Design of Ultra-Wideband Antipodal Vivaldi Antenna for ... - IEEE Xplore

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Abstract—The design of an ultra-wideband antipodal Vivaldi antenna to operate at frequency range from 2.1 GHz-27 GHz for microwave imaging applications is ...
Design of Ultra-Wideband Antipodal Vivaldi Antenna for Microwave Imaging Applications Mahdi Moosazadeh and Sergey Kharkovsky Institute for Infrastructure Engineering, University of Western Sydney Penrith NSW 2751, Australia [email protected] Abstract—The design of an ultra-wideband antipodal Vivaldi antenna to operate at frequency range from 2.1 GHz-27 GHz for microwave imaging applications is proposed in this paper. It includes regular triangular slits and opening rate of the aperture that extends the lower end of frequency band limitation and improves the radiation characteristics. Good radiation characteristics in terms of improvement of front-to-back ratio and gain are achieved. A prototype of the proposed antenna is fabricated and tested. The result of measurement of magnitude of reflection coefficient is in a good agreement with the simulated result. Feasibility of this antenna for the use in microwave imaging technique to obtain high-range resolution images of construction materials is demonstrated. Index Terms—ultra-wideband, antipodal Vivaldi antenna, gain, front-to-back ratio, microwave imaging.

I. INTRODUCTION Microwave imaging technique has been extensively investigated and utilized for different applications [1]. The aims of these applications include, for example, the detection of breast cancer [2] and detection and evaluation of flaws in composite materials and concrete members [3-4]. Ultrawideband (UWB) technology with prominent characteristics such as high resolution, reasonable penetration depth inside materials, high data rate and speed more than 100 Mbs has been applied to imaging systems [5]. The antennas for the UWB systems should have wide bandwidth, light weight, high efficiency, and small size. For specific applications, directional UWB antennas are demanded to provide high-range resolution images of targeted objects inside materials. Recently, UWB printed antennas with different designs and applications have been developed [6, 7]; however, their radiation patterns are omnidirectional. Vivaldi antenna which was firstly proposed by Gibson in 1979 [8] is a good candidate for imaging applications since the Vivaldi antennas have broad bandwidth, planar structure, and directional radiation pattern. Vivaldi antennas developed to the co-planar Vivaldi antenna, antipodal Vivaldi antenna, balanced antipodal Vivaldi antenna; each of them have their own advantages and disadvantages. The co-planar Vivaldi antennas have wide bandwidth; however, their bandwidth is proportional to their radiating length; as a result, Vivaldi antenna becomes big when it is demanded for specific applications. In comparison with co-planar Vivaldi antenna, an antipodal Vivaldi antenna (AVA) has much wider impedance bandwidth

which was proposed by Gazit in 1988 [9]. Various designs are applied on geometry of the AVA to extend low end of frequency band [10-16]. To further extend low end of frequency band, AVA with exponential-shaped slot edge on top and bottom layer of the substrate [17], L-shaped slot [18], Ushaped tapered slot edge [19], tapered slot edge [20], circularshaped extension along with slot-load [21], and tapered slot load [22] have been proposed. However, these AVAs have low gain at lower frequencies. To improve antenna gain at lower frequencies, a corrugation edge technique is carried out in [23]; however, its bandwidth is 2.1 GHz-10 GHz and antenna gain is still low at lower frequencies. In order to further elevating antenna gain at the lower frequencies, a modified AVA is proposed in this paper. The modification of AVA includes regular triangular-shaped slits and opening rate of the aperture and it is referred to as RTSAVA-B in this paper. The proposed RTSAVA-B is capable of extending low end of frequency band limitation for S11 < -10 dB and has relatively high front-to-back ratio (F-to-B) at the higher frequencies, and gain at the lower frequencies. Finally, the proposed antenna is tested with an automated microwave imaging system. For this purpose a sample consisted of a 250-mm concrete block with air-filled groove and a 60-mm thick construction foam slab with a metal disk is used. The images of sample show indications of the groove in concrete and the disk in the foam slab at different depths. II. ANTENNA DESIGN Figure 1(a)-(b) indicates the geometry of the proposed CAVA and RTSAVA-B designed on Rogers RO4003C with dielectric relative permittivity of 3.38, loss tangent of 0.0027, thickness of 0.508 mm, and copper thickness of 35 µm. The total size (Lsub×Wsub) of the proposed RTSAVA-B is 96×100 mm2. The parameters of the proposed RTSAVA-B are optimized by CST Microwave Studio. The designed antennas are fed through microstrip line. In order to match the 50-Ω coaxial line, feedline width, Wf, is set to be 1.14 mm. In design tapered lines of the top and bottom layers in the proposed antenna, quarter elliptical arcs of different major and minor axis ratio is used. The major radii of the two ellipses are a1 and a2, and minor radii are b1 and b2. The ground plane section of the presented antenna is formed from the intersection of a square conductor with dimension of Wg and two anti-faced quarter circles with radius of r to obtain the transition from microstrip to parallel strip line. The simulated magnitude of the

