Wideband Magneto-Electric Dipole Antenna for 60-GHz ... - IEEE Xplore

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 7, JULY 2015

Wideband Magneto-Electric Dipole Antenna for 60-GHz Millimeter-Wave Communications Mingjian Li and Kwai-Man Luk

Abstract—A new wideband magneto-electric dipole antenna is proposed for 60-GHz millimeter-wave applications. This antenna features wideband and stable gain characteristics. The low cross polarization and low back radiation are obtained owing to its complementary antenna structure. The prototype of the single element was built using the low-cost single-layer printed circuit board (PCB) technology. The proposed antenna exhibits an impedance bandwidth of 51% (SWR ≤ 2) and a gain of approximately 8 dBi. Index Terms—Complementary antennas, millimeter-wave antennas, wideband antennas, 60-GHz radios.

I. I NTRODUCTION Nowadays, the push toward higher carrier frequencies is imperative, as such an advancement would fulfill the desire of massive data transfer and address the lack of free frequency spectrum in the currently allocated bands. The dawn of the commercial millimeter-wave technology era has come [1]. Microstrip patch array is a competitive choice in the design of millimeter-wave antenna. To overcome the narrow bandwidth problem of the microstrip antenna, many techniques were reported in the literature, such as the L-probe feed [2], stacked patches [3], U-slot patch [4], cavity backed structure [5], and aperture coupled feed [6]. Most of these designs are realized on low temperature co-fired ceramic (LTCC) substrates, exhibiting wide impedance bandwidth. However, they are high in fabrication cost, compared with the designs printed on conventional printed circuit boards. In addition, other types of antennas have been developed for the millimeter-wave band, including cavitybacked antenna [7], tapered slot antenna [8], [9] and fan-like antenna [10]. In [7], a short backfire antenna is excited by a bowtie dipole, and therefore has a wide impedance bandwidth of 57%. In [8], [9], it was demonstrated that the tapered slot antenna can be designed in millimeter-wave band. By using zero-index metamaterial, the design in [9] achieves a high gain of about 12.6 dBi. In [10], two tapered slot elements are combined to configure an end-fire fan-like structure to broaden the radiation beamwidth. Although these designs exhibit promising wideband or high-gain characteristic, they are not very convenient to be used in array environment due to the large antenna size or end-fire radiation. In 2006, a wideband antenna element, designated as magnetoelectric (ME) dipole, was invented by Luk and Wong [11]. Subsequently, a series of ME dipole antennas was developed for various wideband wireless applications [12], [13]. Recently, a millimeterwave ME dipole antenna was proposed by Ng et al. in 2011 [14]. The antenna is fabricated by using standard PCB technique, and therefore is low in fabrication cost. Instead of employing the Γ-shaped probe to excite the antenna [11], a T-shaped coupled strip was applied in Manuscript received July 10, 2014; revised March 23, 2015; accepted April 12, 2015. Date of publication April 22, 2015; date of current version July 02, 2015. This work was supported in part by the Research Grants Council of the Hong Kong SAR. [Project No. 9041677 (CityU 119511)] The authors are with the State Key Laboratory of Millimeter Waves, City University of Hong Kong, Hong Kong SAR, China (e-mail: mingjiali2@ um.cityu.edu.hk). 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.2015.2425418

