Printed Slot Loaded Bow-Tie Antenna With Super ... - IEEE Xplore

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 12, DECEMBER 2013

Printed Slot Loaded Bow-Tie Antenna With Super Wideband Radiation Characteristics for Imaging Applications Okan Yurduseven, David Smith, and Michael Elsdon Abstract—A super wideband printed modified bow-tie antenna loaded with rounded-T shaped slots fed through a microstrip balun is proposed for microwave and millimeter-wave band imaging applications. The modified slot-loaded bow-tie pattern increases the electrical length of the bow-tie antenna reducing the lower band to 3.1 GHz. In addition, over the investigated frequency band up to 40 GHz, the proposed modified bow-tie pattern considerably flattens the input impedance response of the bow-tie resulting in a smooth impedance matching performance enhancing the reflection coefficharacteristics. The introduction of the modified ground plane cient printed underneath the bow-tie, on the other hand, yields to directional far-field radiation patterns with considerably enhanced gain performance. and E-plane/H-plane far-field radiation pattern measurements The have been carried out and it is demonstrated that the fabricated bow-tie antenna operates across a measured frequency band of 3.1–40 GHz with an average broadband gain of 7.1 dBi. Index Terms—Bow-tie antenna, imaging, microstrip balun, slot loading, super wideband.

I. INTRODUCTION Antennas offering ultra-wideband (UWB) radiation characteristics have been the subject of much research in recent years. Various antennas covering UWB have been studied in the literature including travelling-wave antennas, such as Vivaldi antennas; frequency-independent antennas,suchasbiconicalantennas;self-complementaryantennas,such as logarithmic spiral antennas; and multiple resonance antennas, such as fractal antennas. Among these, printed planar antennas have received much attention due to their planar structure, making them easily mountable on planar surfaces, and simple low-cost manufacturing. Bow-tie antennas, which are the planar version of biconical antennas, have the advantage of being considerably compact in size and offering good time domain and broad-band frequency domain radiation characteristics [1], [2]. UWB bow-tie geometries studied in the literature can be given as slot [3], [4], double-sided [5], self-complementary [6] and self-grounded [7]. Bow-tie antennas are widely used in ground penetrating radar (GPR) applications [8], [9]. Recently, UWB GPR systems have found many applications in the detection of subsurface objects and high resolution non-destructivescanning/imaging of various structures,such as concrete -band imaging blocks, roads and pavements [10]. Beyond UWB, of moving targets [11], K-band imaging for airborne radar systems in terrain reconnaissance applications [12] and millimeter-wave band (30–300 GHz) imaging for security and surveillance [13] have been the subject of much research. Traditionally, for microwave and millimeter wave band imaging systems, separate antennas are used increasing the complexity and the cost of the imaging system. In view of this, the design of a super wideband low-profile bow-tie antenna with desirable linear phase response, directional radiation characteristics across a wide frequency band and good time domain impulse response would be a significant contribution -band and millimeter-wave band imaging to low-cost UWB, applications. However, very little work exists on the design of bow-tie Manuscript received March 21, 2013; revised May 20, 2013; accepted August 31, 2013. Date of publication September 10, 2013; date of current version November 25, 2013. The authors are with the Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne, NE1 8ST, U.K. (e-mail: okan. [email protected]). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2013.2281353

Fig. 1. Fabricated modified bow-tie antenna (a) x-y plane and (b) x-z plane.