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reflection coefficient, S11, of the proposed CAVA and RTSAVA-B is shown in Fig. 2. First, we investigated S11 of the CAVA, indicating that cutoff frequency is 4.3 GHz and a poor impedance matching is attained around 5.5 GHz, 21 GHz, and 23 GHz. In order to extend low end of frequency band limitation and to improve impedance matching, triangularshaped slits and increasing opening rate of the aperture, B, are applied. It can be seen from Fig. 2 that low end of frequency band of the RTSAVA-B is 2.1 GHz and impedance matching has been improved around 5.5 GHz, 21 GHz and 23 GHz. The optimized parameters of the proposed RTSAVA-B are as follows: Wa1 = 22.73 mm, a1 = (Wsub+Wf)/2, a2 = (Wsub-Wf)/2, b1 = 132.5 mm, b2 = 31 mm, Wf = 1.14 mm, Wg = 15 mm, X1 = 5 mm, Y1 = 5.5 mm, S = 2Y1, r = (Wg-Wf)/2, Wa2 = 53.19 mm, B = 20.

(a)

is fed through an end launch SMA which works up to 27 GHz. Figure 3(a) is photograph of the fabricated RTSAVA-B with the SMA end launch connector. The measured and simulated S11 is depicted in Fig. 3(b). As shown in Fig. 3(b), the proposed RTSAVA-B has UWB performance and cover frequency range of 2.1 GHz-more than 27 GHz. There is a good agreement between the simulated and measured results except some discrepancy at some middle and higher bands, which can be attributed to error of fabrication of the antenna and the connector. Figure 4 shows variation gain with frequency of the proposed RTSAVA-B. As reveals in Fig. 4, the gain changes from 2.6 dB to 12 dB across the frequency range between 2 GHz and 27 GHz. It is worth to point that the RTSAVA-B gain at the low frequencies is improved significantly compared with the referenced Vivaldi antennas. For instance, the gain at 4 GHz in [15] and [23] is 6 dB, while it is 7.4 dB for the proposed RTSAVA-B. The E-and H-plane radiation patterns of the CAVA and RTSAVA-B at different frequencies are shown in Fig. 5. According to the Fig. 1, the XY-plane (θ = 90 degrees) and YZ-plane (φ = 90 degrees) are the E-and H-planes, respectively. It should be noted that the maximum radiation occur in Y-direction (Fig. 1) at θ = φ = 90 degrees. As shown in Fig. 5, considerable improvement of F-to-B ratio at frequencies over 9.5 GHz for the RTSAVA-B is achieved compared to the CAVA.

(b)

Fig. 1. (a) Conventional Vivaldi antenna and (b) regular triangular-shaped slits antipodal Vivaldi antenna with opening rate of the aperture.

(a)

(b)

Fig. 3. (a) Photograph of the fabricated RTSAVA-B; (b) Simulated and measured S11 of the proposed RTSAVA-B.

Fig. 2. Simulated S11 of the proposed CAVA and RTSAVA-B.

III. RESULTS AND DISCUSSION In order to validate the simulated result of S11, prototype of the proposed RTSAVA-B is fabricated and tested. The antenna

Fig. 4. Gain of the proposed RTSAVA-B.

