Planar Wideband Loop-Dipole Composite Antenna - IEEE Xplore

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014

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Planar Wideband Loop-Dipole Composite Antenna Wen-Jun Lu, Wen-Hai Zhang, Kin Fai Tong, and Hong-Bo Zhu

Abstract—A novel planar wideband balanced composite antenna is presented. The proposed antenna consists of a symmetrical dipole and an antipodal loop radiator. The return loss, radiation pattern, and gain of the antenna are numerically and experimentally studied. An impedance bandfrom 3.26 to 15 GHz has been width of 129% obtained. Stable end-fire radiation patterns with good polarization purity and flat gain performances are obtained across the operating frequency range. In addition, the time domain responses of the proposed antenna are investigated. The proposed antenna can be used in compact wireless devices for point-to-point applications. Fig. 13. Directivity, gain, and efficiency of T-SIWLSA prototype at the main beam tilt in frequency band.

Index Terms—End-fire radiation pattern, loop-dipole composite antenna, planar antenna, wideband antenna.

I. INTRODUCTION better than at least from 16 to 18 GHz, although the usable bandwidth was reduced to 3% due to main beam tilt frequency dispersion. A 1.86-dB axial ratio, 17-dBi gain, and 80% radiation efficiency at 17 GHz in broadside direction were experimentally obtained. As future research, a new mutual coupling compensation model based on the E-field evaluation in the array aperture will be developed to take into account the radiated mutual effects between adjacent slots. As a consequence, the radiation pattern response is expected to be improved.

REFERENCES [1] L. Yan, W. Hong, G. Hua, J. Chen, K. Wu, and T. J. Cui, “Simulation and experiment on SIW slot array antennas,” IEEE Microwave Wireless Compon. Lett., vol. 14, no. 9, pp. 446–448, Sep. 2004. [2] G. Montisci, “Design of circularly polarized waveguide slot linear arrays,” IEEE Trans. Antennas Propag., vol. 54, no. 10, pp. 3025–3029, Oct. 2006. [3] P. Chen, W. Hong, Z. Kuai, and J. Xu, “A substrate integrated waveguide circularly polarized slot radiator and its linear array,” IEEE Antennas Wireless Propag. Lett., vol. 8, pp. 120–123, 2009. [4] Z. Chen, W. Hong, Z. Kuai, J. Chen, and K. Wu, “Circularly polarized slot array antenna based on substrate integrated waveguide,” in Proc. Int. Conf. on Microw. and Millimeter Wave Tech. ICMMT 2008, 2008, vol. 3, pp. 1066–1069. [5] D. Deslandes and K. Wu, “Integrated microstrip and rectangular waveguide in planar form,” IEEE Microwave Wireless Compon., vol. 11, no. 2, pp. 68–70, Feb. 2001. [6] G. Montisci, M. Musa, and G. Mazzarella, “Waveguide slot antennas for circularly polarized radiated field,” IEEE Trans. Antennas Propag., vol. 52, no. 2, pp. 619–623, Feb. 2004. [7] J. Hirokawa and M. Ando, “Single-layer feed waveguide consisting of posts for plane TEM excitation in parallel plates,” IEEE Trans. Antennas Propag., vol. 46, no. 5, pp. 625–630, May 1998. [8] K. Sakakibara, Y. Kimura, A. Akiyama, J. Hirokawa, M. Ando, and N. Goto, “Alternating phase-fed waveguide slot arrays with a single-layer multiple-way power divider,” Proc. Inst. Elect. Eng. Microw. Antennas Propag. Digest, vol. 144, no. 6, pp. 425–430, 1997. [9] D. Deslandes and K. Wu, “Accurate modeling, wave mechanisms, and design considerations of a substrate integrated waveguide,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 6, pp. 2516–2526, Jun. 2006. [10] J. L. Masa-Campos, S. Klinger, and M. Sierra-Pérez, “Parallel plate patch antenna with internal horizontal coupling lines and mode excitation,” IEEE Trans. Antennas Propag., vol. 57, no. 7, pp. 2185–2189, Jul. 2009. [11] J.-C. Cheng, E. S. Li, W.-F. Chou, and K.-L. Huang, “Improving the high-frequency performance of coaxial-to-Microstrip transitions,” IEEE Trans. Microw. Theory Tech., vol. 59, no. 6, pp. 1468–1477, Jun. 2011.

