IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 3, MARCH 2003 371

A New Ultrawide-Bandwidth Dielectric-Rod Antennafor Ground-Penetrating Radar Applications

Chi-Chih Chen, Member, IEEE, Kishore Rama Rao, and Robert Lee, Senior Member, IEEE

Abstract—A new ultrawide–bandwidth dielectric-rod antennais presented with its application in detecting shallow targets, suchas antipersonnel (AP) mines. The lowest hybrid mode is launchedand guided along a circular-dielectric waveguide. The end of thewaveguide is tapered to a point where electromagnetic waves areradiated out with field behavior similar to that radiated from aHertzian dipole in the forward direction. The low antenna clutterand weak antenna-ground interaction are two unique features. Itsnear-field radiation properties are investigated by directly probingthe fields and by numerical simulation with a three-dimensional fi-nite-difference time-domain technique. Both measurement and nu-merical simulation results are presented for the detection of buriedAP mines using a prototype dielectric-rod antenna operated at afrequency range from 1 to 6 GHz.

Index Terms—Antenna, broad bandwidth, ground-penetrationradar (GPR), landmine, modeling.

I. INTRODUCTION

T HE ground-penetrating radars (GPRs) have been used asnonintrusive instruments for subsurface investigation. For

deep applications, a GPR is usually operated in the kilohertz andmegahertz frequency ranges to achieve greater penetration. Forshallow applications, such as the detection of buried landmines,the 1–6-GHz frequency range gives a reasonable tradeoff be-tween penetration and resolution. For all GPR systems, the an-tenna design plays a vital role in the performance of the system.Broad bandwidth, good efficiency, and low antenna clutter areusually three important requirements of a good GPR antenna.

Most deep-application GPRs place their antennas close to theground surface for better energy coupling and less surface scat-tering. A major drawback of this configuration is the strong an-tenna-ground interaction which can significantly change the an-tenna’s characteristics. This has been shown in the literature,[1]–[3]. Such interaction produces noticeable antenna clutter. Ina well-controlled environment, the effects of the interaction canbe removed through the use of a known buried calibration target,however, in reality, it is not feasible to dig a hole for the place-ment of a calibration target within the mine field. In addition,the performance of the GPR system depends heavily on the sur-face and soil conditions that often vary from time to time andfrom place to place.

It is usually desirable to elevate the GPR antenna off theground for shallow applications because both antenna–target

Manuscript received January 5, 2000; revised April 27, 2000. This work wassupported in part by Fort Belvoir, VA, and by Duke University, Durham, NC,under an Award from the Army Research Office (ARO).

The authors are with ElectroScience Laboratory, The Ohio State University,Columbus, OH 43212 USA.

Digital Object Identifier 10.1109/TAP.2002.802081

and antenna–ground interactions are reduced. However, thereare two major problems. First, the ground surface reflection re-duces both radar efficiency and sensitivity. Second, the surfaceroughness creates undesirable clutter that arrives at the radarover a wide time range due to scattering contributions from alarge portion of the ground. This situation gets worse as the an-tenna height increases. One way to reduce the reflection fromthe ground surface is to radiate the ground at an oblique angle.Unfortunately, this remedy for the surface reflection increasesthe illumination area and causes higher surface clutter levels andwider time spreading making it even more difficult to detect aburied object. A focused-beam illumination was introduced byChenet al. [4] to achieve a greater antenna-to-ground distancewhile keeping the illumination spot on the ground minimized.Although the antenna illumination was normally incident onthe ground producing a strong surface reflection, it was shownthat such a reflection can be easily separated from the scatteredwaves of a shallow target in the time domain. The major disad-vantages of such an antenna are the bulky reflector and the min-imum achievable spot size which is limited to approximately awavelength at the lowest frequency of interest.

In this paper, a new ultrawideband (UWB) dielectric-rodantenna design is introduced for the detection of shallowobjects. This new design provides localized illumination, weakantenna-ground interaction, convenientin-situ calibration,and compact size. A prototype dielectric-rod antenna wasconstructed to operate in the 2–6-GHz frequency range. Therod has a relative dielectric constant of approximately 3,length of 60 cm and width of 7.6 cm. A numerical model ofthis prototype was also constructed with the finite-differencetime-domain (FDTD) method. The FDTD method has beensuccessfully used in the past to model numerous problemsinvolving antennas radiated energy into the ground [5]–[7].In this paper, the propagation and radiation properties of thedielectric-rod antenna were investigated from both direct fieldprobed data as well as numerical simulation data. The resultsverified that broad bandwidth electromagnetic energy waseffectively radiating out of the tip with spherical wavefronts.This prototype antenna has been used for the detection of buriedmines located at Fort A.P. Hill in Virginia, and measurementsfrom this effort will be presented.

