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Microstrip Magnetic Dipole Yagi Array Antenna. With Endfire Radiation and Vertical Polarization. Juhua Liu, Member, IEEE, and Quan Xue, Fellow, IEEE.
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 3, MARCH 2013

Microstrip Magnetic Dipole Yagi Array Antenna With Endfire Radiation and Vertical Polarization Juhua Liu, Member, IEEE, and Quan Xue, Fellow, IEEE

Abstract—A new microstrip Yagi array antenna with endfire radiation and vertical polarization is proposed. The Yagi antenna has a low profile, a wide bandwidth and a high gain. Each element of the Yagi array is based on a new microstrip antenna that has one edge opened and the other three edges shorted, working as a “magnetic dipole antenna”. As opposed to previous microstrip Yagi array antennas, the proposed Yagi antenna could produce a beam radiating at exactly endfire for infinite ground plane, with vertical polarization in the horizontal plane. A coupling microstrip line is introduced between the driven element and the first director element to strengthen the coupling between them, and therefore the front-to-back ratio and bandwidth of the array can be improved. The endfire gain can be enhanced as the number of the director elements increases, in either case where the array has an infinite or a finite ground plane. Index Terms—Bandwidth, endfire, gain, magnetic dipole antenna, microstrip antenna, vertical polarization, Yagi antenna.

I. INTRODUCTION

Y

AGI-UDA antennas have been widely used since they were invented in 1928 by Yagi and Uda [1], because they have a high directivity and only one element needs to be excited. The classical Yagi antenna is based on an array of electric dipoles. A Yagi array of magnetic elements was also studied [2] since it provided a complementary property to that of electric dipoles. For example, a Yagi array of magnetic elements would provide a vertical polarization at endfire while a Yagi array of electric elements provides a horizontal polarization, when the radiating elements of the Yagi arrays are parallel to the horizontal plane. Previously, Yagi array of magnetic elements adopted slot antennas and was fabricated in a rectangular waveguide [2]. However, rectangular waveguide is costly, heavy and bulky. In modern communications, vertical polarization is widely used, since propagation with vertical polarization suffers smaller attenuation loss than that with horizontal polarization when propagating along the ground plane of the earth. In communication systems on a horizontal platform, such as ground-wave communications, vertical polarization is preferred since transmitter and receiver can keep the same vertical Manuscript received July 03, 2012; revised October 01, 2012; accepted November 16, 2012. Date of publication November 29, 2012; date of current version February 27, 2013. J. Liu was with the State Key Laboratory of Millimeter Waves, City University of Hong Kong, Kowloon, Hong Kong. He is now with Sun Yat-sen University, Guangzhou 510275, China (e-mail: [email protected]). Q. Xue is with the State Key Laboratory of Millimeter Waves, City University of Hong Kong, Kowloon, Hong Kong (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2012.2230239

polarization for good connection no matter how transmitter or receiver rotates on the platform. On the other hand, connection would be effected or even be disrupted with the rotation of transmitter or receiver, when the transmitter and the receiver employ horizontal polarization. Monopole antenna is very attractive [3] because it has a simple structure and provides a vertical polarization. With these advantages, the monopole antenna has problems such as a small endfire gain (gain in the horizontal plane) and a high profile of a quarter wavelengths in free space. Monopolar patch antennas [4]–[8] were proposed that can also provide a vertical polarization but have a much lower profile compared with the conventional monopole antenna. However, the endfire gains of the monopolar patch antennas are also small. Microstrip Yagi array antenna with rectangular patch elements was first studied by Huang [9] in 1989 for satellite communication. The antenna can provide circular or linear polarization (including the vertical polarization). Such antenna has advantages of low profile, low cost, simple structure and high gain. A lot of excellent researches were also contributed to improve the front-to-back (F/B) ratio and to increase the gain [10]–[13]. The beam of this type of rectangular patch Yagi array antenna can be tilted away from the broadside. However, the beam direction can not reach completely endfire, because the radiation of a rectangular patch antenna is primarily in the broadside direction [10]. Besides, the rectangular patch Yagi antenna suffers a limitation of the substrate [10] with the relative dielectric constant constrained in a range between and , because the patches can neither be too large nor too small to provide a proper space between them for required mutual coupling. Recently, microstrip Yagi array antennas [14]–[16] based on classical Yagi-Uda dipole array [1] have been proposed. These antennas have a simple structure and low profile and provide a high gain with the main beam radiating at endfire. However, these antennas only generate a horizontal polarization. In this paper, we propose a new Yagi antenna based on microstrip magnetic dipole elements. The Yagi antenna has advantages of low profile, light weight and easy fabrication, as preserved by a common microstrip antenna. Besides, the Yagi antenna has a wide bandwidth and high gain. Compared with previous microstrip Yagi antennas, the presented Yagi antenna can produce a main beam radiating at exact endfire for infinite ground plane, with vertical polarization in the horizontal plane. Each element of the proposed Yagi array is a new microstrip antenna that has one edge opened and the other three edges shorted, working as a magnetic dipole antenna, as shown in Fig. 1. The microstrip magnetic dipole antenna would suppress , compared the interference in transverse directions

