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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013
High-Efficiency 60-GHz Printed Yagi Antenna Array Zouhair Briqech, Student Member, IEEE, Abdel R. Sebak, Fellow, IEEE, and Tayeb. A. Denidni, Senior Member, IEEE
Abstract—A low-cost and highly efficient printed Yagi antenna structure with high radiation efficiency is presented. The proposed prototype consists of two stacked planar layers. The first layer electromagnetically couples energy to the printed Yagi array parasitic elements located on the second layer. A reduced-size 60-GHz prototype (1.0687 0.8015 cm ) is designed to operate over the Industrial, Scientific, and Medical (ISM) band. The measured impedance bandwidth is more than 5 GHz with a gain of 10 dB measured at 60 GHz. With these features, this antenna achieves a low profile that makes it suitable for millimeter-wave monolithic microwave integrated circuits (MMICs) packaging as well as for short-range wireless communications and imaging applications. Index Terms—Microstrip antennas, millimeter-wave antennas, parasitic antennas, Yagi–Uda arrays.
I. INTRODUCTION
I
N RECENT years, there has been a great demand for the implementation of millimeter-wave (MMW) antennas in industrial communications systems for high-data-rate short-range wireless transceiver and imaging systems. The unlicensed band, operating at around 60 GHz, is mainly used for industrial, scientific, and medical (ISM) applications [1]. The 50–75-GHz V-band is used for short-range wireless applications due to the high signal attenuation caused by its absorption by oxygen molecules [2]. This disadvantage is partially compensated for by using high-gain directive antennas. However, this disadvantage can also be seen as an asset since high signal attenuation within the V-band allows for multiples of the same frequency to be reused in the same coverage of wireless personal area networks (WPANs) [3]. As a result of the above-mentioned characteristics, an antenna with a sectorized radiation pattern is essential for the suppression of unwanted interference. This calls for a high-gain quasi-omnidirectional radiation pattern with a beamwidth of around 30 –60 to alleviate the effects of the propagation loss. To achieve such requirements for these applications, an endfire antenna is needed. Yagi–Uda printed microstrip antennas, available in different printed configurations, have long been a preferred choice [4]. Manuscript received July 14, 2013; revised September 08, 2013; accepted September 20, 2013. Date of publication September 24, 2013; date of current version October 08, 2013. This work was supported in part by the Canadian NSERC Discovery Program and King Abdulaziz City of Science and Technology (KACST)—Technology Innovation Center in RFTONICS at King Saud University under a grant. Z. Briqech and A. R. Sebak are with the Department of Electrical and Computer Engineering, Concordia University, Montréal, QC H3G 2W1, Canada (e-mail:
[email protected]). T. A. Denidni is with the Wireless Communications Department, INRS, Montreal, QC H5A 1K6, Canada (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2013.2283199
The first Yagi–Uda printed microstrip antenna contains four patches: driven and reflector elements, and two directors electromagnetically coupled to each other to create a main beam directed 30 –40 from the broadside direction [5]. This design, based on the conventional Yagi–Uda’s principle design, couples electromagnetic energy from the driven patch to the remaining parasitic patches not only through the space between the elements, but also through the surface wave in the substrate. Meanwhile, to enhance the radiation pattern of the antenna, many techniques have been adopted. A design of six vertically stacked layers of printed microstrip patches forming a Yagi array is reported in [6]. In this structure, a patch is added to the top layer of the driven patch, and a single reflector is placed below the driven patch. In [7], the same principle is employed using circular patches at 60 GHz. A printed Yagi antenna, presented by DeJean and Tentzeris [8], features a direct feedline and consists of reflector, driven, and four director elements instead of the two used in [9]. The printed Yagi antenna presented in [8] has a high power efficiency that increases the gain and directivity of the antenna and is suitable for compact integration with monolithic microwave integrated circuits (MMICs). The aim of this letter is to design a low-cost antenna with high radiation efficiency suitable for MMW-MMIC packaging used in wireless communications and imaging systems. Moreover, this letter focuses on investigating the influence of the parasitic printed Yagi array substrate thickness and the size of the Yagi array patches on the antenna performance. The proposed double-layer compact Yagi array antenna operating at 60 GHz is based on the general concept of the printed microstrip Yagi antenna array [4]–[9]. The arrangement of the parasitic elements above the feeding patch antenna elements is widely considered to be an effective technique for improving the radiation efficiency, as examined in [6], [7], and [10]–[14]. The stacked design is commonly used in MMIC structures due to its efficiency in width size reduction. A multistacked parasitic antenna, however, can problematically increase the structure’s height [6]. The design introduced in [7] with six stacked layers achieved 4.2% bandwidth at 60 GHz, but adds more complexity to the fabrication process. Instead of such a multistacked structure, the proposed antenna shown in Fig. 1 introduces only one stacked layer for size and weight reduction while enhancing the antenna performance, and achieves 7.2% in bandwidth at 60 GHz. Parasitic elements in the second (top) layer are designed with the same design principle as the printed Yagi antenna in the first layer. The combination of the coupling between these layers provides an additional increase in the radiated power of the antenna and its radiation efficiency, compared to a single-layer structure. Moreover, the parasitic structure of the second layer creates multifrequency resonance that improves the bandwidth while maintaining the same radiation
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Fig. 2. Photograph of the fabricated prototype: (a) first layer of the Yagi array, (b) parasitic array, (c) combined layers, and (d) calculated electric field distribution.