II. UWB DIELECTRIC-ROD ANTENNA

The use of a dielectric cylinder as a waveguide has beeninvestigated by numerous researchers [8]–[10]. In particular,the propagation attenuation properties of the transverse electric(TE) and transverse magnetic (TM) modes in a dielectric cir-cular rod was studied by Elsasser [10]. These modes have cutoff

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372 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 3, MARCH 2003

Fig. 1. The dielectric-rod with “dipole mode.”

frequencies that limit them to narrow-bandwidth applications.On the other hand, hybrid modes do not have cutoff frequen-cies and, thus, are more suitable for broader bandwidth appli-cations. The mode of interest for our application is the HEmode, which is also sometimes called the “dipole mode” sinceits field distribution is similar to that of an electrical dipole.Fig. 1 illustrates the field behavior of the “dipole mode” given byWalter [11]. The HE mode is guided along the dielectric-rodwith both internal and external fields to satisfy the air-dielec-tric boundary conditions. The energy ratio of the internal andexternal fields have been investigated and can be found in [12].When the rod diameter is much greater than an internal wave-length, most of the energy is guided within the rod with a phasevelocity close to that of a plane wave in an infinite dielectric ma-terial. When the rod diameter is only a fraction of the internalwavelength, most of the energy is guided external to the rodwith a phase velocity close to that of a plane wave in free space.Such behavior suggests some degree of dispersion caused by thephase velocity variation as a function of frequency. Therefore,a proper calibration is required to reduce the dispersion effect.When such a waveguide is terminated properly, the electromag-netic energy then radiates out and thus, gives rise to a dielec-tric-rod antenna.

Earlier work on dielectric-rod antennas have been reported byMueller et al. [12] and Watsonet al. [13] and were referred toas “polyrod antennas” since the rod was made of polystyrene.Mueller also studied the near- and far-field patterns for severaldifferent shapes of polyrod antennas [14], [15]. Almost all pre-vious rod antenna designs were narrow bandwidth devices dueto the narrow band performance of the feed structure for the rodantenna. In addition, those rod antennas were used for far-fieldapplications. The UWB feature of the new dielectric-rod an-tenna is achieved by utilizing a broad bandwidth feed whichonly excites the hybrid HE mode of a dielectric circular wave-guide. The new ultrawide-bandwidth dielectric-rod antenna ismainly used for near-field measurements. As will be shown, thefields radiated from the new rod antenna resembles those froma Hertzian dipole. Therefore, the illumination spot size on theground can be controlled by the antenna height. Another im-portant feature of the new antenna is that it can be accuratelycalibrated. Measurement results for buried antipersonnel (AP)mines showed its superior performance compared to all our pre-vious antennas. Some of these results will be discussed in thispaper. Two key design issues of the new UWB rod antenna in-clude: 1) choosing a proper low-loss dielectric material and rod

Fig. 2. FDTD model for the new circular dielectric-rod antenna above groundwhere a mine-like disk is buried.

diameter to be greater than one waveguide wavelength such thatmost energy is guided within the rod and 2) terminating the rodproperly to reduce end reflections as well as interactions withthe ground interface under measurement. The end could be ter-minated by tapering the diameter down, as adopted in this paper,or by tapering the dielectric constant or the combination of both[18]. The total termination length should be minimized to avoidexcessive movement of the radiation center.

CHEN et al.: NEW UWB DIELECTRIC–ROD ANTENNA FOR GPR APPLICATIONS 373

(a) (b)

(c)

Fig. 3. E plane field distributions. (a) Field guided in the antenna. (b) Spherical wavefront radiated by the antenna. (c) Scattered field from a buried mine-likedielectric target. The relative permittivity and conductivity of the soil are 3 and 0.01 S/m.

III. FDTD NUMERICAL MODELING

A numerical model was created for the prototype rod antennato gain a better understanding of its propagation and radiationproperties. The three-dimensional (3-D) FDTD method [17] isused because of its ability to handle inhomogeneous media andprovide accurate results valid over a wide range of frequenciesat a modest computational cost. Fig. 2 shows the side view andcross-sectional view of the FDTD model near the radiating tipof the circular rod antenna positioned over a circular dielectricdisk buried in soil. The 3-D computational domain is discretizedinto cubic cells where the cell dimension is in. A stairstep ap-

proximation is used to model the circular contour of the dielec-tric rod as well as the narrowing of the diameter of the dielectricrod down to its tip. Unlike a perfectly conducting stairstep, adielectric stairstep produces significantly smaller errors. In ad-dition, the relative dielectric for cells along the stairstep path isset to the average of the relative dielectric of the rod and the freespace region. This “blurring” of the dielectric properties tends tofurther reduce the errors produced by the stairstepping. The timeincrement for the simulation is chosen to satisfy the stability cri-terion [18]. The anisotropic perfectly matched layer (PML) [19]is used to prevent reflections from the truncated boundary. Theconductivity of the PML is chosen to have a quadratic variation