antennas to operate above -band. In this communication, a low-profile printed modified bow-tie antenna fed through a microstrip balun is proposedforwidebandimagingsystemsoperatingacrossameasuredfrequency band of 3.1–40 GHz, simultaneously covering UWB, imaging bands in full and millimeter-wave imaging band partly. II. DESIGN AND FABRICATION OF THE BOW-TIE ANTENNA The fabricated modified bow-tie antenna is shown in Fig. 1. The bow-tie has been printed on a low-loss RT/duroid 5870 substrate, and . The flare angle of the bow-tie arms is 26.2 while each arm has a total length of 16.9 mm as can be seen in Fig. 1(a). Underneath the substrate, a modified ground plane has been introduced to improve the overall gain performance of the bow-tie antenna [14]. The rectangular hollow-shaped pattern of the ground plane illustrated in Fig. 1(a) has been optimized in CST Microwave Studio to ensure that it has minimal effect on the impedance bandwidth of the bow-tie antenna while providing optimum gain performance both of which have been achieved with a printed copper line width of 10 mm. Traditionally, the surface current of a bow-tie antenna is stronger at the edges and flows through the path determined by the edges [15]. In order to widen the frequency band of a bow-tie antenna at the lower band, the electrical length of the current path must be increased. Moreover, the input impedance response over a desired frequency band should remain as flat as possible to perform a smooth wideband impedance matching. In order to achieve this, a novel rounded T-shaped slot loaded modified bow-tie pattern has been designed and is demonstrated in this communication. The proposed modified bow-tie pattern consists of a rounded-T shaped slot with a thin PCB copper line inside as can be seen in Fig. 1(a). The depth of the rounded T-shaped slot is 9 mm with a width of 2 mm. Inside the slot, at a distance of 5.7 mm away from where the slot begins, 1 mm widePCBcopperlinelies,whichisincontactwiththeroundededgeofthe bow-tie arm. This line is responsible for forcing the current to flow within the slot with the help of the rounded 2 mm wide top of the T-shaped slot. This results in a considerably increased electrical length of the current path as demonstrated in this communication. The increased electrical length of the bow-tie arm reduces the lower operation band to 3.1 GHz enabling the antenna to cover UWB. In addition, the proposed slot-loaded bow-tie pattern plays a significant role in further flattening the input impedance response of traditional rounded bow-tie antennas. In Fig. 2, comparison is made between the simulated input impedance curves of the proposed slot-loaded modified rounded bow-tie arm and a traditional solid version as a function

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Fig. 2. Simulated input impedance curves of a triangular monopole, a traditional solid rounded bow-tie and the proposed slot-loaded rounded bow-tie arms.

of antenna length in degrees. Fig. 2 also includes the simulated input impedance curves of a triangular monopole arm for comparison. In Fig. 2, represents the arm length and is the flare angle of bow arm, which are 16.9 mm and 26.2 , respectively, corresponding to an antenna length of 405.6 at the center frequency of 20 GHz. It can be seen in Fig. 2 that rounding the bow-tie edges reduces the fluctuation in the input impedance response making it possible to perform a better impedance matching over a wider frequency band. However, an even further flattening in the input impedance response has been achieved as a result of the proposed rounded-T shaped slot loading, which significantly enhances the impedance matching performance, enabling the proposed modified bow-tie antenna to have stabilized impedance response over the investigated frequency band up to 40 GHz. From the resistance curve obtained for the proposed modified bow-tiearm in Fig. 2 together with the reactance response, which has significantly been flattened and slightly fluctuates around zero enabling a wideband impedance matching to be performed, an average broadband impedance of 150 has been deter, across the investigated mined as bow-tie input impedance, frequency band, 3.1–40 GHz. In order to optimize the radiation performance of a bow-tie antenna, it should be fed using a balanced feeding structure. A balun equals the currents atthebalancedportsofabow-tieantennainamplitudeandopposites in phase [15]. In addition to stabilizing the radiation characteristics, the fabricated microstrip balun illustrated in Fig. 3 has been designed as an impedance matching circuit, matching the impedance at the input SMA port of the balun, which is 50 , to the input impedance of the bow-tie arms, which has been obtained as an average 150 from Fig. 2. A major factor considered in the design process is the requirement of the balun to have a wide impedance bandwidth performance in order to enable the proposed bow-tie antenna to have wideband radiation characteristics. and The balun has been printed on a CER-10 substrate, ,andattachedtothebow-tieatthebottom atrightangleas showninFigs.1(b)and3(d).Itconsistsoftwotypesofplanartransmission lines. The first type is the unshielded U-shaped microstrip transmission line printed on the rear surface of the substrate as can be seen in Fig. 3(b). This transmission line is a combination of two sub-transmission lines, and . The line and line with characteristic impedances of second type is the balanced co-planar stripline with a characteristic , which consists of two short-circuited ground planes impedance of representing the outer conductors of lines and in coaxial equivalent circuit given in Fig. 3(c), printed on the front surface of the substrate as illustrated in Fig. 3(a). The balun matches the impedance at the terminal in Fig. 3(c), which is the input impedance of short-cirX-Y, given as that together in series with the input cuit line AB in parallel with impedance of line . The balun designed for this work is based on a refinement of the balun originally described by Bawer and Wolfe [16]. In [16], transmission line with the same characteristic impedance as transmission line is used to match to the balanced section. In this work, the impedance of transmission line has been optimized to provide a response considerable enhancement in the reflection coefficient of the balun especially at the lower band as shown in Fig. 4 whereas the mm resulting width of transmission line has been fixed at

