Abstractâ In this paper a rolled dipole antenna for low-resolution impulse ground penetrating radar (GPR) is theoretically investigated. The antenna is designed ...
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Rolled Dipole Antenna for Low-resolution GPR A. A. Lestari1,2 , D. Yulian3 , A. B. Suksmono1,4 , E. Bharata1,4 A. G. Yarovoy2 , and L. P. Ligthart2 1
International Research Centre for Telecom and Radar–Indonesian Branch (IRCTR-IB), Indonesia 2 International Research Centre for Telecom and Radar (IRCTR)–TU Delft, The Netherlands 3 Radar and Communication Systems (RCS), Indonesia 4 Bandung Institute of Technology (ITB), Indonesia
Abstract— In this paper a rolled dipole antenna for low-resolution impulse ground penetrating radar (GPR) is theoretically investigated. The antenna is designed for transmission of monocycle pulses with duration of 5 ns (200 MHz central frequency) suitable for low-resolution GPR applications. The dipole is rolled to considerably reduce it length and resistive loading with Wu-King profile is applied for suppression of late-time ringing important for GPR. Using NEC-2 as the numerical tool it is shown that the antenna radiates the pulse with no late-time ringing. Furthermore, by rolling the wires the antenna length is reduced by a factor of 4 with no evident negative impact on the antenna’s characteristics. DOI: 10.2529/PIERS060906104146
1. INTRODUCTION
Ground penetrating radar (GPR) with low resolution is useful for detection of large buried objects or structures such as subsurface caves, bunkers and rivers or canals. Low-resolution GPR generally transmits relatively long transient pulses with central frequency of 200 MHz or lower for obtaining low down-range resolution and sufficient penetration depth required for detecting large subsurface structures. Consequently, large antennas are normally needed for efficient transmission of those pulses. If the dimensions of the antennas are reduced, generally one suffers from degraded efficiency and increased late-time ringing as a result of smaller antenna bandwidth. Minimal late-time ringing is important to avoid masking of radar returns when the targets are shallowly buried. In this paper we investigate a method to substantially reduce the length of a dipole antenna for low-resolution GPR by rolling the wires. Resistive loading with Wu-King profile is then applied to eliminate late-time ringing. Similar approach has been used in [1] to increase antenna bandwidth by slightly rolling the edges of a V-dipole antenna for high-resolution GPR. Here, nearly the whole length of the dipole is rolled resulting in a spiral-like shape. 2. ANTENNA DESIGN
In this work the antenna is designed for excitation with monocycle pulses with duration of 5 ns (having a central frequency of 200 MHz) suitable for low-resolution GPR applications. The geometry of the proposed rolled dipole antenna is shown in Figure 1. The total length of the wire is 296 cm and by rolling the wire as shown in the figure the antenna length is reduced by a factor of nearly 4, resulting in antenna length of only 75 cm. The fraction of the wires starting from 20 cm from the feed point is resistively loaded with resistors according to the Wu-King profile. In each arm of the dipole we employ 65 resistors with 1 cm separation. This number of resistors should be sufficient for proper implementation of the Wu-King profile. The first resistor near the feed point has a value of 200 Ω and at the same time also functions as a secondary source of radiation due to the discontinuity it introduces. It has been demonstrated √ in [2] that when the distance between the feed point and the first resistor is chosen to be c/(4fc εrs ), where c is the speed of light, fc is the central frequency of the exciting pulse and εrs is the relative permittivity of the substrate, in the broadside direction of the antenna radiation from the secondary source combines constructively with radiation from the feed point, resulting in significant increase in the amplitude of the transmitted pulses.
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Figure 1: Geometry of the rolled dipole. The gaps are the locations of resistors for antenna loading. In practice the resistors are soldered across the gaps. The total length of the wire is 296 cm while the length of the rolled dipole is 75 cm, giving a reduction factor of nearly 4. Its height is 31.5 cm. The dipole will be realized as a printed antenna on an FR-4 substrate. 3. SIMULATIONS
The proposed design shown in Figure 1 will be realized as a printed antenna on an FR-4 substrate. By means of the equivalent radius formula introduced in [3] the antenna structure can be approximated with thin wires, for which the one-dimensional integral equation method is well-suited to perform numerical analysis. Therefore, the well-known NEC-2 code is here employed. Influence of the substrate is taken into account by scaling up the antenna dimensions by a factor of the square root of the substrate’s relative permittivity. Time-domain responses of the antenna are obtained by performing frequency sweep over the spectrum of the 5-ns monocycle pulse followed by inverse FFT operation. The frequency sweep was performed from 1 MHz to 1 GHz with 1 MHz step. In Figure 2 the waveform and spectrum of the pulse are presented.
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Transmit waveforms of the antenna have been computed and the near-field waveforms are plotted in Figure 3. The near-field waveforms of the antenna without and with loading are given in Figures 3(a) and 3(b), respectively. It is shown that when the antenna is not loaded the waveform is dominated by internal reflections as expected. We note that in the near-field region the first reflection from the antenna ends is still clearly separated from the main pulse radiated from the feed point. When resistive loading with Wu-King profile is applied, it is demonstrated in Figure 3(b) that
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all reflections are eliminated, leaving the main pulse only. In the far-field region the main pulse and the first end reflection are not anymore separable as observed in Figure 4(a). When resistive loading with Wu-King profile is applied, similarly internal reflections are eliminated, leaving the main pulse with the shape of a triplet as seen in Figure 4(b). Furthermore, in Figure 5 we demonstrate that the influence of the rolled part of the wires on transmit waveforms is noticeable only in the near-field region. -4
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The computed input impedance of the antenna is presented in Figure 6. It is demonstrated that the resonance behavior seen in Figure 6(a) is suppressed by the loading. Moreover, we observe in Figure 6(b) that soil has insignificant impact on the input impedance when the antenna is elevated at least 10 cm as generally is the case in real low-resolution GPR surveys. Currently we are working on feeding and impedance matching techniques for the proposed antenna. In addition, an optimal shield for the antenna is being designed. These works and experimental verifications of the antenna in laboratory and field conditions will be reported in our future papers.
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Figure 6: Free-space input impedance of the rolled dipole: (a) without loading, (b) with loading (Wu-King profile). In (b) the input impedance of the rolled dipole situated 10 cm above a lossy soil is also shown, with the soil parameters: εr = 9, σ = 0.01 S/m. REFERENCES
1. Morrow, I. L., J. Persijn, and P. van Genderen, “Rolled edge ultra-wideband dipole antenna for GPR application,” IEEE APS Int. Symp. Digest, Vol. 3, 484–487, 2002. 2. Lestari, A. A., A. G. Yarovoy, and L. P. Ligthart, “RC loaded bow-tie antenna for improved pulse radiation,” IEEE Trans. Antennas Propagat., Vol. 52, No. 10, 2555–2563, Oct. 2004. 3. Butler, C. M., “The equivalent radius of a narrow conducting strip,” IEEE Trans. Antennas Propagat., Vol. 30, No. 4, 755–758, July 1982.