374 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 3, MARCH 2003

along the direction in which attenuation is desired. The PMLlayers are eight-cells thick on all sides. The antenna is excitedby an directed transient current pulse in the form of a differ-entiated Gaussian given by

(1)

where , , and s.This pulse was chosen because the frequency content of pulseis most significant over the desired operational frequency rangeof 1–6 GHz.

Fig. 3(a) and (b) show the snapshots of the calculated fielddistribution of the copolarization component, i.e., , whenthe wave was propagating down the rod and was radiating intofree-space, respectively. The gray scale indicates the field mag-nitude. It can be seen from Fig. 3(a) that most of the energy isconfined in the dielectric rod as desired. Furthermore, the fielddistribution also indicates a pure mode has been excited. Theenergy then radiates out near the tip with a spherical wavefrontwhich is also compared with a dashed circular arc as shown inFig. 3(b). It is also noticed from Fig. 3(b) that very little re-flection occurs at the tip. This is achieved by a properly shapedtapering. Note that the radiation occurs when the rod diameterbecomes less than the internal wavelength. A longer taper mayfurther reduce the end reflection but would cause the radiationcenter to spread over a larger distance in the desired frequencyrange.

In GPR applications, it is often desirable to have a small illu-mination spot on the ground to minimize surface clutter and alsoachieve a good spatial resolution. The new UWB dielectric-rodantenna can achieve this goal when operating near the ground.The radiated copolarization and cross-polarizationfields at 4 GHz on the ground surface shown in Fig. 2(a) are alsocalculated and shown in Fig. 4(a) and (b). Different contoursindicate different field intensity levels that have been normal-ized against the copolarization field intensity at center of groundsurface. The radiated fields along theand axes are dom-inated by copolarization components as indicated by the neg-ligible cross-polarization fields. The spherical nature of the ra-diated wavefront gives rise to the cross-polarization componentin the off-axis regions. Therefore, the radiation properties of thenew UWB dielectric-rod antenna in the forward direction is verysimilar to a Hertzian dipole. To demonstrate the wide bandwidthproperty of the radiation pattern, the radiated field distributionalong the axis is plotted for different frequencies as shownin Fig. 5. The actual field distribution for the prototype rod an-tenna was also measured and shown in Fig. 6 for comparison.As one can see, the measured data and the calculated data are invery good agreement. It can be seen that the width of the illu-mination spot remains approximately constant above 2.4 GHz,whose guided wavelength is approximately equal to the diam-eter of the rod.

The detection of a shallowly buried nonmetallic AP mine hasbeen a challenging problem due to the combination of domi-nant ground surface scattering, weak target backscattering, andthe close mine–surface distance. This difficulty worsens as themine–soil dielectric contrast decreases or as the size of the minegets smaller. The UWB dielectric-rod antenna has been chosenfor landmine detection over other antennas due to its desirable

(a)

(b)

Fig. 4. The radiated field distributions on a plane normal to the axis of theantenna. (a) Copolarization,E . (b) Crosspolarization,E .

small illumination area and weak antenna–ground interactionfeatures. The latter allows one to perform a good antenna cal-ibration. A FDTD model is developed to evaluate the perfor-mance of the UWB dielectric-rod antenna in detecting a buriednonmetallic AP mine that is simulated by a circular dielectricdisk. The diameter and height of the disk areand in, respec-tively. The relative dielectric constant of the disk is chosen to be3. A realistic relative dielectric constant of 9 and conductivity of0.01 S/m were chosen for the soil. The tip of the antenna is ele-vated above the ground surface. Fig. 3(c) plots the calculated

CHEN et al.: NEW UWB DIELECTRIC–ROD ANTENNA FOR GPR APPLICATIONS 375

Fig. 5. Calculated field distribution along theY axis as a function of frequency.

scattered field 0.431 ns after the dominant wavefront enters theground. The scattered field is defined to be the difference be-tween the field in the presence of the disk and the field in theabsence of the disk. From the figure, it can be observed that themajority of energy is transmitted through the disk. The muchweaker backscattered fields then enter the UWB dielectric-rodantenna, which indicates that an AP mine buried in a low-per-mittivity soil will be more difficult to detect. Significant mul-tiple reflections occur between the air–ground and ground–mineinterfaces. The weak antenna–ground interaction is verified bythe fact that no observable multiple reflections occur betweenthe antenna tip and the ground surface.

