HIGH-GAIN planar antennas have been intensively studied

Transcription

1 584 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 2, FEBRUARY 2017 Single-Layer Magnetic Current Antenna Array With High Realized Aperture Usage Rate Based on Microstrip Line Structure Le Chang, Student Member, IEEE, Zhijun Zhang, Fellow, IEEE, YueLi,Member, IEEE, and Magdy F. Iskander, Life Fellow, IEEE Abstract A new antenna structure is proposed to show that a single-layer magnetic current array based on the microstrip line structure can achieve a relatively uniform and large in-phase radiating aperture together with high realized aperture usage rate (RAUR). By alternatively and periodically loading the microstrip line with blocking stubs, the equivalent magnetic currents along one direction are suppressed while the currents along the other direction are left, forming a relatively uniform and in-phase 1-D magnetic current array. The electromagnetic wave is radiated into space while traveling forward along the line. By in-parallel concatenating a series of the 1-D arrays with adjacent arrays presenting mirror relations, a large in-phase radiating aperture is built with a relatively high RAUR performance. Moreover, the shorting walls at the interconnecting planes of the neighboring 1-Darrays can be removed attributing to the antenna arrangement style and the required feed network. Eight rows of the 24-element 1-D magnetic current array fed using a substrate integrated waveguide-based eight-way power divider fabricated using a single substrate board is simulated and built to demonstrate the design strategy. The prototype shows a measured bandwidth of 4.37% and a peak gain of 25.1 dbi at 20 GHz. The RAUR is 8.66/λ 2 0, which is higher than most of the single-layer high-gain antennas. Index Terms High gain, magnetic current array, microstrip line structure, realized aperture usage rate (RAUR), single layer. I. INTRODUCTION HIGH-GAIN planar antennas have been intensively studied for various application scenarios, such as telecommunication system, collision avoidance radar, and high-speed wireless LAN systems. Multiple-layer structures are frequently adopted to realize narrow pencil beam antennas with high performance. Usually, the fully/partially corporate-feed networks with the feed layer and the radiating aperture layer separated from each other are adopted to achieve wideband Manuscript received May 19, 2016; revised August 24, 2016 and October 29, 2016; accepted November 22, Date of publication December 5, 2016; date of current version February 1, This work was supported in part by the National Basic Research Program of China under Contract 2013CB329002, in part by the National Natural Science Foundation of China under Contract and Contract , and in part by the China Post-Doctoral Science Foundation under Project 2015T L. Chang, Z. Zhang, and Y. Li are with the State Key Lab on Microwave and Communications, Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing , China ( zjzh@tsinghua.edu.cn). M. F. Iskander is with the Hawaii Center for Advanced Communications, University of Hawaii at Manoa, Honolulu, HI USA ( iskander@spectra.eng.hawaii.edu). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TAP and high aperture efficiency performance [1] [8]. However, the multiple-layer antennas have a complex structure, and the bonding processes are usually costly and time-consuming. Antennas fabricated using a single substrate board can effectively reduce cost and antenna structure complexity. Single-layer large-scale microstrip arrays [9] [12], substrate integrated waveguide (SIW) slot arrays [13] [16], and parallelplate slot arrays [17], [18] have been intensively investigated. A parallel-fed circular-polarized microstrip array with 192 elements for NASA s deep space mission usage have the peak gain of 28 dbi [9]. A corporate-fed monopulse patch array used in radar tracking systems have the maximum gain of 24.5 dbi [10]. However, the large-scale microstrip arrays suffer from high ohmic and dielectric losses and high leakage rate at high frequency. SIW- or postwall waveguidebased planar arrays solve the high wave leakage problem. Due to the closed structure compared with open planar strip lines, SIW-based transmission line can be basically thick, and this contributes to the reduction of ohmic loss in especially higher frequency bands. The 4 2 coplanar waveguide (CPW)-fed SIWs with each waveguide supporting 16 radiating slots were realized using a single substrate, obtaining