Fig. 3. Balun (a) balanced co-planar stripline, top view (b) unshielded microstrip transmission line, back view (c) coaxial equivalent circuit [16] (d) balun attached to the bow-tie, (e) balun measurement set-up. Dimensions (in mm): .

in a characteristic impedance of , which is equal to the input impedance of the input SMA port of the balun. From Fig. 4, the optimum performance has been obtained when , achieved with mm. When , a transmission line width of as demonstrated in [16], deterioration in rerequiring sponse of the balun below 15 GHz can clearly be seen in Fig. 4 preventing higher than 75 , on the the balun from covering UWB. Increasing level compared to the proposed other hand, improves the average butattheexpenseofthebandwidth.Thisisevident balunwith in Fig. 4, where setting has shifted the low-band from 2.92 GHz to 4.6 GHz, which is not desirable for UWB imaging applications. III. MEASUREMENT RESULTS AND DISCUSSION Fig. 5 demonstrates the and transmission coefficient ( and ) patterns of the fabricated balun terminated in a load impedance of 150 . It should be noted here that as the differential output port of the balun has two single-ended balanced terminals, together with the

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Fig. 7. Simulated and measured Fig. 4. Simulated

response of the balun versus varying

Fig. 5. Simulated and measured

and

with

patterns of the bow-tie antenna.

.

patterns of the balun.

Fig. 8. Comparison between the measured phase responses of the proposed modified bow-tie antenna and the traditional solid version.

Fig. 6. Measured phase response of the fabricated balun.

input port, a 3-port S-parameter analysis, and , has been performed. As illustrated in Fig. 5, there is good agreement between the simulated and measured S-parameter responses and the fabricated dB frequency band of 2.92–40 GHz. The balun operates across a and patterns in Fig. 5 demonstrate simulated and measured a good forward transmission performance with an equal power split between the balanced output ports of the fabricated balun. The measured phase response of the balanced terminals of the balun is shown in Fig. 6. As illustrated in Fig. 6, the phase difference between the balanced outputs is almost constant at 180 over the entire frequency range. patterns of the fabricated bow-tie The simulated and measured antenna fed through the proposed balun are shown in Fig. 7. As demonstrated, the proposed modified bow-tie antenna operates across a meadB impedance sured frequency band of 3.1–40 GHz, offering a bandwidth of 36.9 GHz. This covers the full spectrum of UWB and , and part of the millimeter wave spectrum. Fig. 8 provides a comparison between the measured phase responses of the proposed rounded T-shaped slot loaded bow-tie antenna and the traditional version, which is a rounded-edge solid bow-tie antenna, both of which have been fed through the same proposed microstrip balun. In Fig. 8, the traditional solid bow-tie has considerable distortions in the phase response at the frequency band of 1–3.85 GHz and above 20

Fig. 9. Simulated current distributions (a) 3.1 GHz, (b) 20 GHz, (c) 40 GHz.

