Revisiting Relations for Modeling the Input Resistance of a Rectangular Microstrip Antenna [Antenna Designer´s Notebook]

The input resistance variation of an inset-fed rectangular microstrip antenna has been provided in two formulations: one is the cos⁴ variation, and the other one is a shifted cosine squared function. This article revisits these relations from the design perspectives of a rectangular microstrip antenna (RMSA) and reveals that the cos⁴ formulation for an inset-fed patch provides better matching compared with the shifted cosine squared formula, while it is suitable to use the latter for modeling input resistance in the vicinity of the patch center. A modified shifted cosine squared relation is proposed that provides a feed point position, showing improved matching of the feed line with an RMSA compared with the existing computer-aided design (CAD) formula.