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Published in IET Microwaves, Antennas & PropagationReceived on 14th September 2008Revised on 17th February 2009doi: 10.1049/iet-map.2008.0318

ISSN 1751-8725

Effect of antenna dimensions on the antennafootprint in ground penetrating radarapplicationsA.A. Pramudita1,2 A. Kurniawan1 A.B. Suksmono1

A.A. Lestari11International Research Centre for Telecommunications and Radar – Indonesian Branch (IRCTR-IB) STEI – ITB,Jl. Ganesha 10, Bandung 40132, Indonesia2Atmajaya Catholic University, J1 Jenderal Sudirman 51, Jakarta 12930, IndonesiaE-mail: pramudita@atmajaya.ac.id

Abstract: Ground penetrating radar (GPR) surveys show that antenna characteristic is strongly influenced by soilconditions. The footprint of the antenna is an important parameter for a good detection result. Various conditionsof soil in which a target is buried may change the footprint of the antenna. An antenna with capability to controlits footprint is needed in GPR applications. In this study, the authors investigate several ultra-wideband (UWB)antennas with different dimensions to study the effect of antenna dimension on their footprint. Simulationand experiments show that large (small) antenna dimensions result in a large (small) antenna footprint whenthe observation is located in the near-field region. When the observation is located in the far-field, thefootprint of the antenna becomes large (small) if the dimensions of the antenna are small (large). Thus, thesize of the antenna footprint can be adjusted by varying the antenna dimension. It is applied in this work todevelop a new method for controlling the antenna footprint to deal with varying soil condition.Measurements have been carried out to validate this concept.

1 IntroductionGround penetrating radar (GPR) systems are used forsubsurface investigations which include detection of objectsburied beneath the earth surface such as landmines, cables,pipes and hidden tunnels. In a GPR system, transmit andreceive antennas play a key role [1]. GPR antennas areusually situated very close to the ground surface to alloweffective coupling of electromagnetic energy into theground. However, it is widely known that the antenna isstrongly influenced by the type of the soil and furthermoreantenna with stable footprint and input impedance isneeded to achieve a good detection result [2, 3].

The antenna footprint indicates an effective areailluminated by the antenna on the ground surface orsubsurface. An optimal footprint is usually closely related

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with the size of the object. If the footprint is too large, itmay increase surface clutter as large area that is illuminated.On the other hand, a too small a footprint makes itdifficult to distinguish the objects. Additionally, smallfootprint causes a hyperbola that will be too small in theGPR B-scan, making it difficult to detect the targets [2].

Adaptive footprint adjustment for a GPR antenna is one ofthe most challenging topics in GPR antenna research [4]. Aprevious research described that footprint adaptationcapability of wire bow-tie antenna is achieved by varyingthe flare angle of the antenna which in turn results invariation of the antenna footprint [3].

In this paper, first we investigate the effect of the soilcondition on antenna footprint. Next, we investigate fiveUWB antennas with different dimensions to study the

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effect of antenna dimension on the footprint. The size of theantenna footprint can be adjusted by varying the antennadimension. It is applied in this work to develop a newmethod for controlling the antenna footprint to deal withvarying soil condition.

This research focuses on GPR applications within afrequency range of 100–1000 MHz. The proposed UWBantennas should meet the bandwidth requirement of the100–1000 MHz GPR system and exhibit small late-timeringing. Simulations have been performed both infrequency and time domain for which the method ofmoment (MoM) has been chosen for the numericalmethod using the FEKO software package. Moreover,measurements have been carried out to verify thesimulation results.

