15th International Conference on Ground Penetrating Radar - GPR 2014
Design of an improved dipole antenna for detecting enclosure structure defects by crosshole OPR Xiongyao Xie*, Hui Qin, Rongjie Yao Department of Geotechnical Engineering, College of Civil Engineering Tongji University Shanghai 200092, China Email: *
[email protected];
[email protected];
[email protected]
Abstract-In underground engineering, the enclosure structure
In a crosshole GPR system, transmitting and receiving
is the most critical part to guarantee the safety of construction.
antennas are the most critical parts, whose performances will
However, defects such as voids or cracks might appear in the
influence the performance of the whole system. On the one
structure, which will lead to accidents of foundation pit. In this
hand, the structure of the antenna is restricted by the size of
paper, an improved dipole antenna is designed for detecting
preset holes and the size of its shield for water and pressure
enclosure structure defects by a stepped frequency crosshole
proofing. On the other hand, the antenna should still have a
Ground Penetrating Radar (GPR). In order to obtain more
certain bandwidth to guarantee a wide band measurement of
desirable performance, the antenna parameters including the
the system. Thus, the optional antenna types are constrained,
half-cone angle fJ, the feeding gap g, the cylinder radius r and the cylinder length I are designed and optimized by the FEM method.
and the design of borehole antennas are much more difficult
The optimized antenna has a center frequency of O.52GHz and
than the surface ones.
can work well in the frequency range from 0.45GHz to 1.55GHz. Moreover, the antenna is fabricated and tested in a physical model. The result shows that the designed antenna has good performance
and
can
serve
for
enclosure
structure
defects
detection. Keywords-Ground penetrating radar, dipole antenna, design, enclosure structure, defects
1.
INTRODUCTION
In the exploitation of underground space in big cities, the enclosure structure plays a vital role in maintaining the safety of foundation pit and the surrounding environment, for it keeps soil and water away from foundation pit during excavating [I]. However, due to the complexity of the construction process, the
uncontrollability
of
the
construction
quality
and
the
variation of geological conditions, it is common to see defects such as voids, concrete sandwiched with mud, cracks or segregation appear in enclosure structures, which probably cause water leakage, drift sand or even collapse of the foundation [2]. These accidents will not only jeopardize the
Fig. 1. Sketch map of an enclosure structure
safety of the construction, but also the surroundings like the nearby subways, pipelines or buildings, thus resulting in
As for the shape of an antenna, the traditional half-wave
considerable economic losses. Therefore, to develop a non
dipole antenna might be the best candidate for a crosshole
destructive testing method to find out defects in enclosure
measurement.
structures before excavating procedure is both important and
matching performance can hardly meet the requirements of a
urgent.
wide band GPR system [4], [5]. Many efforts are contributed to
Enclosure
structures
are
usually
made
of
reinforced
improve
concrete, and penetrating into the ground for dozens of meters
However,
performances
its
of
bandwidth
borehole
and
antennas
impedance
for
varied
applications [6]-[10]. However, there are few borehole GPR
like walls [3]. Inside the structures, there are several preset
applications to enclosure structure defects detection. In this
holes used for inclination survey along the depth direction with
paper, an improved dipole antenna is presented by combining a
a diameter of around 70mm, which is shown in Fig. 1. Thus,
biconical antenna with a traditional half-wave dipole antenna
borehole Ground Penetrating Radar (GPR) could be used to
[11].
locate the defects inside the structures.
Simulator is adopted to optimize the parameters of the antenna
In the design phase, the High Frequency Structure
to achieve better bandwidth and impedance matching. Then the
978-1-4799-6789-6/14/$31.00 ©2014 IEEE
723
15th International Conference on Ground Penetrating Radar - GPR 2014 are shown in Fig. 3. From the simulated results, it can be seen
designed antenna is fabricated and its S parameters (Return Loss, VSWR and Phase Response) are measured to evaluate its
that the smaller the half-cone angle is, the input impedance
perfonnance. Finally, a physical model experiment is carried
varies stronger over the sweep frequency. With the half-cone angle increasing, the input impedance curve becomes lower
out to test the antenna's working perfonnance in a stepped
and more gentle. When the half-cone angle reaches 60°, the
frequency crosshole GPR system. II.
input impedance is closest to son, which means that good impedance matching can be achieved at the feeding point. Thus
ANTENNA DESIGN AND OPTIMIZATION
a half-cone angle of 60° is chosen for the antenna.