[14] because the probe can be easily realized on a single-layer PCB. This feeding scheme provides wide impedance bandwidth but leads to a high cross polarization level and a relatively low front-to-back ratio (FBR). In this paper, a new wideband millimeter-wave antenna is investigated. Excited by an L-shaped probe, this element features low cross-polarization and back radiation levels. II. M AGNETO -E LECTRIC D IPOLE A NTENNA D ESIGN As shown in Fig. 1, the proposed antenna consists of four rectangular metallic patches, four sets of via holes and an L-shaped probe. The metallic patches, placed λ/4 (t = 0.787 mm) above the ground plane, perform as two planar electric dipoles. Each set of via holes, consisting of three metallic plated via holes, is located close to the inner corner of a rectangular metallic patch. The via holes together with the ground plane between them form a vertically quarter-wave shorted patch antenna which radiates through the aperture between the metallic patches. The L-shaped probe is composed of a plated via hole and a small patch. The via hole, together with the adjacent via holes, accomplishes a transmission line to transmit signals to the small patch. This patch is placed between two planar dipoles which act as its electrical ground. This combination works as a CPW line, the portion of which across the gap (S1 ) is responsible to couple the electrical energy to the antenna element. Thus, the L-shaped probe can excite the planar dipoles and vertically quarter-wave shorted patch antenna simultaneously. All via holes have a diameter of di = 0.3 mm. A W-type connector (Anritsu: W1-103F) is launched underneath the antenna fixture for transmitting signal to the L-shaped probe. Four metallic screws were used to fix the substrate on the antenna fixture with size of 10 mm × 10 mm. Duroid 5880 substrate with thickness t = 0.787 mm, metal thickness tm = 1/2 oz, εr = 2.2 and tanδ = 0.004 at 60 GHz [15] was chosen as our substrate for its low loss tangent and approximately λ/4 height. Detailed dimensions of the proposed antenna are summarized in Table I. III. O PERATION P RINCIPLE To understand the operating principle of the antenna, the current distributions on the antenna and the electric field on the radiating aperture of the vertically shorted microstrip patch antenna are depicted as shown in Fig. 2, where T is the period of oscillation at 60 GHz. At time t = 0, the current densities on the planar patches reach maximum, which means that the electric dipoles are strongly excited. Hence, the E-plane of the antenna is xoz-plane. At time t = T/4, the electric field density on the radiating aperture of the vertically shorted patch antenna reach maximum, which means the magnetic dipole are strongly excited. Hence, the H-plane of the antenna is yoz-plane. At time t = T/2, the currents on the electric dipoles attain maximum strength again in a direction opposite to that occurred at time t = 0. At time t = 3T/4, the electric field on the aperture of the magnetic dipole attains maximum strength again in a direction opposite to that occurred at time t = T/4. Consequently, the planar electric dipoles and shorted patch antenna are excited alternately with similar strength. Effectively, the equivalent electric and magnetic currents are 90◦ in phase difference, equal in strength, and orthogonal to each other. This demonstrates that the antenna operates as a complementary antenna. IV. E FFECT OF THE G ROUND P LANE Owing to the special characteristic of the complementary antenna, this antenna element has low back radiation. For further suppression

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Fig. 1. Geometry of the millimeter-wave ME dipole antenna.

TABLE I D IMENSIONS FOR THE M ILLIMETER -WAVE ME D IPOLE A NTENNA E LEMENT Fig. 3. Simulated radiation patterns of the millimeter-wave ME dipole antenna element on different ground planes, (a) antenna with a 3.15 mm × 3.15 mm ground plane, (b) antenna with a 5 mm × 5 mm ground plane, (c) antenna with a 10 mm × 10 mm ground plane.

λ is one wavelength in the substrate referring to the center frequency.

Fig. 2. ME dipole antenna operation. (a) Current distributions on the antenna element at different times, (b) electric field on the radiating aperture of the vertically shorted patch antenna at different times.

