Analysis of Reactive Loading in a U-Slot Microstrip Patch Using the Theory of Characteristic Modes [Antenna Applications Corner]

In this article, the effect of reactive loading on a probe-fed, single-layer, U-slot loaded microstrip antenna is investigated using the theory of characteristic modes (TCM). A detailed analysis of reactive loading due to feed location and arm-angle variation is presented. The eigenvalue behavior of the characteristic modes can be modified by varying reactive loading on the U-slot antenna. Thus, it is possible to control certain modes such that they resonate in the desired frequency band. These modes can then be effectively excited to achieve the desired antenna performance. In addition, reactive loading helps to move the impedance loop to the center of the Smith chart. Optimized reactive loading has been shown to produce a modified U-slot structure without any increased cost or complexity. The optimized loaded antenna is wide-band, with a relatively stable radiation pattern.