The structure of the borehole antenna is shown in Fig. 2. The antenna has a symmetric geometry, with the same shape of
300
-€I = 300
the two anns. The ann is made up of a cone and a cylinder
---'ff-- €I
250
attached at the end of the cone. The feeding point is set in the middle of the antenna.
200
For the antenna design, four key parameters should be determined, which are the half-cone angle 8, the feeding gap the cylinder radius
r
and the cylinder length
parameters are decided, the cone height ritane, and the ann length
h
I.
=
450
e = 600 -€I = 750
g, E 150
Once the four
s:;
0
can be expressed as
5
Therefore, when the input impedance of the antenna is close to son, the system will achieve good impedance matching.
According to the characteristic of biconical antennas [12], [13], the half-cone angle plays an important role in the antenna's input impedance. Antennas with different half-cone angles are simulated and
Frequency I GHz
the input impedance of antennas of different half-cone angles
Fig. 4. VSWR of diflerent feeding gaps
724
3 mm
g = 4mm
8
designed. Besides, the High Frequency Structure Simulator is
---'ff-- g
5mm
15th International Conference on Ground Penetrating Radar - GPR 2014 10
C. Antenna radius r
_ / = 100 m m
5
The antenna radius affects the antenna bandwidth [14],
/= 110mm
1 = 120mm
-"
normally, antennas with larger radiuses have wider band.
/=130mm
However, the radius of a borehole antenna is sharply restricted
·5
by the size of the preset holes and its own shield.
co
Antennas with different possible radiuses are simulated and
� ::! .3
the Return Loss is plotted in Fig. 5. Around the center frequency, with the increasing of antenna radius, the working
E
-10 -15
"
5
�
Wooden box
4 3
1.
FIlled with sand
2 1
0
0.5
1.5
2
2.5
Frequency I GHz Fig. 9. Measured VSWR
3
3.5
4
..;,----
Phase Response is measured to evaluate the fidelity of the
PVC pipe
signals. As shown in Fig. 10, the Phase varies linearly over frequency, which means that the waveforms of the emission signals and received signals are good and the fidelity of the
Fig. II. Physical model for antenna test
signals is high.
A stepped frequency OPR system is used to conduct the test. The Agilent Vector Network Analyzer (N9923A) is used
726
15th International Conference on Ground Penetrating Radar - GPR 2014
as the transmitter and receiver. The VNA is set with the
model. Future work will be testing the antenna in real
frequency band of 100 to 3000MHz, the IFBW (Intermediate
enclosure structure detection applications.
Frequency Bandwidth) of 3000Hz, and the sampling number of
VI.
101. Two identical antennas are placed in the two PVC pipes
ACKNOWLEDGMENT
respectively. The transmitting antenna is fixed just under the
The authors would like to acknowledge the fmancial
sand surface in one PVC pipe, while the receiving antenna is
support from the National Basic Research Program of China
moved from the top to the bottom in the other PVC pipe at a
(973 Program:
constant speed.
Foundation of China (41372273), and Shanghai Science and
The recorded OPR profile is presented in Fig. 12. The
2011CBOI3803), National Natural
Science
Technology Development Funds (12231200900,13231200102).
direct wave from the transmitting antenna to the receiving
VII.
antenna can be clearly seen. Behind the direct wave, there are
REFERENCES
some other waves of reflection and refraction. The result indicates the designed antenna has a good performance and can
[1]
A. J. Yao, H. Peng, H. F. Chen and F. D. Ning. "Field Deformation Study on Enclosure Structure and Environment of Subway Shaft Construction under Complex Conditions", in Proc. Second International Conference on Geotechnical and Earthquake Engineering, Chengdu, China,Oct. 2013, pp. 25-27.
[2]
P. Town. "Controlling Water Ingress through Diaphragm Wall Joints", in Proc. Fourth International Conference on Grouting and Deep Mixing. Grouting and Deep Mixing, New Orleans, LA USA, Feb. 2012, pp. 1518.
[3]
B. de Paoli. "Construction and quality control of a 100 m deep diaphragm wall", in Proc. 12th International Conference on Soil Mechanics and Foundation Engineering,Aug. 1989,pp. 1479-1482.
