A simple technique was developed to account for the higher order mode coupling between adjacent coupling slots in the feed waveguide of a planar slot array. The method uses an equation relating the slot impedance to the slot voltage and a reaction integral involving the equivalent magnetic current of the slot aperture and the magnetic field coupled from an adjacent slot.
Most waveguide-fed slot antennas use centered-inclined coupling slots in the feed waveguides. In the proposed method, one uses the Elliott’s design technique to determine tilt angles and lengths of the coupling slots. The radiating slots are modeled as shunt admittances, and the coupling slots are modeled as series impedances.
By using reaction integrals to account for higher order mode coupling between adjacent coupling slots, the “active impedance” of each coupling slot is obtained, which differs from the original impedance by a small amount due to higher order mode coupling. The tilt angle and length of each coupling slot are perturbed so that the active impedance of each slot is the same as the corresponding starting value of the impedance before the iterative process started. This process converges after three or four iterations and uses algebraic equations and fast reaction integrals.
Because of desirable features of waveguide arrays such as low volume, ease of design and manufacture, and ease of deployment, these antennas find applications in aeronautical and space activities. In addition, these all-metal antennas can withstand high radiation environments encountered in space.