a gain higher than 22.8 dbi from to GHz [14]. Twelve 12-element SIWs which were integrated using a compact SIW 12-way power divider was fabricated with a single substrate, achieving a gain of about 22 dbi [16]. Single-layer end-fed and center-fed parallel-plate slot arrays with the aperture efficiency of 39.3% and 47.4%, respectively, were proposed for millimeter wave applications [17], [18]. Singlelayer metallic waveguide arrays consisting of the top slotted plate and bottom ridged plate are another compact high-gain antenna solution [19], [20]. Using E- to H -planes crossjunction power divider, the slot free area reduces, and the aperture efficiency using the center-fed method is improved from 46% in [19] to 50% in [20]. The grid array antennas have high gain performance attributing to their scalability, which can be easily fabricated with a single substrate [21] [23]. A single layer Fabry Perot cavity antenna using high permittivity substrate with the reflecting screen and partially reflective surface printed on both sides has obtained the simulated gain of 14 dbi [24]. Authors have designed several cavity-cascaded antennas based on the microstrip line structure [25] [27]. In this paper, a new structure of a single-layer magnetic current antenna array with high realized aperture usage rate (RAUR) based on X 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 CHANG et al.: SINGLE-LAYER MAGNETIC CURRENT ANTENNA ARRAY 585 Fig. 3. Normalized radiation patterns in the two principal planes when the blocking stub numbers are 8, 16, and 24. (Infinite ground is used here for comparing the directivity fairly.) Fig. 1. Radiation principle: the blocking stubs turn the transmission mode of the microstrip line into radiation mode. Fringing electric fields of (a) microstrip line and (b) microstrip line with blocking stubs. Fig. 2. Geometry of the 1-D magnetic current array. The sidewalls in the substrate are planar structures instead of via holes for brevity. the microstrip line structure is proposed. RAUR is defined as the absolute realized gain value per wavelength square area, and high RAUR shows high aperture efficiency when it is calculated using the realized gain instead of the directivity, taking into account the ohmic, dielectric, and mismatch losses. The 2-D magnetic current array consists of eight 1-D arrays, which are constructed by alternatively loading the microstrip line with twenty-four magnetic current blocking stubs every half wavelength. These blocking stubs can suppress the magnetic currents along one direction, and the rest of the in-phase currents produce effective radiation at broadside. The electromagnetic wave is radiated into space while traveling forward along the line, forming a relatively uniform and large radiating aperture, and leading to a high RAUR. An SIW-based eight-way power divider is used as the feed network, which can effectively suppress wave leakage compared to other open guided wave structures, such as microstrip line or CPW-based networks. The fabricated prototype with a peak gain of 25.1 dbi shows an RAUR of 8.66/λ 2 0,whichis higher than most of the single-layer high-gain antennas. In Section II, we describe the radiation principle of the proposed antenna, and Section III discusses implementation in 1-D array. The design of 2-D array is described in Section IV. Section V includes a description of a developed prototype and presents summary and discussion of measured results. Section VI presents the influence of the fabrication errors on antenna performance. Observations and concluding remarks are included in Section VII. II. RADIATION PRINCIPLE Fringing electric fields of the microstrip line is shown in Fig. 1(a): the fields on one side of the line are phase-reversed Fig. 4. (a) Magnitude and (b) vector electric field distributions of the 24-element 1-D array at 20 GHz. (c) Electric field intensity at 20 GHz along the 1-D array. (The data are sampled in the left side of the center line at the half height inside the 1-D array.) with their counterparts on the other side, and they present the period of half wavelength. Thus, the electromagnetic wave is confined and propagating within the microstrip line. Based on the periodicity of the fields, blocking stubs are loaded every half wavelength for effectively radiating. As shown in Fig. 1(b), a series of the small L-shaped stubs are added at the antinode points of the half-period waves which are denoted by the black arrow lines to suppress their radiation.

