Improved Design of Bowtie Antennas for Ground Penetrating Radar Applications
Abstract
Bowtie antennas are extensively used in commercial GPR due to its wide impedance matching, good radiation properties and planar compact geometry. It is a kind of frequency independent antenna defined only by angles. The conventional bowtie antenna provides an excellent radiation performance over a 4:1 bandwidth. Beyond this, there is a reduction in boresight gain, and the direction of maximum radiation gets tilted. Resistive loading is employed in commercial GPR antennas to reduce the size and to increase bandwidth. Since a considerable amount of power gets dissipated in the antenna's lumped resistors, one has to sacrifice the gain and efficiency, even though these antennas have very good ability to radiate short pulses. This is a significant problem that needs to be addressed. On the other hand, the impedance bandwidth of conventional bowtie antennas are very much wider than 4:1, theoretically infinite. This leads to another concern of the thesis: whether it is possible to concentrate the radiation to the boresight direction for maximum gain so that the full advantage of its impedance bandwidth can be utilized.
Modifications are brought to bowtie antenna so that the entire impedance bandwidth is usable by the radiator, making it truly frequency independent. In this work, four antennas have been designed. For Antenna-1, by applying a resistive-capacitive loading, a wide impedance bandwidth (10:1) is obtained from 0.3—3 GHz. Peak boresight gain is 3.6 dBi. Stable radiation patterns were obtained upto 4:1 bandwidth. Radiation efficiency is better than 55%. In Antenna- 2, by adding planar metamaterial lens to Antenna-1, peak gain is improved by 2.84 dBi and radiation pattern stability improved upto 6.7:1. Radiation efficiency is better than 45%. For Antenna-3, loop loading is employed to a bowtie antenna and super-wide (11:1) bandwidth is obtained from 0.5—5.5 GHz. Obtained peak gain is 5.2 dBi. Radiation pattern is stable and gain is positive in the entire 11:1 band. Radiation efficiency is better than 85% as no resistive loads are applied. Another advantage is the high isolation between the transmit and receive antennas.
Antenna-4 is created by adding parasitic loop directors to Antenna-3. With two directors, the peak gain is improved further by 2.76 dBi. Bandwidth is improved to 0.42—5.5 GHz (13:1). Radiation
pattern is stable and gain is positive in the entire 13:1 band. Radiation efficiency is better than 85%. The superior capability of the designed antennas in detecting various targets, is qualitatively assessed through GPR experiments.
Keywords—Bowtie, metamaterial, antenna, Ground Penetrating Radar, UHF, UWB.
Ajith K. K.
(Roll no.11EC92R07)
