A Compact Ultra-Wideband Vivaldi Antenna for Short-Range Vehicular Communications Using Radial-to-Radial Transition and Matching Transformer

This work presents a compact ultra-wideband (UWB) Vivaldi tapered slot antenna for short-range vehicular communication and broadband UWB applications. The proposed design is developed using two simple performance-enhancement and size-reduction techniques: a radial-to-radial microstrip-to-slot transition and an impedance-matching transformer. The antenna employing only the radial-to-radial transition is referred to as VTSAR, while the final design incorporating both the radial-to-radial transition and the matching transformer is referred to as VTSARMT. Unlike many compact UWB antenna designs that rely on complex structural loading, parasitic elements, or additional decoupling components, the proposed approach improves compactness, impedance bandwidth, and gain through a simple feeding-transition modification supported by systematic parametric analysis. The intermediate VTSAR provides a 9.84% footprint-area reduction relative to the reference VTSA, while the final VTSARMT further increases the footprint-area reduction to 16.67%. The fabricated VTSARMT exhibits measured impedance matching below −10.61 dB from 2.58 to 22.34 GHz, corresponding to a 47.67% measured bandwidth improvement relative to the reference VTSA. The maximum measured realized gain increases from 6.51 dBi for the reference VTSA to 7.45 dBi for the VTSARMT, while the measured total radiation efficiency ranges from 47.7% to 94.08%. An approximately stable measured group delay of 1 ns is obtained across most of the operating band. To assess the effects of vehicle integration, the proposed antenna is further evaluated through CST-based stationary rooftop-mounted two-vehicle and three-vehicle configurations. The analysis includes radiation-pattern assessment, transmission coefficients, and CST-based received-power evaluation. These simulations provide a comparative assessment of stationary directional links rather than experimental validation of a dynamic V2V channel. The CST-based vehicle analysis provides a preliminary assessment of rooftop integration and stationary directional V2V links under controlled simulation conditions.