3 586 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 2, FEBRUARY 2017 Fig. 5. Geometry of the proposed single-layer 2-D magnetic current array, including the antenna array body and feed network. The output impedances of the feed network is controlled by the widths of these output metal slices, and the realized sizes are w 2 = w 9 = 2, w 3 = w 4 = w 5 = w 6 = w 7 = w 8 = 1.8 (mm). TABLE I DETAILED DIMENSIONS OF THE 1-D MAGNETIC CURRENT ARRAY AT 20 GHz TABLE II BROADSIDE DIRECTIVITY AT 20 GHz VARYING WITH THE STUB NUMBER As a result, the fringing fields with identical phase denoted by the red arrow lines are left, leading to effective radiation at broadside. According to the Love s and Schelkunoff s Field Equivalence Principle, these in-phase fringing electric fields are equivalent to magnetic currents as denoted by the red dotted double-arrow lines. The small L-shaped stubs and the magnetic currents have one-to-one corresponding relations, so their numbers are identical. Thus, the blocking stubs turn the microstrip line into a 1-D magnetic current array antenna. Moreover, the antenna has the scalable capacity along one direction to achieve more gains. III. 1-D SINGLE-LAYER MAGNETIC CURRENT ARRAY A. 1-D Single-Layer Magnetic Current Array Geometry Fig. 2 shows the 1-D magnetic current array which is exampled with eight blocking stubs. It is a single layer structure based on the Taconic-TLX 8 substrate (ε r = 2.55, tanδ = , h = 1 mm). There are tiny differences between the schematic diagram shown in Fig. 1(b) and the realized structure: the ends of these blocking stubs are interconnected using two metallic sidewalls to suppress surface wave leakage, and the rear end of the line is shorted to the ground. A lump port is placed at the head end. For the sake of analysis, the sidewalls in the substrate are planar structures instead of via holes. The detailed dimensions of the 1-D magnetic current array are tabulated in Table I. The width and height of the microstrip line are 2 and 1 mm, respectively. The period of the blocking stubs is 5.1 mm that is approximately half wavelength in the substrate at center frequency. The width and length of the stubs are 1 and 3 mm, respectively. It is worth mentioning that the length of the stubs is about quarter wavelength: on one hand, the stubs suppress the radiation of the fields along one direction; on the other hand, the stubs transform the shorted ends generated with the sidewalls to the open ends at the interconnection points in the microstrip line, so the transmission mode of the microstrip line is not affected. Thus, when the proposed 1-D array is fed from one end, the electromagnetic wave is radiated into space while traveling forward along the line, forming a relatively uniform in-phase radiating aperture. In addition, the widths of the microstrip line and the blocking stubs are selected according to the optimized uniformity field distribution. The optimization is conducted using the software ANSYS high frequency structure simulator version 14. B. Scalability The 1-D array has the potential to expand along one direction to obtain a large radiating aperture. Normalized radiation patterns in the two principal planes when the blocking stub numbers are 8, 16, and 24 are depicted in Fig. 3. Infinite ground is used here for comparing the directivity fairly. As seen, the three patterns in the E-plane (yz plane) are almost the same due to the identical transverse dimension, and the main beams in the H -plane (xz plane) become sharper and sharper with the increasing of the stub number, implying the increasing broadside directivity. The broadside directivity at center frequency varying with different stub numbers is tabulated in Table II. The directivity increases as more stubs are loaded and the growth trend follows the logarithmic distribution. The stub number of 24 is a nice compromise between the size and directivity considering the slower and slower directivity growth rate. Further increase of the stubs contributes little to directivity because most of the energy is