It is well known that a magnetic dipole has an ideal omnidirectional pattern in its E-plane and 8-shaped pattern in H-plane. While for an electric dipole, the situation is on the contrary. When they are excited together with the same conditions, a cardioid-shape radiation patterns in both principal planes can be achieved. This concept was firstly proposed to achieve an antenna feed [1]. Then, several studies had been focused on the development of magneto-electric composite antenna in the 1970’s due to its impressive features. A series of analytical models for slot antenna with arbitrary coupled dipoles were generalized in [2]–[5]. The growth of modern wireless communication industry has led to an increasing demand for wideband unidirectional antennas with good performance, such as low cross polarization, low back radiation, and stable gain. Therefore, with reference to the concept of complementary antenna, various wideband electric-magnetic dipoles composite antennas were proposed to fulfill such requirements. These proposed works are based on the combination of different shorted-patches and electric dipoles [6]–[9]. Most of these antennas have large metallic reflectors and they are suitable for base station application. In the case of hand-held devices and mobile terminals without large metallic reflectors, in order to maintain a high link gain in point-to-point applications (i.e., short-range, high-data rate communications [10]) and reduce interference with other on-board circuitry, planar antennas having end-fire radiation are desirable. As compact antennas with broadside omnidirectional radiation characteristics are Manuscript received December 04, 2013; revised December 26, 2013; accepted January 07, 2014. Date of publication January 13, 2014; date of current version April 03, 2014. This work was supported in part by the National Natural Science Foundation of China under grants 61001079 and 61271236, the Program for New Century Excellent Talents in University of Ministry of Education of China under Grant NCET-12-0739, Jiangsu Natural and Science Foundation of Universities under Grant 13KJA510002 and the Research Project of State Key Laboratory of Millimeter Waves under Grant K201413. W.-J. Lu is with the Jiangsu Key Laboratory of Wireless Communications, Nanjing University of Posts and Telecommunications, Nanjing 210003, China and also with the State Key Laboratory of Millimeter Waves, Nanjing, Jiangsu, China (e-mail: [email protected]). W.-H. Zhang and H.-B. Zhu are with the Jiangsu Key Laboratory of Wireless Communications, Nanjing University of Posts and Telecommunications, Nanjing 210003, China (e-mail: [email protected]). K. F. Tong is with the Dept. EEE, University College London, U.K. (e-mail: [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.2014.2299820

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TABLE I DIMENSIONS OF THE PROPOSED ANTENNA AND THE BALUN

used to reduce the driven dipoles’ size [14], [15]. However, the compact size leads to narrow bandwidth limited to about 10%. Therefore, it is a challenging task to develop planar, broadband antennas having end-fire characteristic for compact devices and mobile terminals. In this communication, a novel planar, wideband loop-dipole composite antenna is proposed. It is balanced and composed of a symmetrical dipole and a loop radiator. The broadband characteristics with relatively low cross-polarization and quasi-directional patterns over bandwidth up to about 5:1 can be obtained. Finally, impulse response results also show that the antenna has good time-domain performance. II. ANTENNA DESCRIPTION AND OPERATION PRINCIPLE

Fig. 1. Geometry of the proposed antenna: (a) the proposed antenna, (b) the balun and the whole view; (c) photograph of fabricated prototypes.

not suitable for such applications, end-fire radiation antennas such as Vivaldi antennas [11], [12] that feature high gains, wide impedance bandwidth and directional radiation patterns may be a suitable candidate. However, their large physical size is a critical drawback. Compared with Vivaldi antennas, printed Yagi antenna and quasi-Yagi antennas with end-fire characteristic [13]–[15] have smaller size and more suitable for compact devices. A novel, wideband Yagi antenna is proposed, while its electric size approximates to one wavelength [13]. In order to reduce the whole antenna size, meandered structure can be