Fig. 1. Geometry of the proposed antenna: (a) op view of the parasitic layer, (b) side view of both layers, and (c) top view of the first layer.
characteristics of the first-layer printed Yagi array. Simulated results show that the proposed two-layer structure has an 8.5-GHz impedance bandwidth—compared to a 5.3-GHz bandwidth for a single-layer structure. In addition, the parasitic layer increases radiation efficiency and reduces backlobe radiation.
effective aperture of the antenna. The separation between director elements improves the directivity of the antenna. The 50- feedline characteristic impedance is matched to the first patch using three feedline impedance transitions: 70.4, 100, and 144 . Quarter-wave transformers improve the input impedance matching and chamfering edges minimize the feedline edge radiation. The dimensions of the parasitic Yagi array shown in Fig. 1(a) are: , , , , , , , , , and . The gap between the elements is . The distances between the director1 and director2 elements are and .
II. ANTENNA DESIGN AND CONSTRUCTION
III. PARAMETRIC ANALYSIS AND RESULTS
The configuration of the proposed antenna is illustrated in Fig. 1. The antenna is composed of two layers. The first layer consists of a two-element split reflector patch and two pairs of directors. The position of each element is optimized to achieve better impedance matching and to maximize the antenna directivity. The second printed Yagi array, which is mounted on top of the first layer, contains the reflector, the parasitic driven elements, and an additional pair of two director elements. This technique improves the radiation efficiency when the parasitic elements are positioned above the feeding Yagi array of the first layer. Fig. 1(b) shows a side view of the antenna, where the submm strate thicknesses of layer1 and layer2 are mm, respectively. Both layers have the same and , dielectric constant of Rogers 5880 substrate ( at 10 GHz). Fig. 1(c) shows the top view of the first layer, where the proposed Yagi array elements are mm). The printed designed to operate at 60 GHz ( Yagi array, shown in Fig. 1(c), has the optimized parameters: , , , , , , , , and . . FurtherThe gap between the elements is more, the spaces between director1 and director2 elements and , thereby influencing the are
The proposed antenna was studied numerically using CST Microwave Studio to examine the coupling mechanism between the radiated elements. A photograph of the prototype antenna is illustrated in Fig. 2. Fig. 2(a) shows the first layer of the proposed Yagi array antenna, and Fig. 2(b) shows its second layer. In order to understand the behavior of the proposed antenna structure, the calculated electric field distribution of the printed Yagi array at 60 GHz is presented in Fig. 2(d). In this figure, four different phases are monitored at 0 , 45 , 90 , and 180 . The electrical field is coupled from the driven elements to the surrounding parasitic patches on the first layer. The electrical field is then coupled to the second layer’s parasitic patches. The coupling between the two layers is governed by the thickness of the second layer ( ). This parameter influences the bandwidth and the gain of the antenna, as shown in Fig. 3. The increase in thickness leads to a downshift in resonance frequencies and is therefore a measure to tune the resonant frequency to the desired band and enhances the overall matched impedance bandwidth. Generally, to obtain a strong coupling between driven and parasitic patches on the first layer, the gap between the elements should be less than the substrate thick. Furthermore, the parasitic patch ness: dimensions should be smaller than those of the driven patch, by a ratio of 0.8 to 0.95. Additionally, the scale ratio of the parasitic patches ( ) on the second layer in relation to the scale
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013
Fig. 5. Calculated total radiation efficiency of the single and double layer. Fig. 3. Simulated total radiation efficiency and return loss of the proposed antenna, considering the effect of varying the thickness of the substrate of the parasitic layer.
