Contents I. Introduction
Recently, extremely wide frequency ranges have been applied for radar system such as ground penetrating radar (GPR). GPR is a non-destructive ground survey technique that uses electromagnetic (EM) waves to detect or image the subsurface structures. The technique is based on emitting EM wave into the ground and detects the reflected signal due to the discontinuity of dielectric property of subsurface materials such as rock and soil [1]. The antenna is the one of important element in GPR system that is used to transmit and receive the EM waves. In addition, the performance of GPR system such as penetration depth and resolution are related to operating frequency. It is known that lower frequencies have a longer depth of penetration, while higher frequencies have a higher resolution. Therefore, the compact antennas which provide extremely wide bandwidth have been developed [2]. Various types of ultra-wideband antenna are used in GPR system such as Vivaldi antenna, horn antenna, bowtie antenna, and printed slot antenna. Although, horn antenna and Vivaldi antenna have good radiation characteristics, however, these antennas are large size, heavy weight and difficult to install with GPR system. Bowtie antenna is popularly used in GPR due to having a simple structure. However, this antenna type has a relatively low gain. It is found that the printed wide slot antenna is suitable for GPR application because of its advantages of wide operational bandwidth, high gain, stable radiation pattern, small size, ease of fabrication, and easy for mounting the portable GPR. Therefore, many techniques have been applied to improve the bandwidth of printed wide slot antenna such as increasing the coupling between the wide slot and microstrip feed line by using double or trident-shaped feed [3]-[4], using parasitic element [5], using similar or different shapes of antenna stub and slot [6]-[7].
