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Reflector elements for aperture and aperture coupled microstrip antennas

The design advantages provided by aperture coupled microstrip patches can be very useful in certain planar antenna applications. Several papers have appeared in which a large aperture is used in conjunction with thick substrates to increase the bandwidth of the element. This technique is quite robust, and bandwidths in excess of 50% have been realized. However, one disadvantage of this method is the large amount of radiation produced by the aperture, leading to a radiation pattern with a poor front-to-back ratio. One way to improve the front-to-back ratio is to place a microstrip antenna element behind the aperture as a reflector. The action of this element is akin to that in a Yagi-Uda dipole array. However, the incorporation of a reflector in an aperture coupled patch antenna is much easier to achieve. Proximity coupling between the feedline and the reflecting element is negligible due to the thick foam substrate used, allowing the use of the reciprocity method of analysis. Also, the directive patch elements are shielded from the reflector by the ground plane. Therefore, only interactions between the reflector and the aperture need to be modelled, resulting in a simple analysis. For aperture coupled patch designs with a front-to-back ratio of 10 dB or greater, the introduction of a reflecting element has a negligible effect on the input impedance of the antenna. Therefore, a reflector element can readily be incorporated into existing designs.