Designing of a low frequency ultra wideband(UWB) antenna and its application in ground penetrating radar(GPR) system

In a Ground Penetrating Radar(GPR) system, antennas are one of the most critical components for its successful applications. Generally, suitable antennas for a GPR system application can be roughly divided into two categories: dielectric coupling antennas and air coupling antennas. The former includes various dipole elements, bowtie and spiral antennas and so on. The later mainly includes various wideband horns(e.g. TEM horns) and Tapered Slot Antennas(TSA). For developing a GPR antenna, factors should be considered are operation band-width, gain and efficiency, directivity, size and weight, cost and so on. Because of its large size and weight, the air coupling antennas are usually used in the high frequency band, for instance, above 1 GHz. While as the GPR operation frequency below 1 GHz, dielectric coupling antennas are commonly used. The bowtie antenna with loaded resistors is the most one for low frequency application. According to antenna theory, the length of a bowtie antenna should be taken as half-wavelength of its operation center frequency. Therefore, as the GPR operation frequency is very low, for example, at about 10MHz, the size and weight of antenna will be unacceptable if one still use the normal technique to design such bowtie antenna. In implementing our research project, in order to study the subsurface electromagnetic information between 6-2SMHz, a low frequency GPR antenna system is needed. In this paper, the technique for designing such antenna with less size and weight is discussed. The basic shape of the developed antenna is a kind of bowtie. The center frequency of such antenna is about ISMHZ. The elementary theory for designing such UWB low frequency antenna is mainly from broad band biconical antenna and Wu-King resistor-loaded broadband antenna theory. The antenna developed in this paper includes two layers: the bottom layer is used for radiating electromagnetic energy into underground, named as radiating layer; the up layer, which is called as loaded resistors layer, is used for attenuating the current flow along the antenna arm. The loaded resistors layer is folded from the radiating layer and is backed on the radiating layer, therefore the antenna length can be half reduced. The other advantage of such antenna is that it can partly shield the radiated energy into air, i.e., this kind antenna is partly shielded because of its special construction. Its performance for field site measurement in an actual GPR system is also studied.