The geometry of the proposed antenna is shown in Fig. 1(a). It is designed on a Roger’s RT/duroid 5880 substrate with relative permit, and thickness . The tivity of antenna is composed of a symmetrical dipole and a loop radiator. In order to measure the antenna effectively and rapidly, a balun is needed to accomplish the connection between the antenna and the instruments. A simple balun based on a microstrip line (MSL) to broadside coupled stripline (BCS) transition shown in Fig. 1(b) is used. Then, a SMA launcher is connected to the microstrip line side of the balun so that the antenna can be connected with other instruments, as shown in Fig. 1(c). If the antenna is used in compact devices, i.e., handsets, laptops and other mobile terminals, it can be connected to the circuits via a small size, lumped, and surface-mount balun [16], [17]. The loop is divided into two equal half and each half is printed on either side of the microwave substrate. The two sides are connected by three via-holes of diameter and mutual separation . Parameters of the antenna and the transition balun are listed in Tables I. The whole surface area of the proposed antenna is 27 mm 27 mm. The qualitatively predicted radiation pattern of the antenna is shown in Fig. 2. In the analysis, positive y-direction is considered as the normal direction vector, thus the current flowing along the loop can distributed along be replaced by an equivalent magnetic dipole negative z-direction [18]. The radiation pattern should be co-dominated by the electrical dipole and the equivalent magnetic dipole simultaneously. In this case, cardioid-shaped patterns in both principal planes may be achieved, as shown in Fig. 2. In order to verify the predicted results, simulated surface current distributions at two resonant frequencies, 6.39 GHz and 11.91 GHz, respectively, are investigated and shown in Fig. 3. It is observed that the current is mainly concentrated on the printed dipole and the loop radiator at two frequencies, which indicates that the radiation patterns


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Fig. 2. The predicted radiation pattern.

Fig. 3. (a) Surface current distribution at two frequencies: (a) 6.39 GHz, (b) 11.91 GHz.

will be co-dominated by the electrical dipole and the magnetic dipole at these frequency bands. III. FREQUENCY DOMAIN PERFORMANCE The antenna is simulated by using Zeland’s IE3D and prototypes have been fabricated and measured to verify the numerical results. The of the antenna is measured by using Agilent’s 8720ET vector network analyzer (VNA) and its far-field radiation is measured by using is a NSI’s NSI-800F-10x system. The measured and simulated shown in Fig. 4. As shown in Fig. 4(a), it can be observed that the antenna’s inherent impedance bandwidth is not affected by the balun. The is about measured impedance bandwidth 129% from 3.26 to 15 GHz, which is independent to existence of the MSL-BCS balun. In order to further verify the antenna’s inherent wideband characteristic, two different prototypes (i.e., prototype I and prototype are co-designed with II) having average input impedance of 100 different lumped baluns and fabricated. Due to the limited bandwidth of the commercial available lumped baluns [16], [17], prototypes I and II are designed to operate from 3–8 GHz and from 1.76–9 GHz, of a single antenna respectively. For comparison, the simulated

Fig. 4. Measured and simulated reflection coefficient, (a) original prototype, (b) prototype antenna I fed by NCS2-83+ balun at 3–8 GHz and (c) prototype antenna II fed by BAL-0009SMG balun at 1.8–9 GHz.

(normalized to 100 ) and the co-simulated of the antenna with a balun are plotted in the same figure. As shown in Fig. 4(b) and (c), it is observed that the simulated results are in well accordance to the measured ones. It is verified that the antenna’s impedance bandwidth is not affected by the lumped balun. Hence, the proposed antenna can be determined to have inherent wideband characteristic and its impedance bandwidth is independent to the smooth transition profile of the MSL-BCS balun and the lumped balun as well. The measured gain of the proposed antenna shown in Fig. 5 is slightly better than the simulated one in the low-frequency band. It

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Fig. 5. Measured and simulated peak gain.