of back radiation, the size of the ground plane should be chosen properly. Typically, if the size of the ground plane is about 1λ0 × 1λ0 (5 mm × 5 mm at 60 GHz), a good radiation pattern can be maintained over a wide bandwidth [12]. However, this antenna has to be mounted on a metallic antenna fixture with a size of 10 mm × 10 mm, because the W-type connector (Anritsu: W1-103F) used has a width of 10.2 mm. Because of the thick antenna substrate and large ground plane, the antenna radiation pattern and gain can be deteriorated drastically by the excitation of strong surface wave. The proposed elements with ground planes of different sizes are compared in Fig. 3. It can be seen from Fig. 3(b) that the antenna placed on a 5 mm × 5 mm ground radiates identical E- and H-plane radiation patterns with low cross-polarization and back radiation levels. Fig. 3(a) shows the antenna on a small ground plane with nearly the same size as the planar electric dipoles (3.15 mm × 3.15 mm). As expected, the antenna element still achieves a large FBR of over 11 dB. The reason is that this element was designed according to the complementary antenna concept. As discussed in Section II-C, the two kinds of sources (i.e., a pair of electric dipoles and a vertically shorted patch antenna) of almost equal amplitude are combined together. Consequently, the radiation pattern is nearly identical in the E and H planes, and furthermore, the back radiation is suppressed. Fig. 3(c) shows the proposed antenna placed on a 10 mm × 10 mm ground. It can be observed that the cross polarization and back radiation levels are increased. Compared with the other two scenarios, the patterns in E- and H-planes are not identical at 60 GHz. This is due to the addition of the fields radiated at the edges of the substrate, contributed by the surface wave, with the direct radiation from the antenna element. And when the substrate has a larger size, the effect of the

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Fig. 4. Simulated SWRs and gains of the millimeter-wave ME dipole antenna element on different ground planes.

Fig. 5. Prototype of the millimeter-wave ME dipole antenna.

Fig. 6. Simulated and measured SWRs and gains of the millimeter-wave ME dipole antenna element.

surface wave is getting more significant on the antenna radiation. The simulated SWRs and gains for the three scenarios are shown in Fig. 4. It can be seen that the antenna on the smallest ground exhibits the stable gain of approximately 8 dBi. But the antenna operation band is shifted to higher frequencies from 46 to 70 GHz. The antenna on the 5 mm × 5 mm ground plane has a gain increasing with frequency and a wide impedance bandwidth. The antenna on the 10 mm × 10 mm ground plane also has the same wide bandwidth but has a lower gain of ~ 7.8 dBi owing to the aforementioned strong surface wave.

V. A NTENNA P ERFORMANCE AND D ISCUSSION The antenna prototype is shown in Fig. 5. Measurements on impedance bandwidth, gain and radiation pattern were accomplished by a millimeter wave band Agilent E8361A Network Analyzer with N5260-60003 waveguide T/R module and an in-house far-field millimeter wave antenna measurement system [14]. Fig. 6 shows the simulated and measured SWRs and gains. It can be seen that the measured impedance bandwidth is 51%, with SWR ≤ 2 from 41.5

Fig. 7. Simulated and measured radiation patterns of the millimeter-wave ME dipole antenna element.

to 69.5 GHz, which agrees well with simulations. In the operating frequency range, the measured gain is approximately 8 dBi that is lower than the simulated gain slightly. This may be owing to fabrication errors and the rough surface of the antenna fixture. The measured gain is generally consistent with simulations across the 50–70 GHz frequency band. The radiation measurement was done only within this frequency band instead of the whole operating band, which is due to the fact that our millimeter-wave measurement system can only be operated from 50 to 70 GHz. Fig. 7 shows the simulated and measured radiation patterns. Good agreement between measurements and simulations was achieved. It can be seen that the E- and H-plane patterns are generally symmetrical and exhibit good unidirectional radiation characteristic. Over the operating frequency band, the antenna has cross-polarized radiation of less than −20 dB and back radiation of smaller than −17 dB. Side lobes appear at frequencies above 65 GHz due to the surface wave.


TABLE II C OMPARISON B ETWEEN T HIS W ORK AND A NTENNA IN [14]