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LIU AND XUE: MICROSTRIP MAGNETIC DIPOLE YAGI ARRAY ANTENNA WITH ENDFIRE RADIATION AND VERTICAL POLARIZATION

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Fig. 2. Reflection coefficients for only the driven microstrip magnetic dipole antenna, 4-element Yagi array with microstrip line coupling and 4-element Yagi array without microstrip line coupling.

Fig. 1. Top view and cross section of the microstrip magnetic dipole Yagi array antenna.

with the conventional quarter-wave microstrip antenna that has only one edge shorted and the other three edges opened [17]. Since the width of each element is usually less than , the Yagi antenna would not suffer the limitation of the substrate as does the microstrip rectangular patch Yagi array antenna [10]. A coupling microstrip line is introduced between the driven element and the first director element to strengthen the coupling between them, and therefore the F/B ratio and bandwidth can be improved. The F/B ratio of the proposed antenna is high in the entire operating band. The endfire directivity can be enhanced as the number of the director elements increases even when the Yagi array antenna has more than two director elements, no matter if the ground plane is finite or infinite. This type of antenna can be used for point-to-point communications that need a narrow beam at endfire with vertical polarization. II. OPERATING PRINCIPLES The structure of the proposed antenna is shown in Fig. 1. The Yagi antenna is composed of a driven element (D), a reflector element (R) and a series of director elements (D1 and D2). It is based on the principle of the conventional dipole Yagi array antenna [1] that has the electromagnetic energy coupled from a driven dipole through radiation to parasitic dipoles and then reradiated to produce a directional beam at endfire. However, in view of radiation property, this type of Yagi antenna works more likely as a “magnetic dipole Yagi array antenna”. Besides, the coupling mechanism of the proposed array is not completely the same as the classical dipole Yagi array. Each element of the Yagi array is a microstrip antenna that has only one edge opened and the other three edges shorted. The long shorted edge is to make sure that each microstrip antenna provides a single magnetic element, which produces vertically polarized radiation at endfire. The other two shorted edges at both ends are used to prevent propagation or leakage at these ends. Radiation is then contributed only from the open aperture. Therefore each open aperture can be considered as a “magnetic dipole antenna”. The resonant frequency of each

microstrip magnetic dipole antenna can be controlled by its length and width. Here, we let all these elements have the same width , and then we can tune the length of each element to control its resonant frequency. Only the driven element is fed with a coaxial probe. The first director element D1 is coupled from the driven element (D) mostly through a guided wave under a microstrip line with a width of and a length of , as shown in Fig. 1. Other parasitic elements are coupled through radiation. All elements are constructed on a microwave PCB. The substrate has a thickness of and a relative dielectric constant of . III. MICROSTRIP MAGNETIC DIPOLE ANTENNA In this section, we take the driven element as an example and analyze the microstrip magnetic dipole antenna first. As shown in Fig. 1, the driven element can be considered to be constructed on a half-width microstrip line that has both ends shorted. The half-width microstrip line works in the mode with respect to the direction [18]. Suppose that the antenna works in the mode with respect to the direction. Then the half-width microstrip line shorted at both ends has about a half wavelength in the direction. The wavelength is related to the phase constant in the direction by . The dispersion relation of the half-width microstrip line in the mode has been fully analyzed in [18]. The phase constant can be calculated from [18]. When the half-width microstrip line works in the mode, its normalized phase constant is not a constant value but a function of frequency, and the is always less than the wavenumber in the substrate , where is the wavenumber in free space. Then the width is larger than a half wavelength in the substrate . The resonant frequency can be adjusted by varying the length when the width is fixed. The impedance match can be tuned by the position of the probe. We use HFSS to simulate the basic element. The reflection coefficient of the antenna is shown in dashed line in Fig. 2. The parameters for the driven element are given in Table I. In the simulation, the ground plane is assumed to be infinitely large. It shows that the antenna works in a band from 5.06 GHz to 5.26 GHz, with a center frequency of 5.16 GHz and

