Parasitic Slot Elements for Bandwidth Enhancement of Slotted Waveguide Antennas

The use of extra slots in a slotted waveguide array for improving the impedance bandwidth is investigated. This has not been widely studied previously, probably because the directive pattern is a primary parameter for the waveguide array and extra elements are prone to spoiling the control of the pattern. However, the pattern is not a primary design parameter for most mobile terminals, and here we seek a wide bandwidth for a smaller antenna than the traditional waveguide array, which is suitable for a mobile terminal. The extra slots can be viewed as parasitic elements, analogously to free-space thin wire parasitic, but the designs are not interchangeable with wire antennas. This is because the waveguide - both the internal and external structure - constrains the excitation of the slot elements in a different way to that of the wire antenna analogies. Simple-to-build designs are sought where the slot elements are on only one of the waveguide broad walls. The approach is experimental (simulation) which allows rapid and accurate evaluation of different configurations. We do not optimize a configuration because any optimal design will depend on the specific structure of the mobile terminal supporting the antenna. For the basic stand-alone waveguide, a -6dB impedance bandwidth of about ~30% is demonstrated using four slot elements in a short waveguide.