A Novel wideband VHF antenna for impulse GPR applications

A novel wideband VHF antenna for the impulse ground penetrating radar (GPR) system at 200 MHz central frequency is presented in this article. The antenna improves the impulse GPR system for increasing ability penetration. By using the Lemniscate curve, this novel structure of the proposed antenna achieve better radiation than other bow-tie antennas. In addition, this article also proposes the UWB balanced-to-balanced (balun) transformation line is designed to feed the antenna. The balun is an important element for improving the bandwidth of the antenna. The fabrication of the antenna is only simple but also low cost with FR4 substrate and copper patch. The proposed antenna is designed and fabricated with the successful results.

A Novel wideband VHF antenna for impulse GPR applications

ABSTRACT:
A novel wideband VHF antenna for the impulse ground penetrating radar (GPR) system at 200 MHz central frequency is presented in this article.The antenna improves the impulse GPR system for increasing ability penetration.By using the Lemniscate curve, this novel structure of the proposed antenna achieve better radiation than other bow-tie antennas.In addition, this article also proposes the UWB balanced-to-balanced (balun) transformation line is designed to feed the antenna.The balun is an important element for improving the bandwidth of the antenna.The fabrication of the antenna is only simple but also low cost with FR4 substrate and copper patch.The proposed antenna is designed and fabricated with the successful results.

INTRODUCTION
Ground penetrating radar (GPR) is sometimes called georadar, ground probing radar, or subsurface radar.GPR uses electromagnetic wave propagation and scattering to image, locate and quantitatively identify contrasts in electrical and magnetic properties in the ground.[1].Detectability of a subsurface feature depends upon contrast in electrical and magnetic properties, and the geometric relationship with the antenna.

Quantitative interpretation through modeling can
derive from ground penetrating radar data such information as depth, orientation, size and shape of buried objects, density and water content of soils, and much more.Important component in any GPR system are the transmitter and receiver antennas [2].Antennas radiate electromagnetic energy in the microwave band (UHF/VHF frequencies) when there is a change in the acceleration of the current on the antenna.Antennas also convert electromagnetic waves to currents on an antenna Trang 49 element, acting as a receiver of the electromagnetic radiation by capturing part of the electromagnetic wave [3].
The depth range of GPR system depends on not only the electrical conductivity of the ground but also the transmitted central frequency.The lower frequency will make the deeper penetration.So, the GPR systems requite the designed antenna that has a low central frequency in VHF range.
Recently, there are many researches for improving the deeper penetration of the impulse GPR system.
The antenna is situated above dry sand with relative dielectric permittivity in the 500 MHz-3 GHz range and with very small conductivity [4].
The antenna has a broadband and makes the GPR system to high resolution.However, the UHF central frequencies of this antenna don't improve the range of depth for the impulse GPR system.
Besides, ZOU Aimin, LI Jicai, WANG Keke and CHENG Defu have experimental results show that voltage standing wave ratio (VSWR) of the loaded antenna is less than 2.5 in the band 0-300 MHz [5].However, the value of VSWR make performance of the antenna is not good and it is the trouble for processing signals in the receiver.
In addition, Chen Guo and Richard C.Liu provided Shielded antenna system [6].Although they make a good Transmitting signal with shielding and absorbing materials, their designed antenna is used in a GPR system working at 400MHz central frequency.
In this article, we propose a novel wideband VHF antenna to improve the deep penetration for the impulse GPR system.Unlike the above bow-ties antenna in [5], [6] and [7

THE PROPOSED LEMNISCATE ANTENNA
The proposed antenna has FR4 dielectric substrate and copper patch for the impulse GPR system.We use the Lemniscate cure to create the structure of the antenna.This curve of the patch of antenna is shown in Figure 1    The dimensions of balun are shown in Figure 5 and its values are shown in Table I.

EXPERIMENTAL RESULTS
In this section, we present the measured results of the proposed antenna.The implemented antenna is shown in Figure 19.Từ khóa: Impulse ground penetrating radar (GPR) system, Lemniscate curve, balanced-tounbalanced (balun), bow-tie antenna, Novel wideband VHF antenna.
], the antenna is based on Lemniscate curve to achieve a good radiation.The proposed balun has a broadband and makes a good matching impedance.The dimension of the antenna is smaller than other bow-tie antennas at the same central frequency.The antenna is successfully optimized by CST MICROWAVE STUDIO software.The proposed antenna has the return loss is less than -10 dB and VSWR is less than 2 in band 176-232 MHz.The results show good agreement between simulation and measurement.
. The locus of the point P on the Lemniscate curve can be determined from two focal points F and F' such that 2OF.OF' = a2 (where a is the distance from O to the center focal point F).The equation of Lemniscate curve in Cartesean coordinate is shown [7]:

50 Fig 1 .
Fig 1.The Lemniscate curve The curve Lemniscate of the proposed antenna has length La = 541.3mm, width Wa = 182 mm,

Fig 2 .
Fig 2. Geometry and configuration of the proposed antenna The distance of Lemniscate curve for this antenna is mm and OF = 186.61mm.Like the dipole antenna, the feed line of Lemniscate antenna is located in middle of the wings at S opened point.The proposed antenna uses FR4 dielectric material which has a length Ls

Fig 3 .
Fig 3.Geometry and configuration of the proposed antenna is based on substrate with feed point The microstrip taper balun is designed to transform from the unbalanced structure of the coaxial cable 50 Ω impedance to the antenna

Table II and
Table III compare the results of S11 and VSWR.The results of comparison show good agreement between simulation and measurement.

Table 2 .
Comparison results between simulation and measurement of S11