The need for broadband antennas has grown considerably in recent years, with high-frequency and high-speed data networking becoming increasingly prevalent. This article examines the factors that influence the bandwidth of a microstrip antenna. The extension of the antenna is determined by two key parameters: the thickness of the dielectric substrate and the type of substrate material. This research evaluates the performance characteristics of a rectangular patch microstrip antenna with different substrate thicknesses. The changes in resonant frequency, bandwidth, and return loss characteristics are influenced by varying one parameter while keeping the other constant. The rectangular patch microstrip antenna, designed for a microstrip feed line operating at 9 GHz, was developed using two substrate heights of 1.6 mm and 1.8 mm with a microstrip line feed. The design was implemented using CST Microwave Studio simulation software, resulting in directivity values of 6.94 dBi for the 1.6 mm substrate and 7.12 dBi for the 1.8 mm substrate thickness.......... Keywords:........... Rectangular patch, substrate height, FR-4 (lossy), performance, CST