Broadband microstrip antennas

Microstrip antennas are inherently narrow band. The typical bandwidth of a microstrip antenna is around 4%-7%. A considerable number of experimental approaches have been undertaken to develop microstrip antennas which have a broader impedance bandwidth than a single microstrip element achieves without external matching. The methods employed to increase impedance bandwidth are essentially variations of three approaches: (a) Increasing the antenna volume. This is accomplished by geometry changes which increase the volume under the patch (e.g. increasing the thickness h), decreasing the substrate dielectric constant, or adding additional coupled resonators. (b) The implementation of a matching network. (c) Perturbing the antenna geometry to create or relocate resonances using shorts and slots in the antenna. Kumar and Ray (2003) have compiled a considerable number of microstrip antenna design variations which utilize these approaches, as has Wong (2002). One approach is the use of a groundplane slot, which is excited by a microstrip line below the groundplane, which in turn couples to a microstrip patch which is above the groundplane. This configuration can be adjusted to produce an elegant matching network, low dielectric constant substrate for the patch, and maximized substrate thickness, which provides a broadband input impedance match, and a large impedance bandwidth. One implementation of this type of antenna is called SSFIP for Strip-Slot-Foam-Inverted Patch by Zürcher and Gardiol in the mid-1990s. The design of these antennas is experimental in nature. These researchers state: “Since the various parts of the antenna interact, determining the best design is a long and tedious process, even when carried out systematically.” The parameters which may be used to guide this type of design are found in the literature. Some possible alternatives to the SSFIP matching-network/thick patch design are explored in this Chapter.