GHz whereas the phase response of the proposed modified bow-tie is almost linear across the entire investigated frequency band. The surface current distributions across the bow-tie arms obtained at 3.1, 20 and 40 GHz with the antenna attached to the proposed balun across terminals as shown in Fig. 3(a) are illustrated in Fig. 9. The surface current distribution across the left and right bow-tie arms is balanced and symmetric over the investigated frequency band of 3.1–40 GHz validatingthe accuracyof thebalun.As expected,thecurrent flowing across the edges of the bow-tie is stronger in amplitude and the proposed T-shaped slot-loaded bow-tie pattern forces the current to flow through the shaped modified geometry, increasing the electrical length of the proposed bow-tie antenna, especially at the lower band. In the determination of the time-domain characteristics of the bow-tie performed in CST, the width of the amplitude of the analytic at half-maximum, , has impulse response envelope been investigated [1]. Mathematically, the analytic impulse response


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TABLE I MEASURED MAXIMUM GAIN VALUES WITH AND WITHOUT (NO GROUND PLANE) THE MODIFIED GROUND PLANE

Fig. 10. Simulated impulse response envelope and transmit impulse.

cross-polarization/co-polarization ratio. Table I shows a comparison between the measured maximum gain values obtained with and without the proposed modified ground plane. As can be seen in Fig. 11, the measured E-plane and H-plane far-field radiation patterns demonstrate stabilized directional radiation characteristics across the entire operational frequency band. The difference between the cross-polarization and co-polarization patterns is higher than 10 dB in both E-plane and H-plane at the frequency band of 10–40 GHz. The ratio stays above 5 dB at the lowest frequency of 3.1 GHz, which can be considered acceptable considering the proposed operational impedance bandwidth which is larger than a decade. The contribution of the rectangular hollow-shaped ground plane to the overall gain performance of the proposed modified bow-tie antenna can clearly be seen in Table I. In addition to resulting in directional far-field radiation patterns required for imaging applications as demonstrated in Fig. 11, the introduction of the ground plane has increased the average measured maximum gain from 4.14 dBi to 7.1 dBi. This is an enhancement in the gain performance of the proposed bow-tie antenna by a factor of 71.5%. IV. CONCLUSION

Fig. 11. Measured E-plane and H-plane far-field radiation and cross-polarization/co-polarization ratio patterns taken at (a) 3.1 GHz, (b) 10 GHz, (c) 20 GHz, (d) 30 GHz, (e) 40 GHz.

of the bow-tie, MATLAB as:

, has been obtained through Hilbert transform in

(1) In (1), h(t) is the simulated impulse response data obtained in CST. and the transmit impulse of the antenna The obtained envelope are shown in Fig. 10. of the impulse radiated by the bow-tie has been calculated The ofthesynthesizedexcitinginputimas47ps.Consideringthatthe pulseinCSTwas38ps,althoughaslightbroadeninginthewidthoftheimobtained from the bow-tie combined with pulse occurs, the 47 ps the observed transmit impulse of the bow-tie with minimal distortion ensures a good impulse response performance. Fig. 11 demonstrates the measured E-plane and H-plane far-field radiation patterns of the proposed bow-tie antenna together with the

A microstrip balun fed super wideband printed modified bow-tie antenna, consisting of rounded T-shaped slot loaded bow arms printed on a low-loss RT/duroid 5870 substrate with a modified ground plane underneath, has been proposed. E-plane/H-plane far-field radiation pattern and S-parameter results have been demonstrated and it has been verified that the proposed modified bow-tie antenna brings a considerable improvement in the impedance bandwidth response of traditional printed bow-tie antennas. It results in a significant reduction in antenna resistance variation and an antenna reactance close to zero, thus significantly extending the impedance match. The measured directional far-field radiation patterns and linear phase response across the frequency band of 3.1–40 GHz combined with the obtained good time domain impulse response performance confirm the suitability of the and milproposed bow-tie antenna for UWB GPR, and limeter-wave band imaging applications.

