Characteristics of microstrip patch antennas and some methods of improving frequency agility and bandwidth

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Abstract
This chapter begins with introducing the simple cavity model for analysing microstrip patch antennas with thin substrates. The formulas obtained from this model for rectangular, circular, equitriangular and annular-ring patches are then presented. The radiation pattern, efficiency, directivity, gain, quality factor and impedance of these antennas are illustrated, mainly for the broadside modes. Experimental results are included or referenced where available. After a brief mention of some other geometries, notably the elliptical patch, the chapter proceeds to discuss some methods of improving the frequency agility of microstrip patch antennas. They include the use of varactor diodes, shorting pins, optically controlled diodes, adjustable air gap, stacked geometries and reactive loading. The advantages and disadvantages of these methods are discussed. Finally, the method of increasing the absolute bandwidth by electro magnetic coupling of a fed and a parasitic patch in a stacked geometry is discussed.

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