2 Numerical analysisIn this research, a modified microstrip dipole antenna isinvestigated and described in this section. The design hasbeen discussed in our previous research [5]. The geometryof the UWB antenna is depicted in Fig. 1. Five of suchantennas with different dimensions are printed on an FR-4epoxy dielectric substrate with thickness of 3.2 mm and arelative dielectric constant of 4.4. Resistive loads of 62 V

are used at the end of the modified microstrip dipole andthe input port is connected to SMA connector. Thethickness of the strip (D) of each of the modified dipoleantennas is 3 mm. The complete dimensions of thoseantennas are listed in Table 1. Each of those antennas isdesigned to meet the requirement of a 100–1000 MHzGPR system. As a 5-ns monocycle is the shortest pulsethat can be transmitted in the frequency range between100 MHz and 1 GHz. The spectrum of a 5- ns monocyclehas centre frequency at 350 MHz. Those antennas havebeen optimised for transmitting a 5-ns monocycle pulse(see Table 1).

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In the simulation, the antenna was situated above theground surface with an elevation of 20 mm and weinvestigated the antenna in both near and far fields. Thenear and the far-field regions in the ground can beapproximated by calculating the Fraunhofer region in thesubsurface [6]. The near and far-field regions of theantennas in several ground types are calculated at afrequency of 350 MHz which is the centre frequency of the5-ns monocycle pulse. In this paper, the footprint of thoseantennas is computed and measured at a depth of 40 and300 mm. The depth of 40 and 300 mm represents the nearand far-field of the antennas in Table 1. For GPR, it isimportant to examine the amplitude of the transmittedwaveforms since it indicates the amount of the energytransmitted by the antenna into the ground. For thispurpose, the antenna system can be modelled as a lineartime-invariant system for which the transmitted signal canbe obtained as a product of the computed S21 and thespectrum of the exciting pulse followed by inverse FFToperation [7]. The transmitted signal can be written as

Et ¼ IFFT[S21( f )Sf ( f )] (1)

where Sf ( f ) is spectrum of exciting pulse and IFFT is theinverse FFT operator. An antenna footprint is defined as adistribution of the normalised peak values of transmitwaveforms within a horizontal plane on the ground surfaceor subsurface, which indicates the shape and size of thespot illuminated by the antenna [1, 2]. In many GPRapplications antenna footprints play an important role sinceradar imaging can be improved when the size of thefootprint is comparable to those of the targets, as indicatedby Daniels [1]. When the footprint is too large it gives riseto surface clutter as a spot on the ground is illuminatedlarger than what is really needed. An optimal footprint isalso important to improve target localisation. The antennafootprint can be determined by observing the peak oftime domain transmit waveforms at a grid of pointsdistributed in the xy-plane. In this paper we consider

Figure 1 Geometry of the modified microstrip dipole antenna with different dimensions

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Table 1 Dimension of the UWB antennas

Antenna 1 Antenna 2 Antenna 3 Antenna 4 Antenna 5

S, mm 30.4 36.8 43.2 56 72.4

mostly the 23 dB level of footprints. The footprint of UWBantenna is simulated in different soil types at a depth of 40and 300 mm and grid size of 50 mm (in x and y direction).We consider three ground types, i.e. dry sand (1r ¼ 5.1,s ¼ 0.004 S/m), dry clay (1r ¼ 16, s ¼ 0.03 S/m) andmoist clay (1r ¼ 25, s ¼ 0.06 S/m).

MoM is chosen for the numerical method using theFEKO software package. FEKO is MoM-based simulationsoftware that incorporates planar layered-medium Green’sfunctions, and is capable of simulating antennas of arbitraryshape.

This paragraph describes the characteristics of theproposed antennas which include S11, bandwidth,transmitted waveform, radiation efficiency and polarisation.The computed S11 of the UWB antennas as a function offrequency can be seen in Fig. 2. It can be seen that the S11

characteristic of the proposed UWB antennas meetthe 210 dB level bandwidth requirement. Each of theantennas is excited by a 5-ns monocycle pulse and thecomputed transmitted waveforms at a depth of 300 mm indry sand are shown in Fig. 3.