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TABLE I PARAMETERS FOR YAGI ARRAYS WITH WALL-SHORTING

Fig. 3. Elevation (left) and azimuth (right) radiation patterns for only the driven element at 5.16 GHz.

Fig. 4. Elevation (left) and azimuth (right) radiation patterns for Yagi array with microstrip line coupling.

a narrow bandwidth of about 4.3%, for the reflection coefficient . Since the center frequency is not only determined by the resonant frequency but also affected by the impedance match of the coaxial probe, the resonant frequency would be close to but may be slightly less than the center frequency (5.16 GHz). Using HFSS simulation with “Eigenmode” solution type, it is found that the resonant frequency is 5 GHz. At the resonant frequency 5 GHz, the phase constant can be calculated approximately by (1) The value calculated by (1) is , which is very close to the precise value that is calculated by [18]. The elevation and azimuth radiation patterns for only the driven element working at 5.16 GHz are shown in Fig. 3. The antenna produces an almost omnidirectional pattern in the elevation plane ( plane) in the upper half space. At 5.16 GHz, the directivity at forward endfire is 7.8 dBi. In the azimuth plane ( plane), the antenna produces an “8”-shaped radiation pattern. Vertical polarization at endfire is obtained. The radiation patterns show that the antenna works as a magnetic dipole antenna. IV. SIMULATION ANALYSES OF YAGI ARRAY ANTENNAS A. Yagi Array With Microstrip Line Coupling Consider a Yagi array with four elements that includes a driven element, a reflector and two director elements, with the structure shown in Fig. 1. In the design of the presented Yagi array [3], the sizes of the director elements must be smaller than that of the driven element, while the reflector element should have a relatively large size.

for the

In order to have a strong coupling between the driven element and the first director element, a microstrip line is introduced between them. Then the first director element is coupled mostly by a guided wave through the microstrip line. The microstrip line is not connected directly to the driven element or the director element, but coupled through a very small gap with a width of , to avoid the disturbance to the driven element or the director element. The gap width should be less than or around the substrate thickness in order to have a strong coupling. It would have a stronger coupling when the microstrip line has a larger width . However, the width of the microstrip line should be less than the wavelength in the substrate, to avoid the microstrip line resonating in its mode as a radiator. In our simulation, we choose to avoid the resonant frequencies of the and modes of the coupling patch. The length of the microstrip line has a large impact on the F/B ratio, since the length actually controls the phase delay of the director element. It is this type of separation delay phase, together with added phase component introduced by the reactance of the parasitic elements, contributes to the Yagi array’s beam radiating at endfire. The length of the microstrip line needs to be tuned, with the help of simulation, to obtain a high F/B ratio. As a matter of fact, the reflector element radiates well only at low frequency while the director elements radiates well only at high frequency in the operating band, because the resonant frequency of the reflector element is low while the resonant frequencies of the director elements are high. Therefore, in order to obtain a high F/B ratio in the entire operating band, the reflector element should give a contribution to the F/B ratio at low frequency while the director element should give a contribution to the F/B ratio at high frequency of the operating band. To obtain a high F/B ratio at high frequency, we need to tune the length of the coupling microstrip line.