4 CHANG et al.: SINGLE-LAYER MAGNETIC CURRENT ANTENNA ARRAY 587 Fig. 6. (a) Field distribution of the feed network at 20 GHz. (b) Reflection and transmission coefficients and (c) phase difference of the neighboring output ports. radiated out by the front part and the residual energy in the rear part is rather limited. C. Field Distribution In order to illustrate the operation principle clearly, the magnitude and vector electric field distributions of the 24-element 1-D array at 20 GHz are given in Fig. 4(a) and (b). There existing a half standing wave around each blocking stub, and the vector electric fields enclosed by the dotted ellipses are with the same phase as seen from the enlarged inset, which are equivalent to in-phase magnetic currents as denoted by the blue dotted double-arrow lines. Therefore, the proposed 24-element 1-D array is equivalent to a 24-element 1-D magnetic current array. In order to quantitative describe the field uniformity, the electric field intensity at 20 GHz along the 1-D array is plotted in Fig. 4(c). The data is sampled in the left edge of the center line at the half height inside the 1-D array and the sampling line is with the start and end positions of (0, 1, 0.5) and (123.5, 1, 0.5), respectively. The array structure is drawn above the horizontal axis to clearly show its position. As seen, the electric field intensity exhibits a sinusoidal waveform and each stub corresponds to an antinode point. The field magnitude decreases very slowly and the average intensity drops by about 7 db, indicating a sufficiently uniform field, leading to a uniform 24-element 1-D magnetic current array. IV. 2-D SINGLE-LAYER MAGNETIC CURRENT ARRAY A. 2-D Single-Layer Magnetic Current Array Geometry Geometry of the proposed single-layer 2-D magnetic current array including the antenna array body and feed network is shown in Fig. 5. The substrate used here is the Taconic-TLX 8 substrate (ε r = 2.55, tanδ = ) with the thickness of 1 mm as aforementioned. The antenna array body consists of eight rows of the 24-element 1-D arrays, which are parallelfed using an SIW-based fed network. All the dimensions are the same with the aforementioned 1-D array. The eight 1-D arrays are interconnected into a whole with the neighboring 1-D arrays presenting a mirror relations. Then, using Fig. 7. (a) Magnitude electric field distributions of the proposed single-layer 2-D array at 20 GHz. (b) Electric field intensity at 20 GHz along the eight sub arrays. (The data are sampled along the red dotted lines at the half height inside the 1-D arrays.) differential-fed for the neighboring 1-D arrays, the shorting sidewalls which should be located in the interconnecting planes can be removed, only the two sides and rear end need to be shorted to restrict the electromagnetic wave, leading to a compact antenna structure. The diameter and spacing of the via holes, and distance between the hole center to the edge of the gold covering are 0.4, 0.8, and 0.3 mm, respectively, as shown in the enlarged inset on the left of the array body. The spacing between the neighboring 1-D array is 8 mm, which is approximately 0.53λ 0 (λ 0 is the free space wavelength at 20 GHz). In the final realized structure, an extra via-hole wall is added next to each initial via-hole wall to

5 588 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 2, FEBRUARY 2017 Fig. 8. Fabricated prototype of the proposed single-layer 2-D array. Fig. 10. Measured near-field distributions over the scanning plane at 20 GHz. (a) Magnitude and (b) phase distributions. (The relative size and position of the radiating aperture is denoted with the black dotted rectangle.) Fig. 9. Measured and simulated magnitudes of (a) reflection coefficients and (b) gains of the proposed single-layer 2-D array. further suppress wave leakage, as seen from the prototype in Fig. 8. Radiating aperture of the 2-D antenna array measures mm 64.6 mm (8.67λ λ 0 ). It is worth mentioning that the input active impedances of the eight ports are not identical: due to the different relative positions between the respective port and the rest of the ports, the couplings coming from other ports are different, resulting in different active impedances. However, the active impedance of the four upper ports are the same with the respective four lower ports due to the symmetry. Herein, the active impedances are obtained by assigning the excitation coefficients with uniform amplitude and differential phases for the adjacent ports. These active impedance values are listed in the right of the array body as shown in Fig. 5. A probe-fed SIW-based feed network is adopted to feed the proposed 