Fig. 4 shows a comparison of the transmitted waveformsbetween the UWB antennas and the 350-MHz stripdipole. From Fig. 4, we can see that the UWB antennashave a lower ringing level than the strip dipole. Figs. 2, 3and 4 indicate that the proposed antennas have UWBcharacteristics. As the antennas have been designed withresistive loads to improve the bandwidth and reduce theringing level. Applying resistive loading will of course

Figure 2 Computed S11 of five UWB antennas as a functionof frequency

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reduce the antenna efficiency. As has been shown in [8],for example that the Wu-King loading profile reduces theantenna efficiency until 50%. However, UWBcharacteristics and a low late-time ringing level areimportant in GPR applications and thus, despite reductionin efficiency, resistive loading is here applied. When theantennas are excited by a 5-ns monocycle pulse, simulationshows that resistive loading reduces the antenna efficiencybelow 50% for antenna 1, antenna 2 and antenna 3.Antenna 4 and antenna 5 have efficiency of around 50%, asshown in Table 2. The polarisation of the UWB antennasin Table 1 has also been investigated. It is found that theantennas have relatively constant ratio of co- and cross-polarisation of around 60 dB within a frequency range100 MHz–1 GHz.

In Fig. 5 we present the computed footprint of antenna 2in the xy-plane at a depth of 40 mm in dry sand, dry clay andmoist clay. Fig. 5 shows that the size of the antenna footprintis more reduced by the presence of a ground with higherdielectric constant. Table 3 shows the footprint size of theantennas in Table 1 when observed in three differentground types. In comparison with dry sand, the footprintsize of UWB antennas reduces by about 39% whenobserved in dry clay and by 68% when observed in moistclay. As the antenna footprint varies with the soil type, thissituation complicates post-processing of the GPR data fortarget classification and identification. To deal with thisproblem an antenna with the capability of controlling itsfootprint is needed.

Figure 3 Computed transmit waveform of the antenna at adepth of 300 mm in dry sand

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Table 2 Simulation result of antenna efficiency

Antenna 1 Antenna 2 Antenna 3 Antenna 4 Antenna 5

efficiency, % 45 48 51 52 52

Furthermore, we investigated the influence of antennadimensions on the size of the antenna footprint. To thisend, the footprints of the five UWB antennas withdifferent dimensions have been investigated by simulationand measurement.

In Fig. 6, we present the computed footprint of antenna 1and antenna 2, at a depth of 40 mm in dry sand for which theobservation area is located in near-field region. Fig. 6 andTable 3 together show that the size of the antenna’sfootprint is closely related with the dimensions of theantenna, i.e. the footprint of the antenna becomes large(small) if the dimension of the antenna is large (small)when the observation region is located in the near fieldregion. In Fig. 7, we present the computed footprint ofantenna 1 and antenna 2, at a depth of 300 mm in drysand for which the observation area is located in the far-field region. The simulation result in Table 3 concludesthat the footprint of the antenna becomes large (small) ifthe dimension of the antenna is small (large) when theobservation region is located in the far-field region. Thisresult can be understood by considering the Fouriertransform relationship between aperture distribution andfar-field [5, 9–11]. An antenna with larger aperture willproduce a narrower beam width.

According to the simulation results in Table 3, size ofthe antenna’s footprint is can be adjusted by varyingthe dimensions of the antenna. For example, when thefootprint size should be maintained to be around 80 cm2 ata depth 40 mm, one should use antenna 1 when the

Figure 4 Measured transmit waveform of the antenna at adepth of 300 mm in dry sand

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Figure 5 Computed footprint of antenna 2 at a depth of40 mm in different soil type

a Dry sandb Dry clayc Moist clay

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Table 3 Computed footprint at different ground types (in cm2)

Depth ¼ 40 mm Depth ¼ 300 mm

Dry sand Dry clay Moist clay Dry sand Dry clay Moist clay

antenna 1 84 44 14 1037 573 360

antenna 2 132 76 34 814 390 241

antenna 3 208 140 82 764 327 193

antenna 4 254 162 88 596 306 178

antenna 5 282 178 110 465 237 124

ground type is dry sand, antenna 2 when the ground type isdry clay and antenna 3 when the ground type is moist clay.