IV. UWB ROD ANTENNA CALIBRATION METHOD

As mentioned earlier, calibration is necessary to reduce thedispersion effect for the weakly guided low-frequency compo-nents. First, a reference target was measured by placing a shortconducting rod approximately 5 cm in front of the antenna tipwith its axis parallel to the electric field. Second, any internalreflections caused by the internal mismatches at the cable con-nections, antenna feed, and antenna tip were also measured bysimply aiming the rod into an empty space. The background re-sponse was then subtracted out of the data. A conventional radarcalibration procedure was applied with the reference data andthe exact solution calculated from a numerical technique suchas the moment method. Another conducting rod with a different

size was measured to verify the calibration. Fig. 7 comparesthe radar cross section of the calibrated data and its theoret-ical values obtained from the moment method. The agreementis very good. As one can see, such calibration procedure can beeasily performed in the field.

V. MEASUREMENTEXAMPLES FORBURIED AP MINES

The said prototype antenna was used to measure buried land-mines at a government test site located at the Fort A.P. Hill, VA.The antenna tip was elevated two to three inches off the ground.Frequency-swept data were collected with a network analyzer atintervals of 0.54 cm as the antenna was moved along a straightline by a linear scanner. The data were then converted into thetime domain via the Fourier transform. After each scan, the timeversus position data were displayed to the operator as demon-strated in Fig. 8 where an AP mines, PMA-3, was buried at ap-proximately 5-cm depth near 0.3 meter position. The PMA-3mine has a flat cylindrical body whose height and diameter are36 and 103 mm, respectively. This type of mine contains almostno metal content and is very difficult to detect with a mine de-tector. The gray scale is proportional to the measured voltage.The strong surface reflections are clearly seen at earlier timeregion (top of Fig. 8). This surface reflection can easily be sig-nificantly reduced if desired, using processing techniques [20].It is shown here to demonstrate the unprocessed data. One wayto look at more scattering details is to take the derivative of thedata in the spatial direction. This is also included in Fig. 9. Sucha space-time scattering pattern is related to external and internal

376 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 3, MARCH 2003

Fig. 6. Measured field distribution along theY axis as a function of frequency.

Fig. 7. Comparison of theoretical and measured (calibrated) data for a shortconducting cylinder.

scattering characteristics such as the symmetry, shape, compo-sition, edges, cavities, and thickness. It is also a function of themine orientation. Special two-dimensional (2-D) image recog-nition techniques can be developed to extract various scatteringfeatures and perform mine classification. In the case shown here,

Fig. 8. Calibrated data of a low metallic AP mine, PMA-3 buried at 5-cm depthat the Fort A.P. Hill JUXOCO site using the new dielectric-rod antenna.

the type and location of the mine were known. It should be notedthat such classification approach is made possible by the finespatial and temporal resolution that are achieved by the goodcalibration, broad bandwidth, and small illumination spot-sizecharacteristics of the new dielectric-rod antenna design.

CHEN et al.: NEW UWB DIELECTRIC–ROD ANTENNA FOR GPR APPLICATIONS 377

Fig. 9. Spatial derivative data of a low metallic AP mine, PMA-3 buried at5-cm depth.

VI. SUMMARY

A new UWB dielectric-rod antenna design suitable for thedetection of shallow buried targets is presented. A prototypeantenna was built and tested. Good measurement results wereobtained from government test sites. The wave propagation andradiation properties of this prototype were further verified by anumerical model based on the FDTD technique. Both measureddata and calculated data indicate that the broad bandwidth elec-tromagnetic fields are first guided along the rod and then radi-ated out near the end of the rod. The radiated wave properties aresimilar to those from a Hertzian dipole in the forward direction.It was shown that an accurate calibration can be done easily inthe field. By controlling the antenna height to the ground sur-face, one can also control the illumination spot size and thus,control the surface clutter. To effectively guide the electromag-netic energy inside the rod, its diameter should be greater thanone internal wavelength. The upper frequency bound is limitedby the excitation of other modes.

REFERENCES

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[2] A. S. Abul-Kassem, “Experimental study of the characteristics of a hor-izontal antenna above a dissipative homogeneous earth,” M.S. thesis,Dept. Elect. Eng., Univ. Colorado , Denver, CO, 1972.