LIU AND XUE: MICROSTRIP MAGNETIC DIPOLE YAGI ARRAY ANTENNA WITH ENDFIRE RADIATION AND VERTICAL POLARIZATION

The gap width between the driven element and the reflector element, and the gap width between the first director element and the second director element should be about or larger than the thickness of the substrate, in order to have a good reradiating performance of the parasitic elements. We use HFSS to simulate the Yagi array with four elements. Optimized values for the variables of the array are given in Table I. In the simulation, the ground plane is assumed to be infinitely large. The reflection coefficient is shown in Fig. 2. It shows that the bandwidth of the array with microstrip coupling is from 4.98 GHz to 5.56 GHz, with a fractional bandwidth of about 11%. The bandwidth is much wider than that of the single driven element. Actually, the enhancement of the bandwidth is mostly contributed by the first director element, because the coupling between the driven element and the first director element through a microstrip line is much stronger than the couplings between others. The elevation ( plane) and azimuth ( plane) radiation patterns are shown in Fig. 4. The elevation radiation patterns show that the maximum radiation occurs at exactly endfire for the array in an infinite ground plane. Fig. 4 shows that all the azimuth radiation patterns have a back lobe level less than . The directivity and F/B ratio are shown in solid lines in Fig. 5. The F/B ratio here represents the ratio of the radiation at forward endfire to that at backward endfire. It shows that the Yagi array has a directivity from 10 dBi to 14 dBi, and an F/B ratio from 10 dB to 24 dB, in the band from 4.98 GHz to 5.56 GHz.

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Fig. 5. Directivity and F/B ratio for the Yagi arrays with and without microstrip line coupling.

Fig. 6. Geometry for the Yagi array without microstrip line coupling.

B. Yagi Array Without Microstrip Line Coupling When the coupling microstrip line between the driven element and the first director element is removed (with the structure shown in Fig. 6), the performance of the array would not be so excellent. A Yagi array without microstrip line coupling (with the parameters given in Table I) is simulated. Fig. 2 shows that the operating band of the array without microstrip line coupling is from 4.94 GHz to 5.36 GHz, which is narrower than that of the array with microstrip coupling. The directivity and F/B ratio of the array without microstrip line coupling are shown in dashed lines in Fig. 5. It shows that at high frequency the directivity of the array without microstrip line coupling is lower than that of the array with microstrip line coupling, because the coupling microstrip line would have more power coupled from the driven element to the first director element especially at high frequency. It also shows that the F/B ratio of the array without microstrip line coupling is not as high as that of the array with microstrip line coupling, especially at high frequency. Without a microstrip line coupling, the distance between the apertures of the driven element and the first director element must be small in order to have a strong enough coupling [10]. However, the small distance between the two apertures would result in that the array fails to generate a high F/B ratio [3]. On the other hand, when employing a coupling microstrip line, the distance between the apertures of the driven element and the first director element can be adjusted freely and a high F/B ratio can be obtained.

Fig. 7. Geometry for the microstrip Yagi array antenna with shorting vias (not to scale).

V. EXPERIMENT FOR 4-ELEMENTS YAGI ARRAY In order to make the fabrication easier, closely-spaced shorting-vias or shorting-pins can be used to replace the shorting-walls [19]. Geometry of the corresponding Yagi array with via-shorted is shown in Fig. 7. Based on the simulation of the microstrip Yagi array antenna with shorting-wall (Section IV-A), we can design and fabricate a microstrip magnetic dipole Yagi antenna with shorting-vias conveniently. Corresponding parameters for the Yagi array with shorting-vias is given in Table II. Note that the ground plane simulated in Section IV-A is infinitely large, while here the ground plane is finite. The photo of the fabricated Yagi array

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Fig. 8. Photo for the microstrip Yagi array antenna with four elements. Corresponding geometries are shown in Fig. 7.

Fig. 9. Reflection coefficients for the microstrip Yagi array antenna shown in Fig. 8.

TABLE II PARAMETERS FOR YAGI ARRAYS WITH VIA-SHORTING

Fig. 10. Simulated and measured radiation patterns for the microstrip Yagi array antenna at 5.1 GHz shown in Fig. 8.

