at the series junction and causing power division preferentially into the branch. When illuminated, as by a laser or LED, the photoconductive layer becomes a conductor instead of a dielectric, and the dieletric 'increase' in the dimension is eliminated. Instead, the conductive material actually decreases the cross-section, to thereby reduce the actual impedance of the branch at the junction point. This reduces the amount of coupling below that for an unmodified waveguide branch. Thus, the amount of coupling into the branch at the junction is increased by the dielectric constant when the photoconductor is dark, and decreased by the conductivity when illuminated. A waveguide directional coupler includes one or more such controllable branches. Redundant light emitting diodes are located in slots adjacent the central seam of the directional coupler housing, for fine control of the coupling factor. Control may be applied to move nulls in the coupler isolation to reduce interference in antenna arrays, to adjust coupling to achieve improved channel-to-channel isolation, or to compensate for aging, or other changes of amplifiers or other circuit components.