2-D array from one end, promoting the single layer operation. It produces uniform amplitude and alternative reversed phases, and the eight output ports active impedances are matched to the former active impedances. As shown in Fig. 5, eight microstrip lines are protruded out from the broad walls of the SIW [28]. Utilizing the SIW-based feed network can effectively suppress wave leakage compared with the counterparts made of open transmission lines, and this merit is especially significant for high frequency usage. The width of the SIW is 6.1 mm, corresponding to the cutoff frequency of 16 GHz. The guide wavelength is mm, which approximately accommodates two 1-D arrays in lateral direction. Geometry of the input port (port1) is illustrated in the enlarged inset on the bottom-right corner: the via hole with a diameter of 0.4 mm acts as the feeding probe, which sits at a metal ring with a width of 0.2 mm. The circular ring with a width of 0.3 mm as denoted in red is the lump port, and the electric integration line is denoted by the blue arrow line. As to practical implement, the inner conductor of the semirigid cable crosses the circular ring and is soldered to the metal ring, and the outer conductor is soldered to the outer edge of the circular ring. The output ports are matched to different impedances by selecting proper widths of the output metal slices, and the optimized widths are w 2 = w 9 = 2 mm,

6 CHANG et al.: SINGLE-LAYER MAGNETIC CURRENT ANTENNA ARRAY 589 Fig. 11. Measured and simulated normalized radiation patterns in the two principal planes of the proposed single-layer 2-D array. w i = 1.8 mm(i = 3 8). Other dimensions of the eight output structures are identical and labeled in the enlarged inset on the top-right corner. Field distribution of the feed network at 20 GHz is depicted in Fig. 6(a), eight half-period standing waves are distributed along the waveguide, and each port is located around the antinode point of the respective standing wave, indicating the adjacent ports are phase-reversed. The magnitudes of the eight half-period waves look nearly identical, implying the uniform output magnitude distribution. Transmission characteristics of the network are given in Fig. 6(b). The magnitude of the reflection coefficient at around 20 GHz is better than 18 db, and the transmission coefficients fluctuate within 0.8 db at 20 GHz, ranging from 9.7 to 10.5 db. The loss rate including the ohmic and dielectric losses is only 17.71%, which corresponds to an absolute loss value of about 7.5 db (the loss value of db denotes lossless while 0 db denotes total loss). The phase differences of adjacent outputs are shown in Fig. 6(c), which fluctuate around 180 with a phase error less than The SIW-based feed network presents good performance. B. Field Distribution The magnitude and vector electric field distributions are illustrated in Fig. 7(a). The excitation phases of the neighboring 1-D arrays are phase-reversed and the phase distribution is marked in the right position of Fig. 7(a). In this way, virtual shorted planes as denoted by the black dashed lines are produced in the interconnecting planes of the neighboring 1-D arrays. The seven virtual shorted walls together with the three via-holes walls restrict the electromagnetic wave of the each 1-D array resonant within its own respective space, without any interfering with each other. In this way, the realistic shorted walls which should be existed in these dashed lines positions can be omitted, resulting in a compact antenna structure. On the other hand, the adjacent 1-D arrays are mirrored with each other. Finally, all the fringing fields are with the identical phase, as confirmed by the vector fields in the enlarged inset, forming a relatively uniform radiating aperture. As mentioned in Section II, the proposed single-layer 2-D array is equivalent to an 8 24 magnetic current array. The electric field intensity at 20 GHz along the eight sub arrays is plotted in Fig. 7(b) to quantitative describe the field uniformity of the 2-D array. The data are sampled along one side of each center line as denoted by the red dotted lines (l1 l8) which locates at the half height inside the 1-D arrays, and the stubs on this sides have the same loading locations. As with the case of the 1-D array, the electric field intensity exhibits a sinusoidal waveform and