Fig. 11. Measured radiation patterns for tenna shown in Fig. 8.

is shown in Fig. 8, in which shorting-pins are used to replace the shorting walls. The reflection coefficients are shown in Fig. 9. Measured results show that the antenna works in a band from 4.87 GHz to 5.55 GHz, with a fractional bandwidth of about 13.1%. Simulated results agree very well with the measured data. Measured and simulated results for the elevation ( plane) and azimuth ( plane) radiation patterns at 5.1 GHz are shown in Fig. 10. Simulated patterns agree very well with measured ones. Measured results show that the cross polarization in the main elevation plane ( plane) is less than . Actually, such cross polarization is due to the fabrication error. Simulated results for the cross polarization in the elevation plane is

for the microstrip Yagi array an-

too small to be observed. In the horizontal plane ( plane), the cross polarization is lower than . Fortunately, the maximum cross polarization does not occur in the main elevation plane. Measured results for the elevation and azimuth radiation patterns at other frequencies are shown in Fig. 11. It shows that the main beam is about 50 from the broadside but not completely endfire, due to the finite ground diffraction effect. Nevertheless, the main beam is closer to endfire than that (with the peak direction between from the broadside) of the rectangular patch Yagi array [10]. The back lobe is less than in the band from 4.87 GHz to 5.55 GHz. The peak and endfire gains are shown in Fig. 12 for the array in finite ground plane. The endfire gain for the array with infinite ground plane is also shown for comparison. Simulated results agree well with measured ones. Measured results show that the

LIU AND XUE: MICROSTRIP MAGNETIC DIPOLE YAGI ARRAY ANTENNA WITH ENDFIRE RADIATION AND VERTICAL POLARIZATION

Fig. 12. Peak and endfire gains for the microstrip Yagi array antenna shown in Fig. 8.

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Fig. 14. Reflection coefficient for the microstrip Yagi array antenna with twelve elements shown in Fig. 13.

Fig. 13. Photo for the microstrip Yagi array antenna with twelve elements.

peak gain for the array with finite ground plane is about 10 dBi in the band from 4.87 GHz to 5.55 GHz. Due to the diffraction effect of the finite ground plane, the endfire gain of the array with finite ground plane is about from 4 dBi to 6 dBi in the working band, which is about 6 dB lower than the endfire gain of the Yagi array with infinite ground plane.

Fig. 15. Simulated and measured radiation patterns for the microstrip Yagi array antenna with twelve elements shown in Fig. 13 at 5.1 GHz.

VI. EXPERIMENT FOR 12-ELEMENT YAGI ARRAY When the director elements increase, the endfire gain can be enhanced, no matter in the case of infinite ground plane or finite ground plane. We add eight director elements to the 4-element Yagi array (Section V), and have the sizes of the added eight director elements the same as that of the second director element of the 4-element Yagi array. Parameters for the Yagi array with twelve elements are given in Table II. The photo of the fabricated antenna is shown in Fig. 13. Results for the reflection coefficients are shown in Fig. 14. It shows that the array works in a band from 4.85 GHz to 5.45 GHz, which is a little narrower than that for the Yagi array with four elements (Section V). Simulated and measured results for the elevation ( plane) and azimuth ( plane) radiation patterns at 5.1 GHz are shown in Fig. 15. It shows that the simulated results are close to the measured ones. In the azimuth radiation pattern (right plot in Fig. 15), the cross polarization is about , which is slightly larger than that of the 4-element Yagi array (right plot in Fig. 10). Fortunately, the highest cross polarization does not occur in the forward endfire or in the main elevation plane. In the main elevation plane ( plane), measured results show that the cross polarization is less than . Measured results

Fig. 16. Measured radiation patterns for the microstrip Yagi array antenna with twelve elements shown in Fig. 13.

for the radiation patterns for other frequencies are shown in Fig. 16. It shows that the main beam points at an angle of about 70 from broadside, which is closer to endfire than that for the 4-element Yagi array. Figs. 15 and 16 show that the array has a small back lobe level less than . Results for the peak and endfire gains for the antenna with finite ground plane are shown in Fig. 17. Measured results for the peak gains are close to simulated data. Measured results show that the peak gains are between 10.5 dBi and 12.2 dBi in the band from 4.85 GHz to 5.5 GHz, which is only about 1 dB higher than that for the 4-element Yagi array. The endfire gains, however, are between 7.4 dBi and 10.4 dBi in the band from 4.85 GHz to 5.5 GHz, which are 4 dB higher than that for the 4-element Yagi array. Endfire gain for the antenna with infinite ground plane is also shown for comparison. It shows that the endfire gain for the 12-element array with infinite ground plane

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REFERENCES

Fig. 17. Peak and endfire gains for the microstrip Yagi array antenna with twelve elements shown in Fig. 13.

is from 13.2 dBi to 16.3 dBi in the operating band, which is about 4 dB higher than that for the 4-element array with infinite ground plane. Therefore, the endfire gain can be enhanced using more director elements in either case of finite ground plane or infinite ground plane, although the peak gain is not enhanced as significantly as well for the array with finite ground plane. Note that the effect of increased director element would become weaker as the number of directors increase, as is generally found in the classical dipole Yagi antenna [1].

