AN EFFICIENT ULTRA-WIDEBAND BOW-TIE ANTENNA A.A. Lestari, A.G. Yarovoy, L.P. Ligthart International Research Centre for Telecommunications-transmission and Radar (IRCTR) Delft University of Technology Mekelweg 4, 2628 CD Delft, The Netherlands Phone: 31-15-2782496, Fax: 31-15-2784046, Email:
[email protected] ABSTRACT In this paper, a transient bow-tie antenna for Ground Penetrating Radar applications is discussed. This antenna exhibits good efficiency and an ultra-wideband property due to a combination of a tapered capacitive and a resistive loading. The tapered capacitive loading is realized by constructing an array of slots on the antenna surface and the resistive loading is obtained by making use of microwave absorber blocks placed on the slotted surface of the antenna. Using a 0.8-ns monocycle as input pulse, the amplitude of its transmit waveform is 70% higher than that of a conventional bow-tie antenna of equal dimension. The transmit waveform is a nearly symmetrical triplet with a late-time ringing level of lower than -33 dB. INTRODUCTION A wide range of time-domain applications such as impulse Ground Penetrating Radar (GPR) requires an ultra-wideband antenna system capable of transmitting properly short transient pulses. In certain GPR applications for detection of surfacelaid and shallowly-buried objects such as landmines, the antenna should exhibit a very low level of late-time ringing in order to avoid masking of radar targets. Late-time ringing is caused by the antenna's narrow bandwidth and internal reflections. A conventional method to reduce it (thus increasing the antenna's bandwidth) is the implementation of a resistive loading. A major drawback of this method is that antenna efficiency might degrade down to as low as 29% [1]. For many ultra-wideband applications this can be a serious disadvantage due to severe restrictions of the allowed transmit power usually imposed by local authorities. For this reason it is desired to have an ultra-wideband antenna with good radiation efficiency. In this paper we propose a novel technique to significantly improve antenna bandwidth while at the same time radiation efficiency can still be kept high. The proposed technique implements a combination of a tapered capacitive loading and a resistive loading. The tapered capacitive loading is realized by an array of slots constructed on the antenna surface. The slots should have linearly increasing widths towards the antenna open ends in order to create a capacitively-tapered loading. The resistive loading is realized by employing microwave absorber blocks placed on the slotted surface of the antenna. For experimental verifications, we applied the technique on a circular-end bow-tie antenna. It has been experimentally found that this technique proves effective to considerably improve bandwidth as well as radiation efficiency. A substantial understanding of the physical processes within the antenna on which this technique is applied, is gained through Finite-Difference TimeDomain (FDTD) and Method of Moments (MoM) simulations. FDTD SIMULATIONS As a very detailed structure is difficult to simulate by FDTD because of computer memory limitations, a model of a circular-end bow-tie antenna with a low-profile taper is used. As shown in Fig. 1, the taper consists only of 10 slots with linearly increasing widths towards the open end. This simple model is intended only to give the basic understanding of transient radiation from such taper. The exciting pulse is a monocycle having a spatial length smaller than the antenna's length in order to obtain a clear separation of radiation from the feed point and open end. The calculated transient electric field in the broadside direction of the tapered bow-tie antenna and a conventional bow-tie antenna of the same size is plotted in Fig. 2. It can be seen that in the case of the tapered bow-tie antenna, the first open-end reflection disappears and replaced by a ringing with much lower amplitudes due to the presence of the slots. This result indicates the potential of this kind of taper to significantly reduce open-end reflections. Intuitively, it is believed that a taper consisting of more slots would result in a smaller ringing. However, since we are not able to properly simulate a finer taper by FDTD, verifications are carried out experimentally. Moreover, it can also be seen in Fig. 2 that the amplitude of the transmit waveform of the tapered bow-tie antenna is about two times higher than that of the conventional bow-tie antenna. This is caused by the fact that radiation from the tapered bow-tie antenna is basically a superposition of radiation from two locations: the feed point and the first slot. This implies
that the shape and amplitude (thus radiation efficiency) of the transmit waveform can be optimized by adjusting the distance between the feed point and the first slot. MoM SIMULATIONS Analysis of the actual taper which will be constructed on an experimental antenna is carried out by MoM in frequency domain. Using MoM, a very detailed structure of the taper can be modeled since it requires much less computer memory than FDTD. The actual taper consists of 33 slots with linearly increasing widths towards the open end. The triangular mesh of the tapered bow-tie antenna shown in Fig. 3 is employed in conjunction with the Rao-Wilton-Glissson basis function [2]. For this simulation, 2849 unknowns are involved. The calculated distribution of surface current density on the tapered bow-tie antenna and a conventional bow-tie antenna of equal size is given in Fig. 4 and Fig. 5, respectively. It can be seen in Fig. 5 that on the surface of the conventional bow-tie antenna, currents are concentrated at the feed point and along the two edges, which agrees with [3]. In contrast to this, Fig. 4 shows a current concentration at the feed point and on the slotted surface of the tapered bow-tie antenna. The result clearly indicates that each slot serves as a secondary source of radiation which most likely is the main cause of the late-time ringing seen in Fig. 2. This suggests that by applying a resistive loading such as an absorber block on the slotted