each stub corresponds to an antinode point. On one hand, all the eight intensity curves are nearly overlapped, so the field intensity of the eight sub arrays are almost identical. On the other hand, the field magnitude decreases very slowly and the average intensity drops by about 10 db from the head to the rear, indicating a sufficiently uniform field along each sub array. Therefore, the field distribution of the 2-D array is sufficiently uniform to obtain a high RAUR. V. ANTENNA PROTOTYPE AND MEASURED RESULT The prototype of the proposed single-layer 2-D antenna array fabricated using a single substrate board is shown in Fig. 8. The commercial semirigid cable with the type of SMA-JB2-SFT is used to feed the antenna, and a series of magnetic beads are adopted to prevent the radiation caused by the surface current on the outer surface of the outer conductor. Reflection coefficient was measured using a N5247A vector network analyzer (10 MHz 67 GHz), the gains and radiation patterns were measured using the NSI 2000 system by Nearfield Systems Inc. The simulated and measured magnitude of reflection coefficients are shown in Fig. 9. The simulated bandwidth is 510 MHz (2.54%, GHz), while the measured bandwidth is 870 MHz (4.37%, GHz). Although discrepancy exists around 19.7 GHz, the measured result is better. The measured and simulated broadside gains are given in Fig. 9(b), they have good agreement. The measured and

7 590 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 2, FEBRUARY 2017 Fig. 12. Radiation directivity patterns under the (a) stub width and (b) length errors of ±0.05 mm. Fig. 13. Reflection and transmission characteristics of the SIW-based eight-way power divider (a) with the dimension errors of mm, (b) without errors, and (c) with the dimension errors of 0.05 mm. simulated 3 db gain bandwidths are both about 3% from 19.7 to 20.3 GHz. The simulated peak gain is dbi at GHz while the measured is 25.1 dbi at 20 GHz. The RAUR defined based on the realized gain is used in this paper to evaluate the performance of high-gain antennas. The realized gain which includes the ohmic loss, dielectric loss, and mismatch loss is the performance of concern in practical applications. RAUR is defined as the absolute realized gain value per wavelength square area, and the upper limit value equals to 4π (under the circumstances of 100% radiation efficiency). Generally, it is impossible to excite each element in parallel of the single-layer antenna due to the space limitations while keep the element spacing less than 0.7 wavelength to avoid grating lobe [29]. As a result, the aperture amplitude distribution is not uniform, leading to a low RAUR. RAUR comparison of the proposed antenna with other single-layer high-gain antennas with similar gain, frequency, and size is listed in Table III. As seen, the RAUR of the proposed antenna is 8.66/λ 2 0, whereas the rest of the single-layer high-gain antennas are not exceeding 7.22/λ 2 0. This indicates that the proposed antenna can achieve a higher realized gain using the same radiating area. The proposed antenna is especially beneficial for high-gain application scenarios. The measured near-field magnitude and phase distributions of the electric field at 20 GHz are plotted in Fig. 10. The scanning plane with the size of 405 mm 305 mm locates about 50 mm away from the radiating aperture, and the sampling step is 5 mm. The black dotted rectangle denotes the relative size and position of the proposed antenna. Within this area, the measured magnitude looks nearly uniform and the measured phase looks identical, indicating a local quasiplane wave area, so high gain is obtained. The measured and simulated normalized radiation patterns in the two principal planes are plotted in Fig. 11. The far-field pattern is obtained from the measured near-field sampling data by using fast Fourier transformation. Here, the patterns within the elevation angle range of ±60 are given. It is observed that the measured results agree perfect well with the simulation. The measured 3-dB beamwidths in the E- andh -planes are 11.8 and 7, respectively, which are the same with the simulated results. The measured peak side lobe levels are and db in the E-andH -planes, respectively, and the cross-polar levels