VII. CONCLUSION A new type of Yagi array based on microstrip magnetic dipole antennas is proposed and studied. The Yagi array has a low profile, a wide bandwidth and a high gain. The Yagi array can produce a beam radiating at exactly endfire for infinite ground plane, with vertical polarization in the horizontal plane. A coupling microstrip line is introduced between the driven element and the first director element, in order to yield a high F/B ratio and a wide bandwidth. A microstrip Yagi array with four elements is designed and measured. Measured results show that the antenna has a fractional bandwidth of about 13.1% for a profile of 0.027 wavelengths in free space . Due to the finite ground diffraction effect, the antenna with finite ground plane produces a beam radiating not exactly at endfire but close to endfire. The antenna produces a peak gain of about 10 dBi, an endfire gain between 4 dBi and 6 dBi, and a back lobe level less than , in the working band. A microstrip Yagi array with twelve elements is also designed and measured. The endfire gain can be enhanced greatly as the number of director elements increases, even for the case of finite ground plane. Measured results show that the antenna has a fractional bandwidth of about 11.7% for a profile of 0.027 wavelengths in free space . The antenna can produce a peak gain between 10.5 dBi and 12.2 dBi, an endfire gain between 7.4 dBi and 10.4 dBi, and a back lobe level less than , in the working band.

[1] H. Yagi, “Beam transmission of the ultra short waves,” Proc. IRE, vol. 16, pp. 715–741, Jun. 1928. [2] R. J. Coe and G. Held, “A parasitic slot array,” IEEE Trans. Antennas Propag., vol. 12, no. 1, pp. 10–16, Jan. 1964. [3] C. A. Balanis, Antenna Theory Analysis and Design, 3rd ed. Hoboken, NJ, USA: Wiley, 2005. [4] L. Economou and R. J. Langley, “Patch antenna equivalent to simple monopole,” Electron. Lett, vol. 33, no. 9, pp. 727–728, Apr. 1997. [5] C. Delaveaud, P. Leveque, and B. Jecko, “New kind of microstrip antenna: The monopolar wire-patch antenna,” Electron. Lett, vol. 30, no. 1, pp. 1–2, 1994. [6] K.-L. Lau, P. Li, and K.-M. Luk, “A monopolar patch antenna with very wide impedance bandwidth,” IEEE Trans. Antennas Propag., vol. 53, no. 2, pp. 655–661, Feb. 2005. [7] A. Al-Zoubi, F. Yang, and A. Kishk, “A broadband center-fed circular patch-ring antenna with a monopole like radiation pattern,” IEEE Trans. Antennas Propag., vol. 57, no. 3, pp. 789–792, Mar. 2009. [8] J. Liu, Q. Xue, H. Wong, H. Lai, and Y. Long, “Design and analysis of a low-profile and broadband microstrip monopolar patch antenna,” IEEE Trans. Antennas Propag., vol. 61, no. 1, pp. 11–18, Jan. 2013. [9] J. Huang, “Planar microstrip Yagi array antenna,” in IEEE Antennas and Propag. Soc. Int. Symp., Jun. 1989, vol. 2, pp. 894–897. [10] J. Huang and A. Densmore, “Microstrip Yagi antenna for mobile satellite vehicle application,” IEEE Trans. Antennas Propag., vol. 39, no. 7, pp. 1024–1030, Jul. 1991. [11] G. R. DeJean and M. M. Tentzeris, “A new high-gain microstrip Yagi array antenna with a high front-to-back (F/B) ratio for WLAN and millimeter-wave applications,” IEEE Trans. Antennas Propag., vol. 55, pp. 298–304, Feb. 2007. [12] G. R. DeJean, T. T. Thai, S. Nikolaou, and M. M. Tentzeris, “Design and analysis of microstrip bi-Yagi and quad-Yagi antenna arrays for WLAN applications,” IEEE Antennas Wireless Propag. Lett., vol. 6, pp. 244–248, 2007. [13] T. T. Thai, G. R. DeJean, and M. M. Tentzeris, “Design and development of a novel compact soft-surface structure for the front-to-back ratio improvement and size reduction of a microstrip Yagi array antenna,” IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 369–373, 2008. [14] W. R. Deal, N. Kaneda, J. Sor, Y. Qian, and T. Itoh, “A new quasiYagi antenna for planar active antenna arrays,” IEEE Trans. Microwave Theory Tech., vol. 48, no. 6, pp. 910–918, Jun. 2000. [15] N. Kaneda, W. R. Deal, Y. Qian, R. Waterhouse, and T. Itoh, “A broadband planar quasi-Yagi antenna,” IEEE Trans. Antennas Propag., vol. 50, no. 8, pp. 1158–1160, Aug. 2002. [16] P. R. Grajek, B. Schoenlinner, and G. M. Rebeiz, “A 24 GHz high-gain Yagi-Uda antenna array,” IEEE Trans. Antennas Propag., vol. 52, no. 5, pp. 1257–1261, May 2004. [17] S. Pinhas and S. Shtrikman, “Comparison between computed and measured bandwidth of quarter-wave microstrip radiators,” IEEE Trans. Antennas Propag., vol. AP-36, no. 11, pp. 1615–1616, 1988. [18] J. Liu, D. R. Jackson, and Y. Long, “Propagation wavenumbers for mode,” IEEE Trans. half- and full-width microstrip lines in the Microwave Theory Tech., vol. 59, no. 12, pp. 3005–3012, Dec. 2011. [19] Y. Cassivi, L. Perregrini, P. Arcioni, M. Bressan, K. Wu, and G. Conciauro, “Dispersion characteristics of substrate integrated rectangular waveguide,” IEEE Microw. Wireless Compon. Lett., vol. 12, no. 9, pp. 333–335, Sep. 2002.