3 ExperimentsThe UWB antennas shown in Fig. 1 have been constructedfor experimental investigations. The measured S11

Figure 6 Computed footprint of the UWB antennas. Thefootprint is located at a depth of 40 mm in dry sand

a Antenna 1b Antenna 2

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characteristics of the UWB antennas are plotted in Fig. 8.It can be seen that the measurement result in Fig. 8 agreeswith the simulation result in Fig. 2. Both simulation andmeasurement results show that the UWB antennas meetthe prescribed bandwidth requirement. Moreover, themeasurement results of the transmitted waveform of theUWB antennas are shown in Fig. 9 in which a comparison

Figure 7 Computed footprint of the UWB antennas. Thefootprint is located at a depth of 300 mm in dry sand

a Antenna 1b Antenna 2

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of the transmitted waveform between the UWB antennas andthe 350 MHz strip dipole is given. Both the simulation andmeasurement of the transmitted waveform show that theantenna with smaller dimension has transmitted waveformwith lower amplitude. The time domain radiation pattern(i.e. the maximum amplitude of the radiated pulsemeasured at equal distance around the antenna) of theantennas show that the antennas radiate maximumelectromagnetic energy in the boresight direction of theantenna. As an example of time domain patterns, Fig. 9shows a the time-domain pattern of antenna 3.

The footprint measurement setup in our GPR test rangefacility is illustrated in Fig. 10. Our GPR test facilityconsists of a GPR test range, PC-controlled scanner, aUWB sensor and a vector network analyser. The test rangewas constructed as a wooden sandbox of 3 m � 3 m �1.6 m and was filled with dry sand with relative permittivityof 5.1 [12]. The relative permittivity of the sand is

Figure 8 Measured S11 characteristic of the UWB antennas

Figure 9 Measured time-domain radiation pattern ofantenna 3

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relatively high since it consists of mainly granite with arelative permittivity of 5–7 [1]. Furthermore, the highhumidity in Indonesia also plays a role in increasing thevalue of the permittivity of the sand.

Figure 10 Measurement setup at the GPR test rangefacility (d ¼ 40 and 300 mm)

Figure 11 Measurement result of antenna footprint fromUWB antennas. The footprint is located at a depth of40 mm in dry sand

a Antenna 1b Antenna 2

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The transmitted signal has been measured at a number ofsampling coordinate points distributed in the xy-plane witha grid size of 5 cm by positioning the antenna at each of thegrid point using the PC-controlled scanner. The antennafootprint has been measured at a depth of 40 and 300 mmin dry sand. Figs. 11 and 12 show the footprintmeasurement results in the near and far-field regions. Wecan see from the two figures that the size of the antennafootprint is closely related with the antenna dimension.Fig. 11 shows the measured footprint of antenna 1 andantenna 2 at a depth of 40 mm in dry sand whereas the

Figure 12 Measurement result of antenna footprint fromUWB antennas. The footprint is located at a depth of300 mm in dry sand

a Antenna 1b Antenna 2

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measured footprint size of all five UWB antennas at a depthof 40 mm in dry sand is listed in Table 4. By comparing thesize of the footprints at the 23-dB level, we can concludethat in the near-field region the footprint of the antennabecomes large (small) if the dimension of the antenna islarge (small). In addition, Fig. 12 shows the footprint ofantenna 1 and antenna 2 at a depth of 300 mm in dry sand.The footprint size of the UWB antennas at a depth of300 mm in dry sand can be seen in Table 4. It is obviousthat the footprint of the antenna becomes large (small) if thedimension of the antenna is small (large) when observed infar-field region and this result has been predicted bysimulation. However, the size of the measured footprint issmaller than that in simulation when observed at a depth of40 mm and the size of the measured footprint is larger thanthat in simulation when observed at a depth of 300 mm.This discrepancy might be contributed by the UWB sensorused in measurement since it is difficult to include thesensor model in simulation. Despite the slight discrepancybetween the measurement and the simulation results, thesimulation can still give a good prediction of themeasurement results. From the simulation and measurementresults, one can conclude that the size of the antennafootprint can be adjusted by varying the effective aperture ofthe antenna. This concept forms the basis for developing anew method for controlling the antenna footprint. An arrayof several UWB antennas with different dimensions iscurrently under investigation for implementation in a GPRsystem. The array system should be supported by a switchingmechanism that is used to select the active element in arrayconsider to the footprint that is needed.