has been observed that the average measured peak gain is 4.56 dBi. However, at frequency band between 10 and 12 GHz, the measured peak gain is lower than the simulated one. One of the main reasons is the pattern degradation caused by the high-order modes radiation [9], another one is the balun’s spurious radiation [19]. The simulated and measured radiation patterns in E- and H-planes at 3.6, 7.4 and 13.4 GHz are depicted in Fig. 6. It is observed that the simulated and measured results agree reasonably well with each other. As observed from Fig. 6(a) and (b), it is seen that when the antenna is operating at the low frequency band, its radiation pattern is mainly dominated by the loop radiator. In this case, the loop behaves more like a slot, i.e., the slot mode, and z-direction will be the maximum radiation direction. With the operating frequency increasing, the radiation pattern is co-dominated by the simultaneously excited electric dipole and magnetic dipole, i.e. the loop-dipole mode. Thus, the cardioid-shaped radiation pattern can be obtained in both principal planes, as shown in Fig. 6(c), (d), which agree with the qualitatively predicted results in Fig. 2 and Fig. 3. At higher frequencies, i.e. 13.4 GHz, ripples and side lobes appear due to the excitation of high-order slot modes and loop-dipole modes, as shown in Fig. 6(e), (f). However, the main beam is still pointing to the positive y-direction. The relatively high cross-polarization level (i.e., up to 9 dB within the main beam) is caused by the spurious radiation of the balun [19]. The measured cross-polarization level is higher than the simulated one, and the measured cross-polarization levels in the main beam are generally lower than 10 dB at 3.6 GHz and 7.4 GHz. Two factors may lead to high cross-polarization levels at high frequency band. The first one is the balun’s spurious radiation [19] which may contribute the E-field components along the y-direction. Another one is skewed E-field components along the z-direction distribution on the cross section of the substrate [12]. Although, the cross-polarization level slightly degrades in the high band, a stable main beam pointing to the positive y-direction can be obtained within the whole frequency range. IV. TIME DOMAIN PERFORMANCE As observed from the performance in frequency-domain, it is seen that the proposed antenna has ultra-wideband characteristics. If the antenna is applied in pulsed radio systems, its time-domain response is also an important feature [19], [20]. In this section, the transfer function (magnitude and group delay) was firstly measured within 3–15 GHz by using 8720ET VNA as shown in Fig. 7. A pair of the proposed antennas is used as the transmitting and axis with each receiving antennas. The antennas are aligning along

Fig. 6. Simulated and measured radiation patterns of the proposed antenna: (a) yz-plane, 3.6 GHz, (b) xy-plane, 3.6 GHz, (c) yz-plane, 7.4 GHz, (d) xy-plane, 7.4 GHz, (e) yz-plane, 13.4 GHz, (f) xy-plane, 13.4 GHz.

Fig. 7. Transfer function measurement setup.

other separating at a distance in an indoor propagation environment with arbitrary scatters in it. The measured transfer function in frequency domain will be transformed into time-domain transfer response function by using inverse fast Fourier transform (IFFT). The transmitting (i.e., input) signal is selected to be a fifth derivative of Gaussian pulse which satisfies the FCC spectral mask for indoor UWB applications, which is defined as (1)


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REFERENCES

Fig. 8. Normalized transmitting and receiving antenna signal waveform.

where is the peak power spectral density and equals 51 ps to comply with FCC mask [9], [21]–[23]. To evaluate the transmission performance of the pulse, a pulse fidelity function defined in [24]–[26] is used. (2) is the transmitting (i.e., input) signal, is the received where (i.e., output) signal and is a delay which is varied to make the numerator in (2) maximum. Then, the pulse defined by (1) is convoluted with the time-domain transfer response function to obtained the output . In the calculation of , all signals are normalized for comsignal parison. The normalized receiving pulse and the transmitting pulse are compared in Fig. 8. It is shown that the pulses agree well with each other. Minor ripples and signal ringing are also observed, which are caused by the spectrum leakage at low frequency band. The calculated pulse , which means the signal distortion is acceptable fidelity is [26] in the transmission of UWB signals. V. CONCLUSION A novel wideband balanced loop-dipole composite antenna with end-fire radiation has been investigated. Operation bandwidth of with stable quasi-direcmore than 129% tional radiation patterns and low cross-polarization level has been achieved. Compared to most of proposed magneto-electric dipole antennas, the presented antenna has advantages of small size (i.e., , denotes the lowest cut-off guided-wave wavelength), simple structures and easily fabrication. Without employing a large ground plane or reflector, broadband end-fire radiation characteristic can be obtained. Moreover, it is shown that the proposed antenna has low pulse distortion effects. Therefore, it may be a good candidate in compact portable or hand-held wideband and ultra-wideband wireless devices for point-to-point applications. ACKNOWLEDGMENT The authors would like to thank Prof. Y.-M. Bo, Nanjing University of Posts and Telecommunications, for his beneficial discussions and comments on this communication. They are also in debt to the anonymous reviewers for their professional and warm-hearted comments on this communication.

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