Ground Size Optimization for High Boresight Gain in Offset-Fed Wide Slot Antennas

This paper presents a microstrip-fed wide rectangular slot antenna that achieves improved boresight gain while functioning in the $\mathbf{6 - 8. 5} \mathbf{~ G H z}$ frequency band. The radiation and impedance characteristics of the slot antenna for distinct dimensions of ground plane are examined and analyzed. It is observed that optimizing the ground plane size enhances the gain of a wide rectangular slot antenna. This occurs when direct waves from the slot radiator interfere appropriately with the waves that are diffracted from the ground plane edges. The optimized rectangular slot antenna utilizing an FR-4 substrate, with a slot size of $0.31 \lambda_{0} \times 0.22 \lambda_{0}$, has overall dimensions of $1.2 \lambda_{0} \times \mathbf{0. 7 7} \lambda_{0} \times \mathbf{0. 0 4} \lambda_{0}$, where $\lambda_{0}$ denotes the free-space wavelength at the midpoint frequency of the operating band. Measurements of the fabricated antenna prototype reveal an impedance bandwidth ranging from 5.96 to 8.54 GHz and a bidirectional radiation pattern. The antenna exhibits an average boresight gain of 5.33 dBi within this range, with a gain variation of 1.35 dBi. The key advantage of the proposed method is the precise selection of the ground plane size, which allows for nearly uniform gain improvement across the entire impedance bandwidth, while maintaining a bidirectional radiation pattern.