2.2 GHz

9.5 GHz

11.5 GHz

16 GHz

mm concrete block covered by a 60-mm thick construction foam slab. The block possesses fine and coarse (stones with diameter of up to 20 mm) aggregates and artificially created grooves on the top surface while a 28-mm metal disk was embedded in the foam slab at the distance of 36 mm from the surface of concrete block as shown in Fig. 6(a)-(b). Fig. 6(b) shows that one groove was wide (referred as to Groove 1) and another was narrow (referred as to Groove 2) with dimensions at the surface of about 70 mm by 60 mm by 20 mm and 90 mm by 25 mm by 20 mm, respectively. It should be noted that the edges of grooves were not uniform and their walls were tilted. When the grooves were covered by the foam slab, they considered as cavities in a composite structure. A customermade imaging system with a commercial Agilent performance network analyzer (PNA) and a scanning mechanism [24] was used to scan the samples and to measure the microwave reflection coefficient over a plane of the sample. Three- and two-dimensional (3D and 2D) images of the sample were generated using the measured data and a SAR algorithm [25]. The sample irradiated by the antenna at the entire frequency band (2.1 GHz–27 GHz) at the standoff distance (distance between the antenna and the top surface of the specimen) of 10 mm, and scanned area of 150 mm by 150 mm was located above the wide part of the cavity. Figs.6(c)-(d) show two horizontal slices of the 3D image of the sample when microwave signal was focused at the location of disk (Fig. 6(c)) and the surface of concrete block (Fig. 6(d)). The following observations can be made from Fig. 6. First, indication of the disk without any indication of the cavity is clearly visible in Fig. 6(c). Fig. 6(d) demonstrates indication of the cavity which is slightly distorted by indication of the disk since a metal disk is a strong target. In general, these images demonstrate a highrange resolution of the proposed antenna along with the SAR algorithm. Foam slab 36

Groove 2

Groove 1

Scanned area Projection of the disk

23 GHz (a)

(b)

(c)

(d)

27 GHz Fig. 5. H-plane (left) and E-plane (right) radiation patterns of the CAVA (dashed-line) and RTSAVA-B (solid-line) at different frequencies.

IV. MICROWAVE IMAGING WITH RTSAVA-B In this section applicability of the proposed RTSAVA-B for wideband imaging applications is demonstrated using a 250-

Fig. 6. Cross-sectional view of the sample (a) side view and (b) top view of the top face of the block under the foam slab, and images when microwave signal is focused at the location of (c) disk and (d) the surface of concrete block.

V. CONCLUSION An UWB antipodal Vivaldi antenna with triangular-shaped slits is presented and employed with microwave imaging system. By applying triangular-shaped slits and increasing opening rate of the aperture, low end of frequency band at S11 < -10 dB limitation is extended to the 2.1 GHz and impedance matching has been improved over bandwidth. The proposed antenna exhibits higher gain at the low frequencies than the other referenced Vivaldi antennas. Moreover, an improvement of F-to-B ratio is obtained. Applicability of the proposed antenna for high-range resolution imaging of construction materials for the purpose of detection of flaws within these materials is demonstrated. The proposed antenna can be used with imaging technique for non-destructive testing and evaluation of construction composite materials and structures. REFERENCES [1] M. Pastorino, Microwave imaging Hoboken, NJ, USA: Wiley, 2010. [2] T. Yin, F.H. Ali, “Adaptive combining via correlation exploration for ultrawideband breast cancer imaging,” IEEE Antennas Wireless Propag. Lett.,Vol. 14, pp. 587-590, 2015. [3] S. Kharkovsky, and R. Zoughi, “Microwave and millimeter wave nondestructive testing and evaluation - Overview and recent advances’, IEEE Instrum. Meas. Mag., vol. 10, no. 2, pp. 26-38, 2007. [4] M. Moosazadeh and S. Kharkovsky, “Development of the antipodal Vivaldi antenna for detection of cracks inside concrete members,” Microw. Opt. Technol. Lett., vol. 57, no. 7, pp. 15731578, 2015. [5] L. Xu, E. J. Bond, B. D. Van Veen, and S. C. Hagness, “An overview of ultra-wideband microwave imaging via space-time beamforming for early-stage breast-cancer detection,” IEEE Antennas Propag. Mag., vol. 47, no. 1, pp. 19–34, Feb. 2005. [6] M. Moosazadeh and Z. Esmati, “A CPW-fed antenna with the circular shaped form with the slots for UWB application,” Microw. Opt. Technol. Lett., vol. 53, no. 12, pp. 2945-2949, 2011. [7] Z. Esmati and M. Moosazadeh, “Design of compact dual bandnotched UWB antenna using slotted conductor-backed plane,” Arab. Journal Sci. Eng. (AJSE), vol. 39, no. 6, pp. 4707-4713, 2014. [8] P J. Gibson, “The Vivaldi aerial,” 9th European Microwave Conference, UK, 1979, pp. 101-105. [9] E. Gazit, “Improved design of the Vivaldi antenna,” Inst. Elect. Eng. Proc. H, vol. 135, no. 2, pp. 89-92, Apr. 1988. [10] C. Daniel, J. Lilly, and D. T. Auckland, “A compact, costeffective 4-40 GHz antenna,” IEEE Int. Symp. Antennas Propag. Soc. (APSURSI), 2011, pp. 1237-1238.

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