surface, one would be able to effectively reduce the ringing without essentially affecting the main radiation from the feed point and the first slot. This understanding forms the basic idea of the technique, i.e., combining an array of slots (tapered capacitive loading) and absorbers (resistive loading) to suppress ringing, together with the exploitation of radiation from the feed point and the fist slot to enhance radiation efficiency. MEASUREMENTS For experimental verifications, a tapered bow-tie antenna with a flare angle of 90° and a length of about 50 cm has been built using a photo-etching technique on a thin epoxy substrate, as shown in Fig. 6. The slots have increasing widths towards the open end with a step of 0.1 mm and a center-to-center spacing of 5 mm. As input pulse, we employ a monocycle with duration of 0.8 ns. This pulse is sufficiently short to guarantee that open-end reflections do not overlap with the main pulse (which comes from the feed point and the first slot). Another advantage of this pulse is that its spectrum contains only very small low frequency components that cannot be radiated properly by an antenna of such dimension. We have experimentally found that by changing the distance between the feedpoint and the first slot an optimization can be done to obtain a minimum level of late-time ringing. For this optimization absorber blocks are utilized to suppress unwanted radiation which is responsible for late-time ringing, by placing them on one of the slotted surfaces of the antenna. The result has been reported in [4] in which it was shown that a level of ringing of lower than -40 dB can be achieved. However, the transmit waveform in [4] is asymmetrical. For some purposes such as target identification, usually it is important to use a transmit waveform which can be easily characterized [5]. Generally, this can be achieved if the waveform is symmetrical. An optimization result to obtain a symmetrical transmit waveform with a very low level of late-time ringing (lower than -33 dB in free space) is shown in Fig. 7. As can be seen, the waveform is a nearly symmetrical triplet with amplitude which is 70% higher than that of a conventional bow-tie antenna of equal size. For this result, the distance between the feed point and the fist slot has been experimentally determined to be 6 cm. The frequency characteristics of the tapered bow-tie antenna (with absorbers) can be analyzed by defining the receiving sensitivity as follows
Sr =
he Z in + Z L
where he, Zin, and ZL are the antenna effective length, the antenna input impedance and the load impedance, respectively. Fig. 8 shows the plot of the receiving sensitivity of the tapered bow-tie antenna (with absorbers) and a conventional bow-tie antenna. In contrast to the conventional bow-tie antenna, the tapered bow-tie antenna does not exhibit any high-Q resonance in the 1 - 3 GHz frequency band. CONCLUSIONS A novel technique to design an efficient ultra-wideband antenna is introduced. This technique employs a combination of a tapered capacitive loading and a resistive loading. The tapered capacitive loading is realized by an array of slots constructed on the antenna surface. For the resistive loading, absorber blocks are used by placing them on the slotted surface of the antenna. Applying this technique on a transient bow-tie antenna results in a transmit waveform with amplitude which is
70% higher than that of a conventional bow-tie antenna of the same dimension. The transmit waveform is a nearly symmetrical triplet with a level of late-time ringing of less than -33 dB. ACKNOWLEDGEMENTS This work was supported by the Dutch Foundation for Technology and Science (STW) within the framework of the project "Improved Ground Penetrating Radar Technology" and "Advanced Re-Locatable Multi-Sensor System for Buried Landmine Detection". REFERENCES [1] T.P. Montoya, G.S. Smith, "A study of pulse radiation from several broad-band loaded monopoles", IEEE Trans. Antennas Propagat., vol. AP-44, no. 8, pp. 1172-1182, Aug. 1996. [2] S.M. Rao, D.R. Wilton, A.W. Glisson, "Electromagnetic scattering by surfaces of arbitrary shape", IEEE Trans. Antennas Propagat., vol. AP-30, no. 3, pp. 409-418, May 1982. 500
Tapered bow tie Conventional bow tie
400 300 200 Ex 100 (V/m) 0 -100 -200 -300 -400
Fig. 1. FDTD model of the low-profile taper.
0
0.5
1 Time (ns)
1.5
2
Fig. 2. Radiated transient field from a bow-tie model in Fig. 1 and a conventional bow-tie antenna of equal dimension, calculated by FDTD.
Freq. = 1 GHz 0.15
0.1
Y (meter)
0.05
0
-0.05
-0.1
-0.15 0
Fig. 3. Triangular mesh of the tapered bow-tie antenna for MoM simulations (only one arm is shown).
0.05
0.1
0.15 X (meter)
0.2
0.25
Fig. 4. Distribution of the surface current density on the tapered bow-tie antenna, calculated by MoM.
0.15
0.1
Y (m)
0.05
0
-0.05
-0.1
-0.15 0
0.05
0.1
0.15
0.2
0.25
X (m)
Fig. 5. Distribution of the surface current density on a conventional bow-tie antenna, calculated by MoM.
Fig. 6. An experimental tapered bow-tie antenna.
0
200 Tapered bow tie Conventional bow tie
-5 Magnitude of Sensitivity (dB)
Measured induced voltage (mV)
150
100
50
0
-50
-100
-10
-15
-20
-25
-30
-35 1
1.5
2
2.5
3 3.5 Time (ns)
4
4.5
5
5.5
Fig. 7. Measured radiated transient field from the tapered bow-tie (with absorbers) and a conventional bow-tie antenna in broadside direction.
tapered bow tie conventional bow tie 0
0.5
1
1.5 2 2.5 Frequency (Hz)
3
3.5
4 9
x 10
Fig. 8. Receiving sensitivity of the tapered bow-tie antenna (with absorbers) and a conventional bow-tie antenna.
[3] C.J. Leat, N.V. Shuley, G.F. Stickley, "Triangular-patch model of bowtie antennas: validation against Brown and Woodward", IEE Proc. Microw. Antennas Propagat., vol. 145, no. 6, pp. 465-470, Dec. 1998. [4] A.A. Lestari, A.G. Yarovoy, L.P. Ligthart, "Capacitively-tapered bowtie antenna", Proc. (CD-ROM) Millennium Conference on Antennas & Propagat. (AP 2000), Davos, Switzerland, Apr. 2000. [5] C.E. Baum, "The SEM representation of scattering from perfectly conducting targets in simple lossy media", Detection and Identification of Visually Obscured Targets, ed. C.E Baum, Taylor and Francis, 1999.