REFERENCES [1] W. Wiesbeck, G. Adamiuk, and C. Sturm, “Basic properties and design principles of UWB antennas,” Proc. IEEE, vol. 97, pp. 372–385, Feb. 2009. [2] R. Compton, R. McPhedran, Z. Popovic, G. Rebeiz, P. Tong, and D. Rutledge, “Bow-tie antennas on a dielectric half-space: Theory and experiment,” IEEE Trans. Antennas Propag., vol. 35, pp. 622–631, Jun. 1987. [3] A. Mehdipour, K. M. Aghdam, R. F. Dana, and A. R. Sebak, “Modified slot bow-tie antenna for UWB applications,” Microw. Optical Technol. Lett., vol. 50, pp. 429–432, Dec. 2007. [4] E. S. Angelopoulos, A. Z. Anastopoulos, and D. I. Kaklamani, “Ultra wide-band bow-tie slot antenna fed by a CPW-to-CPW transition loaded with inductively coupled slots,” Microw. Optical Technol. Lett., vol. 48, pp. 1816–1820, Jun. 2006. [5] T. Karacolak and E. Topsakal, “A double-sided rounded bow-tie antenna (DSRBA) for UWB communication,” IEEE Antennas Wireless Propag. Lett., vol. 5, pp. 446–449, Dec. 2006.

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[6] L. Chun-Chen, “Compact bow-tie quasi-self-complementary antenna for UWB applications,” IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 987–989, 2012. [7] J. Yang and A. Kishk, “A novel low-profile compact directional ultrawideband antenna: The self-grounded bow-tie antenna,” IEEE Trans. Antennas Propag., vol. 60, pp. 1214–1220, Mar. 2012. [8] G. E. Atteia, A. A. Shaalan, and K. F. A. Hussein, “Wideband partially-covered bowtie antenna for ground-penetrating-radars,” Progr. Electromagn. Res., vol. 71, pp. 211–226, 2007. [9] A. A. Lestari, E. Bharata, A. B. Suksmono, A. Kurniawan, A. G. Yarovoy, and L. P. Ligthart, “A modified bow-tie antenna for improved pulse radiation,” IEEE Trans. Antennas Propag., vol. 58, pp. 2184–2192, Jul. 2010. [10] L. Li, A. E.-C. Tan, K. Jhamb, and K. Rambabu, “Buried object characterization using ultra-wideband ground penetrating radar,” IEEE Trans. Microw. Theory Tech., vol. 60, pp. 2654–2664, Aug. 2012. [11] Y. Zhang, W. Zhai, X. Zhang, X. Shi, X. Gu, and J. Jiang, “Moving target imaging by both Ka-band and Ku-band high-resolution radars,” in Proc. SPIE, 2011, vol. 8179, Article ID 81790R, 7 pages. [12] W. P. Waite and H. C. Macdonald, “Vegetation penetration” with K-band imaging radars,” IEEE Trans. Geosci. Electron., vol. 9, pp. 147–155, Jul. 1971. [13] R. Appleby and R. N. Anderton, “Millimeter-wave and submillimeterwave imaging for security and surveillance,” Proc. IEEE, vol. 95, pp. 1683–1690, Aug. 2007. [14] S. Noghanian and L. Shafai, “Control of microstrip antenna radiation characteristics by ground plane size and shape,” in Proc. Inst. Elect. Eng. Microwaves, Antennas and Propag., Jun. 1998, vol. 145, pp. 207–212. [15] J. Volakis, Antenna Engineering Handbook, 4th ed. New York, NY, USA: McGraw-Hill, 2009. [16] R. Bawer and J. J. Wolfe, “A printed circuit balun for use with spiral antennas,” IRE Trans. Microw. Theory Tech., vol. 8, pp. 319–325, 1960.

A Four-Beam Pattern Reconfigurable Yagi-Uda Antenna C. Kittiyanpunya and M. Krairiksh Abstract—This communication presents a pattern reconfigurable antenna which is based on the Yagi-Uda configuration that can change the radiation beam into four directions. The function of the parasitic elements can be altered between a director and a reflector with PIN diodes while the driven element can be altered between two perpendicular directions with RF switches. To be able to use this proposed antenna on an electric conductor, such as on the vehicle rooftop, the antenna is thus mounted on electromagnetic band gap (EBG) structure which serves as a magnetic conductor. The gain is approximately 5 dBi and the field trial exhibited the diversity gain of 17 dB over the fixed beam antenna. As a result, the proposed antenna possesses high potential for further development into a TV signal reception on moving vehicles. Index Terms—EBG, magnetic conductor, mobile TV antenna, pattern reconfiguration, PIN diode, reconfigurable antenna, RF switch, Yagi-Uda antenna.