8 CHANG et al.: SINGLE-LAYER MAGNETIC CURRENT ANTENNA ARRAY 591 TABLE III RAUR COMPARISON OF THE PROPOSED SINGLE-LAYER 2-D ARRAY WITH OTHER SINGLE-LAYER HIGH-GAINANTENNAS In the future, we will further improve the realized gain using CPW-based center-fed approach. REFERENCES are both better than 30 db in the two planes. VI. INVESTIGATION OF THE FABRICATION TOLERANCES The tolerances against the mechanical fabrication errors on the directivity pattern of the 1-D array and the S-parameters of the eight-way power divider are investigated. The machining precision is about 0.05 mm, so the dimension errors of ±0.05 mm are assumed for simulation. Variation of directivity pattern due to the stub length and width errors in the 1-D array is shown in Fig. 12. As seen, the patterns are hardly affected under the dimension errors of ±0.05 mm, so the radiation performance of the 1-D array is stable and robust. Variations of reflection and transmission characteristics due to the dimension errors of w 2 9 in feeding circuit are presented in Fig. 13. The transmission amplitude and phase ranges are marked in Fig. 13 using red bold font. Under the case with errors of mm and 0.05 mm, the transmission amplitude ranges change from db to db and db, respectively, and the transmission phase ranges change from 15.2 to 14.7, and 15.6, respectively. Thus, the performance of the feeding circuit is also robust to the fabrication errors. In a word, the proposed structure is robust enough to the fabrication errors, leading to good and stable antenna performance. VII. CONCLUSION A new structure of single-layer high-gain magnetic current antenna array based on the microstrip line is proposed. By alternatively loading several side-by-side microstrip lines with a series of magnetic current blocking stubs, and arranging the stubs of the neighboring lines in a mirrored way, the proposed 2-D antenna array is built. 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9 592 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 2, FEBRUARY 2017 [22] L. Zhang, W. Zhang, and Y. P. Zhang, Microstrip grid and comb array antennas, IEEE Trans. Antennas Propag., vol. 59, no. 11, pp , Nov [23] L. Zhang, Y. Zhang, and Y. Lu, 30-dBi gain microstrip grid array antenna at 24 GHz on a single-layer substrate, in Proc. IEEE APS, Orlando, FL, USA, Jul. 2013, pp [24] T. K. Nguyen, D. C. Park, and I. Park, A single layer high gain Fabry Perot cavity antenna, in Proc. Global Symp. Millim. Waves (GSMM), Montreal, QC, Canada, May 2015, pp [25] L. Chang, Z. Zhang, Y. Li, and Z. Feng, All-metal antenna array based on microstrip line structure, IEEE Trans. Antennas Propag., vol. 64, no. 1, pp , Jan [26] L. Chang, Y. Li, Z. Zhang, and Z. Feng, A compact all-metallic cavity-cascaded antenna, Electron. Lett., vol. 52, no. 6, pp , Mar [27] L. Chang, Z. Zhang, Y. Li, and Z. Feng, Wideband triangular-cavitycascaded antennas, IEEE Trans. Antennas Propag., vol. 64, no. 7, pp , Jul [28] X. Chen, W. Hong, T. Cui, Z. Hao, and K. Wu, Substrate integrated waveguide elliptic filter with transmission line inserted inverter, Electron. Lett., vol. 41, no. 15, pp , Jul [29] A. U. Zaman and P. S. Kildal, Wide-band slot antenna arrays with single-layer corporate-feed network in ridge gap waveguide technology, IEEE Trans. Antennas Propag., vol. 62, no. 6, pp , Jun Yue Li (S 11 M 12) received the B.S. degree in telecommunication engineering from Zhejiang University, Hangzhou, China, in 2007, and the Ph.D. degree in electronic engineering from Tsinghua University, Beijing, China, in He was a Visiting Scholar at the Institute for Infocomm Research, Agency for Science, Technology and Research, Singapore, in 2010, and the Hawaii Center of Advanced Communication, University of Hawaii at Manoa, Honolulu, HI, USA, in In 2012, he joined the Department of Electronic Engineering, Tsinghua University, as a Post-Doctoral Fellow. In 2013, he joined the Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, PA, USA, as a Research Scholar. Since 2016, he has been with Tsinghua University, where he is currently an Assistant Professor with the Department of Electronic Engineering. He has authored or co-authored over 60 journal papers and 30 international conference papers, and holds 13 granted Chinese patents. His current research interests include metamaterials, plasmonics, nanocircuits, electromagnetics, mobile and handset antennas, MIMO and diversity antennas, and millimeter-wave