A comparison between our work and the design in [14] is shown in Table II. Different from the antenna in [14], the proposed antenna utilizes a pair of planar dipoles and a number of shorting pins arranged in L shape. More importantly, an L-shaped probe, instead of a T-strip [14], is employed to excite the antenna. Hence, the proposed antenna exhibits a wider impedance bandwidth and a lower cross-polarization level. Furthermore, the proposed antenna can be used in an array environment by using the coplanar waveguide feed network presented in [2]. Since this coplanar network can operate properly regardless of the substrate thickness, the antenna array can be built on a double-sided single-layer substrate with a large thickness of 0.787 mm. This array with a much higher gain not only provides wide impedance and gain bandwidths but also is low in fabrication cost. A design guideline of the proposed antenna is recommended as follows. 1) According to the center frequency of the operating band (λ), choose a proper substrate with a thickness of ∼ λ/4. 2) Design two identical planar dipoles which are shorted to the ground by using shorting pins. Choose the dimensions of the dipole arm with P1 ≈ 0.33λ, P2 ≈ 0.36λ. 3) Locate the shorting pins as close as possible to the inner corner of the rectangular metallic patches. 4) Design an L-shaped probe. Fine tune Fp , Fw and Fl to achieve a wide impedance bandwidth. 5) Carefully tune the gaps between the dipole arms, S1 , to further broaden the impedance bandwidth.

VI. C ONCLUSION A new wideband millimeter-wave magneto-electric dipole antenna at 60-GHz band, consisting of two planar dipoles and a shorted patch antenna, has been presented. The proposed antenna not only provides a wide impedance bandwidth of over 50% and a stable gain of approximately 8 dBi but radiates unidirectionally with low cross-polarization and back radiation levels. The operating principle of the antenna and the effect of the ground plane have been investigated. It is believed that this antenna is a promising candidate for low-cost wideband millimeter-wave communication systems.

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[5] K.-S. Chin, W. Jiang, W. Che, C.-C. Chang, and H. Jin, “Wideband LTCC 60-GHz antenna array with a dual-resonant slot and patch structure,” IEEE Trans. Antennas Propag., vol. 62, no. 1, pp. 174–182, Jan. 2014. [6] S. B. Yeap, Z. N. Chen, and X. Qing, “Gain-enhanced 60-GHz LTCC antenna array with open air cavities,” IEEE Trans. Antennas Propag., vol. 59, no. 9, pp. 3470–3473, Sep. 2011. [7] S.-W. Qu, K. B. Ng, and C. H. Chan, “Waveguide fed broadband millimeter wave short backfire antenna,” IEEE Trans. Antennas Propag., vol. 61, no. 4, pp. 1697–1703, Apr. 2013. [8] N. Ghassemi and K. Wu, “Planar high-gain dielectric-loaded antipodal linearly tapered slot antenna for E- and W-band gigabyte point-topoint wireless services,” IEEE Trans. Antennas Propag., vol. 61, no. 4, pp. 1747–1755, Apr. 2013. [9] M. Sun, Z. N. Chen, and X. Qing, “Gain enhancement of 60-GHz antipodal tapered slot antenna using zero-index metamaterial,” IEEE Trans. Antennas Propag., vol. 61, no. 4, pp. 1741–1746, Apr. 2013. [10] M. Sun, X. Qing, and Z. N. Chen, “60-GHz end-fire fan-like antennas with wide beamwidth,” IEEE Trans. Antennas Propag., vol. 61, no. 4, pp. 1616–1622, Apr. 2013. [11] K. M. Luk and H. Wong, “A new wideband unidirectional antenna element,” Int. J. Microw. Opt. Technol., vol. 1, no. 1, pp. 35–44, Jun. 2006. [12] K. M. Luk and B. Wu, “The magnetoelectric dipole, a wideband antenna for base stations in mobile communications,” Proc. IEEE, vol. 100, no. 7, pp. 2297–2307, Jul. 2012. [13] J. J. Xie, S. L. Deng, and Y. Z. Yin “A wideband magneto-electric dipole antenna using CPW structure,” Prog. Electromagn. Res. C, vol. 41, pp. 217–226, 2013. [14] K. B. Ng, H. Wong, K. K. So, C. H. Chan, and K. M. Luk, “60 GHz plated through hole printed magneto-electric dipole antenna,” IEEE Trans. Antennas Propag., vol. 60, no. 7, pp. 3129–3136, Apr. 2012. [15] D. Liu, B. Gaucher, U. Pfeiffer, and J. Grzyb, Advanced Millimeter-Wave Technologies: Antennas, Packaging and Circuits. Hoboken, NJ, USA: Wiley, 2009.