[3] P. Hayes, “Electromagnetic behavior of transmission lines and resonantwires very near a lossy dielectric interface,” Ph.D. dissertation, Electro-Science Lab., The Ohio State Univ., Columbus, OH, 1983.

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[6] Y. Nishioka, O. Maeshima, T. Uno, and S. Adachi, “FDTD analysis of re-sistor-loaded bow-tie antennas covered with ferrite-coated conductivitycavity for subsurface radar,”IEEE Trans. Antennas Propagat., vol. 47,pp. 970–977, June 1999.

[7] T. P. Montoya and G. S. Smith, “Land mine detection using a groundpenetrating radar ased on resistively loaded vee dipoles,”IEEE Trans.Antennas Propagat., vol. 47, pp. 1795–1806, Dec. 1999.

[8] J. R. Carson, S. P. Mead, and S.A. Schelkunoff, “Hyper-frequency waveguides—Mathematical theory,”Bell Syst. Tech. J., vol. XV, pp. 311–333,1936.

[9] C. H. Chandler, “An investigation of dielectric rod as wave guide,”J.Appl. Phys., vol. 20, pp. 1188–1192, 1949.

[10] W. M. Elsasser, “Attenuation in a dielectric circular rod,”J. Appl. Phys.,vol. 20, pp. 1193–1196, 1949.

[11] C. H. Walter,Traveling Wave Antennas. New York: McGraw-Hill,1965, p. 214.

[12] G. E. Mueller and W. A. Tyrrell, “Polyrod antennas,”Bell Syst. Tech. J.,vol. XXVI, pp. 837–851, 1947.

[13] R. B. Watson and C. W. Horton, “The radiation pattern of dielectricrods—Experiment and theory,”J. Appl. Phys., vol. 19, pp. 661–670,1948.

[14] J. J. Panakel and G. E. Mueller, “An investigation of polystyrene rodantennas,” Antenna Lab., The Ohio State Univ., Columbus, OH, Tech.Rep. 510-11, June 1954.

[15] G. E. Mueller, “Dielectric antennas,” Antenna Lab., The Ohio StateUniv., Columbus, OH, Tech. Rep. 434-5, Feb. 1952.

[16] C.C. Chen, K. Rama Rao, and R. Lee, “A tapered-permittivity Rod An-tenna for Ground Penetrating Radar Applications,”J. Appl. Geophys.,vol. 47/3-4, pp. 309–316, Sept. 2001.

[17] K. S. Yee, “Numerical solution of initial boundary value problems in-volving Maxwell’s equations in isotropic media,”IEEE Trans. AntennasPropagat., vol. AP-14, pp. 302–307, 1966.

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Chi-Chih Chen (S’92–M’97) was born in Taiwan,R.O.C., in 1966. He received the B.S.E.E. degreefrom the National Taiwan University, Taiwan,R.O.C., and the M.S.E.E. and Ph.D. degrees fromThe Ohio State University, Columbus, in 1988,1993, and 1997, respectively.

In 1997, he joined the ElectroScience Laboratory,Ohio State University as a Postdoctoral Researcherand became a Senior Research Associate in 1999.His main research interests include the developmentof ground-penetrating radar, ultrawide-bandwidth

antenna development, radar-signal processing, target detection, and classifica-tion. In recent years, his research activities have focused on the detection andclassification of buried landmines, unexploded ordinance, and undergroundpipes.

Dr. Chen is a Member of Sigma Xi and Phi Kappa Phi. He is currently servingas Chairman of the IEEE Joint APS/MTT Columbus Chapter.

Kishore Rama Raoreceived the B.E. degree in 1995, from Anna University,Chennai, India, and the M.S.E.E. degree, in 1998, from The Ohio State Univer-sity, Columbus, where he is currently working toward the Ph.D. degree.

Robert Lee (M’90–SM’01) received the B.S.E.E.degree from Lehigh University, Bethlehem, PA, andthe M.S.E.E. and Ph.D. degrees from the Universityof Arizona, Tucson, in 1983, 1988, and 1990,respectively.

From 1983 to 1984, he was a Microwave Engineerwith the Microwave Semiconductor Corporation,Somerset, NJ. From 1984 to 1986, he was a Memberof the Technical Staff at Hughes Aircraft Company,Tucson, AZ. From 1986 to 1990, he was a ResearchAssistant at the University of Arizona. During

summers from 1987 to 1989, he worked at Sandia National Laboratories,Albuquerque, NM. Since 1990, he has been at The Ohio State University,Columbus, where he is currently a Professor. His research interests include theanalysis and application of finite methods to electromagnetics.

Prof. Lee is a Member of the International Union of Radio Science (URSI).He was a recipient of the URSI Young Scientist Award in 1996.