Fig. 6. Measured and calculated reflection coefficient of the single and double layer.
of the proposed Fig. 4. Simulated total radiation efficiency and return loss antenna, considering the effect of varying the parasitic patches’ size.
size of the first layer should be between 0.7 and 1, as shown in Fig. 4. When the size of the second layer’s parasitic elements is greater than that of the driven layer, multiple resonate frequencies are created within the same ISM band. Furthermore, when the parasitic elements of the second layer have a smaller size, the resonance frequency of the first layer dominates. Notably, to enhance the coupling between the two layers, the parasitic patch’s size for the second layer should be smaller than the driven patch. This also helps reduce the backlobe radiation and enhances both the antenna’s matched impedance bandwidth and its radiation efficiency. In a printed Yagi antenna, the dielectric constant governs the guided wavelength, patch size, separation distance, and the gaps between patches. The antenna elements are coupled both through space and by a surface wave in the substrate. As a result, to enhance the constructive mutual coupling for the Yagi radiating elements, the dielectric constant and the thickness of the substrate are carefully chosen to improve the antenna’s performance. Simulated results show that the proposed double-layer structure increases the 10-dB matched impedance bandwidth by more than 3 GHz and improves the radiation efficiency over the ISM band, as shown in Fig. 5. Fig. 6 shows the measured and calculated reflection coefficient of the single- and double-layer configurations. The reflection coefficient was measured with the TRL-kit for a more accurate calibration of the two configuration models. The measured impedance bandwidth ( dB) covers frequencies from 60.7 to 65 GHz with 7.2% at 60 GHz. Furthermore, about
Fig. 7. Illustration of the anechoic chamber up to 110 GHz, showing the antenna mounted on a Southwest End Launch V-connector and the setup for measuring the radiation pattern.
5 GHz can be achieved using the two close resonances of the driven and parasitic elements. Notably, the resonant frequency is shifted up due to possible variations in the material properties at 60 GHz and the effects of fabrication tolerance. The material used in the simulation is RO5880. It has measured at 10 GHz with a loss tangent of . These parameters are typically frequency-dependent and may be different at 60 GHz. Moreover, based on the measured results of the reflection coefficient for several prototypes, we observed that the material permittivity of the RO5880 substrate is found to be slightly smaller than . In Fig. 6, the of the single- and double-layer cases are calculated using the dielectric constant taking into account potential variation with frequency. The Yagi array radiation pattern is measured in an anechoic chamber, as shown in Fig. 7. The copolarized radiation patterns in the E-plane ( -plane) and H-plane ( -plane) are measured over the ISM band, at 59–64 GHz, as illustrated in Fig. 8. The 3-dB beamwidth of both the H-plane and E-plane are about 25 –40 , and the antenna achieves a radiation efficiency greater
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The backside radiation is associated with the size of the ground plane. A large-sized ground plane reduces the backlobe and sidelobes that are caused by the fringing field in the far edges between the radiating elements and the ground plane. The backlobe is partially reduced by optimizing the reflector elements of the printed Yagi antenna IV. CONCLUSION A 60-GHz double-layer printed Yagi array antenna has been proposed, fabricated, and tested. The antenna performances of the proposed antenna and a single-layer Yagi antenna are compared and analyzed. By utilizing a full Yagi parasitic array, we showed improvement in both impedance bandwidth and radiation efficiency. Furthermore, the parasitic layer was shown to be a robust method that can be implemented in many planar models to enhance impedance matching, gain, F/B ratio, and radiation efficiency. Consequently, these features make the antenna potentially useful for MMW-MMIC packaging circuits, short-range wireless communications, and imaging applications. REFERENCES
Fig. 8. Measured and calculated radiation pattern results of the ISM band, from 59 to 64 GHz.
than 94% at 60 GHz. The front-to-back (F/B) ratio at 60 GHz in the simulated results achieves 29 dB for the double layer, while it is 23 dB for the single layer. The measured absolute gain of the proposed prototype is 10 dB with an E-plane F/B ratio of more than 18 dB. The radiation behavior of a printed Yagi array is very sensitive to various array parameters. The peak of the main beam is tilted by 25 –45 from the broadside direction toward the direction of the director patches. This kind of behavior occurs due to the strong coupling between the driven and parasitic patches. The measured radiation pattern results in Fig. 8 show that the main beam is tilted by 20 –40 over the ISM band with a slight difference from calculated results. This difference is potentially caused by several factors, such as fabrication tolerances, variations of the substrate dielectric constant within the V band, and the effect of connectors during the radiation pattern measurements. Simulated results show that the tilted beam angle depends on the substrate thickness, dielectric constant, gap distance, patch separation, and the size of the parasitic patches. These parameters show a high degree of interrelation effect on the main beam tilted angle direction. Thus, for the E-plane radiation pattern, the minor deviation angle increases when the frequency increases, as shown in Fig. 8.
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