4 ConclusionEffect of antenna dimensions on the antenna footprint hasbeen investigated both in theory and in experiment. Thesimulation and measurement give the same conclusion, i.e.the size of the antenna footprint closely related with theantenna dimension. The footprint of the antenna becomeslarge (small) if the dimension of the antenna is large (small)when observed in near-field region. Contrary to that, thefootprint of the antenna becomes large (small) if thedimension of the antenna is small (large) when observed infar-field region. This result can be used in GPR applicationto deal with varying soil conditions which may causevariation of antenna footprint. It has been shown that thesize of the antenna footprint can be adjusted by varying theantenna dimensions. This concept is currently underinvestigation for developing a new method for controllingthe antenna footprint for GPR application.

Table 4 Measurement result of the antenna footprint in dry sand

Depth (mm) Antenna 1 Antenna 2 Antenna 3 Antenna 4 Antenna 5

40 76 107 156 219 258

300 1184 974 911 837 752

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5 AcknowledgmentThe authors are thankful to Prof. Leo Ligthart and Prof.Alex Yarovoy from IRCTR Tu-Delft for their support andsuggestions. A.B Suksmono is supported by ITB AlumnaeAssociation International Research Grant (HRIA).

6 References

[1] DANIELS D.J.: ‘Ground penetrating radar, IEE radar sonar’(Navigation and Avionics Series, Institution of Engineeringand Technology, London, UK, 2004, 2nd edn.)

[2] LESTARI A.A.: ‘Antennas for improved ground penetratingradar: modeling tools, analysis and design’. PhD thesis,Delft University of Technology, 2003, The Netherlands

[3] LESTARI A.A., YAROVOY A.G., LIGTHART L.P.: ‘Adaptive wire bow-tie antenna for GPR applications’, IEEE Trans. AntennasPropag., 2005, 53, (5), pp. 1745–1754

[4] YAROVOY A.G., MEINCKE P., DAUVIGNAC J.: ‘Development ofantennas for subsurface radars within ACE’. Proc. Int.Conf. Ultra-wideband, Singapore, September 2007

[5] PRAMUDITA A.A., KURNIAWAN A., SUSKMONO A.B.: ‘Modifieddipole antenna for UWB SFCW-GPR’. Proc. Int. Conf.Electrical Engineering and Informatics (ICEEI’07)’,Bandung, Indonesia, June 2007

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[6] KRAUS D.J., MARHEFKA R.J.: ‘Antennas’ (McGraw-Hill, 2002,3rd edn.)

[7] LESTARI A.A., YAROVOY A.G., LIGTHART L.P.: ‘Ground influenceon the input impedance of transient dipole and bowtieantennas’, IEEE Trans. Antennas Propag., 2004, 52, (8),pp. 1970–1975

[8] WU T.T., KING R.W.P.: ‘The cylindrical antenna withnonreflecting resistive loading’, IEEE Trans. AntennasPropag., 1965, AP-13, pp. 369–373

[9] HASSANI H.R.: ‘Method of moment analysis of finitephased array of aperture coupled circular microstrippatch antenna’, Prog. Electromagn. Res. B, 2008, 4,pp. 197–210

[10] FERRARA F., GENNARELLI C., GUERRIERO R., RICCIO G.: ‘An efficientnear-field to far-field transformation using the planar widemesh scanning’, J. Electromagn. Waves Appl., 2007, 21, (3),pp. 341–357

[11] COSTANZO S., MASSA G.D.: ‘Direct far field computationfrom bi-polar near field samples’, J. Electromagn. WavesAppl., 2006, 20, (9), pp. 1137–1148

[12] LESTARI A.A., YULIAN D., LIARTO L., ET AL.: ‘GPR antenna testfacility at IRCTR-IB’. Proc. Fourth Int. Workshop onAdvanced Ground Penetrating Radar, June 2007

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