On the reconfigurable Yagi-Uda antenna, attempts have been made to develop the antennas which are reconfigurable in frequency [3] and/or pattern. The antennas in [4]–[6] were designed according to the principle of Yagi - Uda antenna by switching the state of PIN diodes inserted on parasitic elements between “ON” and “OFF” to force them act as reflectors or directors. Even though the beam is reconfigured and becomes multi-directional, it still fails to cover the horizontal plane. Gray et al. experimented with an electronically steerable Yagi-Uda microstrip patch antenna array based on a four element Yagi-Uda patch antenna [7]. The four antennas forming the array are located radially from a single reflector patch. By switching the four elements, the main beam of the array is steerable in the azimuth plane and support circular polarization radiation. In [8] circular polarization was formed by two identical reconfigurable linear Yagi - Uda patch arrays lying orthogonally to each other around a single driven patch element. Switching among the four modes changes the radiation pattern and thereby makes coverage of the horizontal plane possible. Nevertheless, as TV reception employs horizontal polarization, it is thus difficult to mount a low-profile antenna on a metallic surface, such as the rooftop of a vehicle, due to the boundary condition in which reflection-phase cancels out the electric field on the surface. Meanwhile, the EBG structure [9]–[12] acts as a magnetic conductor that addresses the problem of mounting a horizontal polarization antenna on an electric conductor. Recently, studies on the reconfigurable antennas over the EBG ground plane have been undertaken [13], [14], none of which nevertheless deals with antennas that produce the radiation patterns in directions surrounding the azimuth plane. This work presents a pattern reconfigurable antenna which can alternate the main beam into four-direction radiation. The structure is based on the Yagi-Uda antenna, whose driven element and parasitic elements are switched to change the main beam direction. This proposed antenna has been installed on EBG structure that serves as a magnetic conductor to obtain a pattern reconfigurable antenna for horizontal polarization. II. PRINCIPLE AND DESIGN The objective of this work is to investigate the pattern reconfigurable antenna with horizontal polarization mountable on the metallic roof-top of a vehicle. The radiation pattern must be reconfigurable to receive the high field strength of UHF signal from the TV station as the vehicle moves. The authors have proposed the Yagi-Uda antenna with a driven element that can be altered between two perpendicular directions using SPDT RF switches (AS186-302LF), while the function of parasitic elements can be altered between a director and a reflector with PIN diodes (BAP51-02). The proposed antenna is mounted on a mushroom-like EBG structure. A. Four-Beam Pattern Reconfigurable Yagi-Uda Antenna in Free Space

I. INTRODUCTION The Yagi-Uda antenna [1] is an end-fire array using a single feed and it has been in use for TV reception worldwide. Due to its fixed unidirectional radiation pattern, the Yagi-Uda antenna is unsuited to mounting on vehicles. Thus, the solution lies in a reconfigurable antenna [2] that is able to cover the full azimuth plane. Manuscript received December 27, 2012; revised September 09, 2013; accepted September 15, 2013. Date of publication September 20, 2013; date of current version November 25, 2013. The authors are with Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2013.2282914

In this research work, the operating frequency is 562 MHz and the proposed antenna consists of brass tubes, foam substrate, SPDT RF switches and PIN diodes. The geometry of the proposed four-beam pattern reconfigurable Yagi-Uda antenna with integrated SPDT RF switches and PIN diodes is shown in Fig. 1(a). The schematic of the circuit board of RF switches is shown in Fig. 1(b), where RF switch legs H1, H2, H3, and H4 are respectively soldered to monopoles D1, D2, D3, and D4. Monopoles D1 and D3 and monopoles D2 and D4 form two perpendicular dipoles. The output signal from the RF switches is connected to the coaxial cable where inner and outer conductors are at I and O, respectively. The biasing circuit for a PIN diode and its photograph are shown in Fig. 1(c). The antenna design is based on the Yagi-Uda antenna. The initial design was as shown in [15] in

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