Juhua Liu (M’12) was born in Heyuan, Guangdong, China, in 1981. He received the B.S. and Ph.D. degrees in electrical engineering from Sun Yat-sen University, Guangzhou, China, in 2004 and 2011, respectively. From 2008 to 2009, he was a Visiting Scholar with the Department of Electrical and Computer Engineering, University of Houston, Houston, TX, USA. From September 2011 to September 2012, he was a Research Associate with the State Key Laboratory of Millimeter Waves, City University of Hong Kong, Hong Kong, China. In 2012, he joined Sun Yat-sen University, as a Lecturer. His present research interests include microstrip antennas, substrate integrated waveguide antennas, leaky-wave antennas, periodic structures, and computational electromagnetics.

LIU AND XUE: MICROSTRIP MAGNETIC DIPOLE YAGI ARRAY ANTENNA WITH ENDFIRE RADIATION AND VERTICAL POLARIZATION

Quan Xue (M’02–SM’04–F’11) received the B.S., M.S., and Ph.D. degrees in electronic engineering from the University of Electronic Science and Technology of China (UESTC), Chengdu, in 1988, 1990, and 1993, respectively. In 1993, he joined the UESTC, as a Lecturer. He became a Professor in 1997. From October 1997 to October 1998, he was a Research Associate and then a Research Fellow with the Chinese University of Hong Kong. In 1999, he joined the City University of Hong Kong where he is currently a Professor in the Department of Electronic Engineering. He also serves the University as the As-

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sociate Vice President (Innovation Advancement and China Office), the Deputy Director of CityU Shenzhen Research Institute, and the Deputy Director of State Key Lab of Millimeter Waves (Hong Kong). He has authored or coauthored over 200 internationally referred journal papers and over 80 international conference papers. His research interests include microwave passive components, active components, antenna, microwave monolithic integrated circuits (MMIC), and radio frequency integrated circuits (RFIC), etc. Prof. Xue is an elected member of the IEEE MTT-S AdCom. He is now an Associate Editor of the IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, and is an Editor of the International Journal of Antennas and Propagation.