antennas and arrays. Dr. Li was a recipient of the Young Scientist Award from URSI General Assembly in 2014, the Outstanding Doctoral Dissertation of Beijing Municipality in 2013, and the Principal Scholarship of Tsinghua University in technology. Le Chang (S 16) received the B.S. degree in electronics and information engineering from Xidian University, Xi an, China, in He is currently pursuing the Ph.D. degree in electrical engineering with Tsinghua University, Beijing, China. His current research interests include antenna design and theory, particularly cavity-cascaded antenna arrays, transmitted arrays, leaky-wave antennas, and millimeter-wave and terahertz antennas based on MEMS silicon micromachining Zhijun Zhang (M 00 SM 04 F 15) received the B.S. and M.S. degrees from the University of Electronic Science and Technology of China, Chengdu, China, in 1992 and 1995, respectively, and the Ph.D. degree from Tsinghua University, Beijing, China, in In 1999, he was a Post-Doctoral Fellow with the Department of Electrical Engineering, University of Utah, Salt Lake City, UT, USA, where he was appointed as a Research Assistant Professor in In 2002, he joined the University of Hawaii at Manoa, Honolulu, HI, USA, as an Assistant Researcher. Later, he joined Amphenol T and M Antennas, Vernon Hills, IL, USA, in 2002, as a Senior Staff Antenna Development Engineer, and was then promoted as an Antenna Engineer Manager. In 2004, he joined Nokia Inc., San Diego, CA, USA, as a Senior Antenna Design Engineer. In 2006, he joined Apple Inc., Cupertino, CA, USA, as a Senior Antenna Design Engineer, and was then promoted as a Principal Antenna Engineer. Since 2007, he has been with Tsinghua University, where he is currently a Professor with the Department of Electronic Engineering. He has authored Antenna Design for Mobile Devices (Wiley, 2011). Dr. Zhang served as an Associate Editor of the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION during and the IEEE Antennas and Wireless Propagation Letters during Magdy F. Iskander (F 93 LF 12) was a Professor of Electrical and Computer Engineering and the Engineering Clinic Endowed Chair Professor with the University of Utah, Salt Lake City, UT, USA. He joined the University of Hawaii at Manoa, Honolulu, HI, USA, in 2002, where he is currently a Professor of Electrical Engineering and the Director of the Hawaii Center for Advanced Communications, College of Engineering. He is also the Co-Director of the NSF Industry/University Co-operative Research Center with four other universities. He has authored over 250 papers in technical journals, holds nine patents, and has made numerous presentations at national/international conferences. He authored and edited several books, including the textbook Electromagnetic Fields and Waves (Prentice Hall, 1992, and Waveland Press, 2001; second edition 2012), and four books published by the Materials Research Society on Microwave Processing of Materials. His research in computational and biomedical electromagnetics and wireless communications was funded by the National Science Foundation, National Institute of Health, Army Research Office, U.S. Army CERDEC, Office of Naval Research, and several corporate sponsors. Dr. Iskander was a recipient of many awards for excellence in research and teaching, including the University of Hawaii Board of Regents Medal for Excellence in Research in 2013, the Board of Regents Medal for Teaching Excellence in 2010, and the Hi Chang Chai Outstanding Teaching Award in 2011 and 2014, which is based on votes by graduating seniors. He was also a recipient of the IEEE MTT-S Distinguished Educator Award in 2013, the IEEE AP-S Chen-To Tai Distinguished Educator Award in 2012, and the Richard R. Stoddard Award from the IEEE EMC Society in He received the Northrop Grumman Excellence in Teaching Award in 2010, the American Society for Engineering Education Curtis W. McGraw National Research Award in 1985, and the ASEE George Westinghouse National Award for Excellence in Education in He was the President of the IEEE Antennas and Propagation Society in 2002, a Distinguished Lecturer, and a Program Director of the Electrical, Communications, and Cyber Systems Division at the National Science Foundation. He has been the Founding Editor of the Computer Applications in Engineering Education Journal (Wiley), since 1992.