Wideband antenna array for step-frequency ground penetrating radar

This work investigates the properties of a novel wideband air-launched antenna array for high speed 3-dimensional GPR surveys. It consists of boomerang-shaped monopole antenna elements mounted on a layer of microwave absorbing material, backed by a ground plane. The antenna element provides a very flat frequency response ranging from 200 MHz to 3 GHz. This allows us to take full advantage of the step-frequency radar's property of covering a wide frequency range without switching between different frequency band antennas. The transmitting and receiving elements are arranged on a V-shaped structure where the tip of the V is pointing towards the ground. This structure gives high isolation between the transmitter and receiver and suppresses the multiple reflections between antenna and ground surface. The antenna was studied both numerically and experimentally to characterize the electromagnetic behavior. The main design objective was to maximize the radiation in the downward direction while minimizing the coupling between the transmitter and receiver. Based on these studies, more than 40 dB of isolation between transmitting and receiving antenna has been obtained. Furthermore, the flat frequency response ensures a clean impulse response with low ringing which is very important when measuring thin pavement layers. The paper also presents field measurement results from a high-speed road survey.