**1.Introduction**

 After the year 2002, ultra-wideband (UWB) systems have gained popularity mainly when the US Department of Federal Communications Commission (FCC) allocated a license-free spectrum for industrial and scientific purposes. FCC is doing the greatest step in opening new doors of researches for UWB in the field of wireless communications and microwave imaging [1, 2]. UWB device is defined as any device operating in absolute bandwidth greater than 500 MHz or fractional bandwidth greater than 0.2 of central frequency [3]. The frequency band ranges of UWB extended from 3.1 to 10.6 GHz that have expected the applications in the fields of wireless body area networks (WBAN), wireless local area networks (WLAN), wireless interoperability for microwave access (WiMAX), wireless personal area networks (WPAN), and ground-penetrating radar (GPR) technology where wide bandwidth is required [4]. GPR is the major applications of UWB technology, which

is in large degree used in military and civilian applications such as water detection and land mines [5]. There are many UWB antennas have been designed for GPR applications. The study based on the lower-frequency band is conducted mainly to increase the penetration depth, while the designing in the higher-frequency band is performed to achieve high-resolution imaging for GPR systems. Some of the researches focused on the entire UWB frequency range to further improve the bandwidth, while others focused on enhancement of the antenna gain [6]. Moreover, GPR is also used in remote-sensing techniques as nondestructive testing of concrete and detection of trapped people under debris or in opaque environment [7]. For the achievement of UWB GPR systems, the performance of various antenna designs, such as bow-tie antenna [8], spiral antenna [9], loaded dipole antenna [10], TEM horn antenna [11], tapered slot antenna (TSA) [12, 13], and Vivaldi antenna [14, 15], has been evaluated.

 Ultra-wideband antenna is one of the preferred antennas for some applications in the microwave imaging, object measurement technology, and noninvasive testing (NIT) [4–15]. In this era, the microstrip antenna has the advantages of small size, high gain, and low cost for good performance in some applications [6–10]. Groundpenetrating radar (GPR) has been utilized by emitting an electromagnetic wave directed into the ground, and the buried objects cause reflections of the emitted wave that are then detected by the receiver system. This is contrasted in the electrical properties as the signal reflection coefficient and their related phase [11–17]. GPR has an ability to detect the electrical inhomogeneity of metal and dielectric object in the presence of surrounding soil or sand [18]. GPR system can exist at the same location for the transmitting and receiving, and there are four types of GPR system: quasi-monostatic radar if there is no separation distance between transmitter and receiver, monostatic radar if there is single antenna performs both transmit and receive operations [19], bistatic radar if the transmitter and receiver have separate distance, and multistatic radar if a radar system involves one or more transmitting platforms and multiple receiving platforms [20]. GPR systems have been classified as the time domain (impulse radars) and continuous wave (CW) radar [21]. CW radar transmits the signal, which can be frequency-modulated continuous wave (FMCW), or creates the resulting signal as a combination of monochromatic steps through a certain band of frequencies, referred as stepped frequency continuous wave (SFCW) [22]. GPR systems usually work at central frequencies below 1 GHz, and large bandwidth is needed for a better depth resolution and detailed echo. The use of impulse wideband systems involves some technical problems, such as Doppler processing, propagation fading, interference rejection, wave clutter, detecting birds on or near the water surface, and radar interference.