[4]
S. Ebihara and Y. Hashimoto. "MoM analysis of dipole antennas in crosshole borehole radar and field experiments", Ieee Transactions on Geoscience and Remote Sensing,Vol. 45,pp. 2435-2450,Aug. 2007.
[5]
K. 1. Ellefsen and D. L. Wright. "Radiation pattern of a borehole radar antenna", in Proc.Ninth International Conference on Ground Penetrating Radar,Columbus, Ohio USA, Apr. 2002,pp. 68-73.
[6]
F. M. Sagnard. "Design of Compact UWB Planar Monopole Antennas for Cross-Hole Radar Application", Ieee Geoscience and Remote Sensing Letters,Vol. 6,pp. 816-819,Oct. 2009.
[7]
F. M. Sagnard. "Design of a Wideband Antenna for a Narrow Borehole Radar Using FDTD Modeling", Ieee Geoscience and Remote Sensing Letters,Vol. 6, pp. 553-557,Jul. 2009.
[8]
H. Y. Liang, H. C. Yang and J. Zhang. "A Cylindrical Conformal Directional Monopole Antenna for Borehole Radar Application", Ieee Antennas and Wireless Propagation Letters, Vol. 11, pp. 1525-1528, Dec. 2012.
[9]
S. Ebihara and Y. Wada. "Investigation of Wideband Coaxial-Fed Circular Dipole Array Antenna in a Borehole", in Proc. 6th International Workshop on Advanced Ground Penetrating Radar (IWAGPR), Aachen, Germany, Jun,2011,pp. 1-5.
work well in the stepped frequency crosshole OPR system.
E
o
2
4
6
8
10
12
Travel Time I ns
14
16
18
Fig. 12. Recorded GPR profile
V.
CONCLUSIONS
Tn order to apply the crosshole OPR to the enclosure structure defects detection, an improved dipole antenna is designed for the stepped frequency OPR system. To obtain both wide band and slim size of the antenna, the biconical antenna and traditional dipole antenna are combined.
[10] M.Sato and T.Takayama. "A Novel Directional Borehole Radar System Using Optical Electric Field Sensors",IEEE Transactions on Geoscience and Remote Sensing,Vol. 8,pp. 2529-2535,Aug. 2007.
Four key parameters including the half-cone angle e, the feeding gap g, the cylinder radius r and the cylinder length
I are
optimized. The half-cone angle has a significant impact on the
[11] M. X. Su, G. Tian, Z. F. Zeng, X. G. Xue, L. Huang and S. H. Shi. "The Making and Application Eflect Analysis of One Facility Ground Penetrating Radar Antenna", Progress in Geophysics, Vol. 23, pp. 295299+300, Feb. 2008. (in Chinese)
input impedance. The bigger the angle is, the more gentle the input impedance change over frequency. The feeding gap has no obvious effect on the performance of the antenna, especially
[12] C. L. Lin. "Approximate Analysis on a Wide Angle Biconical Antenna", Journal on Communications,Vol. 1,pp. 72-78,Jan. 1984. (in Chinese)
the band width. The radius mainly determines the antenna working bandwidth. Larger radius results in wider working
[13] Q. Wang, C. 1. Ruan and H. Y. Wang. "Simulation study on the electromagnetic characteristics of the biconical antennas for finite length and arbitrary", Chinese Journal of Radio Science, Vol. 18, pp. 704-708, Dec. 2003. (in Chinese)
frequency band, but is restricted by the size of the preset holes. The cylinder length controls antenna's center frequency and is designed smaller than the value of theoretical analysis. Taking
[14] Y. F. Zhao, J. F. Gong and 1. S. Huang. "Capability research of thick dipole antenna", Journal of Naval University of Engineering, Vol. 17,pp. 84--87,Apr. 2005. (in Chinese)
into account the parameters together, an antenna with e=60°, g=3mm, r=24mm and 1=II0mm is designed. From
the
measured
results,
the
antenna
has
good
[15] M. H. Jamaluddin, M. K. A. Rahim, M. Z. A. A. Aziz and A. Asrokin. "Microstrip dipole antenna analysis with diflerent width and length at 2.4 GHz", in Proc. Asia-Pacific Conference on Applied Electromagnetics, Johor,Malaysia, Dec. 2005,pp. 41-44.
perfonnances in the frequency range from 0.45 to 1.550Hz with a center frequency of 0.520Hz. Moreover, the antenna works well in a stepped frequency OPR system on a physical
727
