Enhanced Realized Gain of a Compact Coplanar Vivaldi Antenna on a High-Permittivity Substrate Using Wide Tilted Rectangular Slots

Vivaldi antennas are widely used in broadband applications such as ground-penetrating radar, medical imaging, and ultra-wideband radar systems, where end-fire realized gain strongly affects system performance. However, compact Vivaldi antennas generally suffer from poor low-frequency gain because the antenna aperture becomes electrically small. In this study, we propose a compact coplanar Vivaldi antenna designed on a high-permittivity substrate (εr = 9.9) with overall dimensions of 150 × 210 mm. To enhance the end-fire realized gain at low frequency without increasing the size, wide tilted rectangular slots are introduced into the radiating flares. Although slot loading is a well-established technique for improving antenna performance, previously reported designs have primarily employed narrow slot geometries. Full-wave simulations show that slots with widths corresponding to approximately 10% of the antenna aperture improve the realized gain by up to approximately 5.7 dB in the 550–1500 MHz band, substantially outperforming conventional narrow-slot configurations. Surface current analysis indicates that the slots increase the effective current path length and change the surface current distribution around the slot edges. The resulting current distribution is interpreted to enhance the constructive superposition of the radiated fields in the end-fire direction. The proposed antenna achieves S11 < −10 dB across most of the operating band above 650 MHz and exhibits a peak realized gain exceeding 9 dBi. These results demonstrate that slot width is critical for gain enhancement in compact broadband Vivaldi antennas. All performance values reported in this study are obtained from full-wave simulations.