W-band Microstrip Reflectarray with Double-cross Element for Bandwidth Improvement Ji Hwan Yoon, Young Joong Yoon
Woo-sang Lee, Joon-ho So
Department of Electrical and Electronic Engineering Yonsei University Seoul, Korea
[email protected],
[email protected]
Agency for Defense Development Daejeon, Korea
Abstract—A broadband microstrip reflectarray is designed with the proposed element at W-band. The element consists of two cross-shaped resonators. Based on the analysis on the resonance mechanism of the double-cross element, the reflection phase of the element is controlled to achieve broadband characteristics by adjusting the dimensions of the element. Using the proposed element, a reflectarray with diameter of 50 mm is designed at f0 = 95 GHz. At f = f0, the peak gain of 31.73 dBi is achieved with aperture efficiency of 66.1%. Also, 1-dB gain bandwidth of 15.9% is achieved at W-band. Keywords—Broadband; Reflectarray; Reflector; W-band;
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l1 g1
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I. INTRODUCTION
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Reflector antennas are used in various applications where highly directive radiation patterns are required. Reflectors are designed to direct the illuminated field from feeders to the target direction by shaping the shape of the reflector based on geometrical optics. However, such curved shape increases fabrication cost and the volume of the antenna. To overcome the shortcomings of the reflector antennas, reflectarray antennas with printed elements were proposed [1]. Compared to the curved reflector antennas, microstrip reflectarrays can be fabricated in low-profile with simple PCB etching process for both single- and dual-reflector configurations [2]. Unfortunately, the gain bandwidth of the microstrip reflectarrays is limited due to the narrow band characteristics of the elements and different spatial phase delay from the feeder to the reflectarray surface [3]. For reflectarrays with typical patch element with variable size, the achievable bandwidth is less than 7% [1]. To improve the bandwidth characteristics of the reflectarrays, various types of elements have been proposed including multi-layer stacked structures [3], [4] and single-layer multi-resonant structures [5]í[7]. However, the multi-layer elements require extra fabrication and material costs. The multi-resonant single-layer elements require an additional foam layer or electrically thick substrate to achieve broadband characteristics (typically, larger than 0.12Ȝ0, where is Ȝ0 the free space wavelength at the design frequency f0). Thus, in this paper, a broadband reflectarray element with electrically thin substrate is proposed for W-band application. At the design frequency of f0 = 95 GHz, the element is designed on a substrate with thickness h = 0.254 mm (=0.08Ȝ0).
Fig. 1 The proposed double-cross element.
With the proposed element, 1-dB gain bandwidth of 15.9% is achieved in W-band. In this work, CST Microwave Studio has been used for full wave analysis and design. II. PROPOSED ELEMENT A. Element Configuration The proposed element consists of two cross-shaped resonators as in Fig. 1, where the first cross with length l0 is shown in blue square and the second cross (which is divided into four L-shaped resonators) with length l1 is shown in red square, respectively. The element spacing p is 1.6 mm (0.507Ȝ0 at f0 = 95 GHz). The reflection phase is controlled by changing l0 with l1 being proportional to l0 with the following relation. l1 = r1×l0+w1+g1
(1)
where r1 = 0.72. The gap distance between the center and side crosses g0 is fixed to 0.75 mm and g1 = 0.16 mm. The widths of the crosses are w0 = 0.1 mm and w1 = 0.17 mm, respectively. The proposed element is designed on a substrate with a thickness h = 0.254 mm, relative permittivity İr = 2.2, and loss tangent tanį = 0.0009. B. Operating Mechanism In Fig. 2, the reflection phase of the double-cross element when l0 = 1.5 mm is shown. The reflection phase has been
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(a)
(b)
Fig. 2 The reflection phase of the double-cross element.
calculated assuming that the element is surrounded by an infinite array of identical elements (the same type and size), and the y-polarized incident wave is propagating to the normal direction of the element surface. It is observed that the reflection characteristic exhibits two resonances at f1 = 59.05 GHz and f2 = 85.5 GHz. In Fig. 3, the electric field and surface current at each resonance frequency are shown. It is observed that the at f1, strong electric field is observed between the first cross edges of the adjacent elements in y-axis (Fig. 3(a)), and surface current is mainly induced at the vertical part of the first cross (Fig. 3(b)). At f2, electric field is strong at the gap between the first cross and the second cross (Fig. 3(c)), and surface current is mainly induced at the second cross (Fig. 3(d)). Based on these observations on the operating mechanism of the double-cross element, it is possible to control the reflection phase of the element to achieve broadband characteristics by adjusting the dimensions of the crosses. The final dimensions of the double-cross element are given in Section II. A, and the resultant reflection phases at f = 80, 85, … 110 GHz are shown in Fig. 4. It is observed that almost linear and parallel reflection phase curves are obtained in Wband for the range of l0 from 0.9 mm to 1.4 mm, which is desirable for broadband reflectarray design.
(c) (d) Fig. 3 The electric field and surface current at the resonant frequencies of the double-cross element: (a) electric field at f1 = 59.05 GHz, (b) electric field at f2 = 85.5 GHz, (c) surface current at f1, and (d) surface current at f2.
Fig. 4 The reflection phases of the double-cross element at various frequencies.
Although the reflection phases Fig. 4 are calculated under normal incidence of plane wave, the element can be under oblique incidence in actual reflectarray environment. Thus, the reflection phase curves under various angles of incidence for both TE and TM polarizations are obtained at f = 95 GHz, as in Fig. 5. Within the incidence angle (șinc) of 30°, the reflection phase deviation from the reflection phase curve under normal incidence is small enough to be ignored. When șinc = 15°, the maximum value of the reflection phase differences between the normal incidence and oblique incidence are 11.0° (TE) and 12.2° (TM), respectively. When șinc = 30°, the maximum phase differences between the normal incidence and oblique incidence are 25.1° (TE) and 25.4° (TM) , respectively. III. REFLECTARRAY DESIGN Using the designed double-cross element, a broadband microstrip reflectarray is designed at W-band. The design frequency is f0 = 95 GHz and the reflectarray is designed to achieve a single collimated beam at bore-sight. As a feeder, a
Fig. 5 The reflection phases of the double-cross element under various angles of incidence at f = 95 GHz.
circular dual-mode horn antenna with the aperture diameter Df = 7.4 mm is used. The distance between the phase center of the feeder and the reflectarray center is F = 50 mm (§ 15.8Ȝ0). The diameter of the reflectarray is D = 50 mm, thus, F/D = 1. The
Fig. 6 The required reflection phase distribution for the reflectarray design at f = 95 GHz.
Fig. 8 The radiation patterns of the double-cross element reflectarray at f = 95 GHz.
Fig. 9 The gain against frequency of the double-cross element reflectarray.
REFERENCES
Fig. 7 The designed double-cross element reflectarray.
total number of the elements is 725. The required reflection phase distribution on the reflectarray surface at f0 is shown in Fig. 6, and the designed reflectarray is shown in Fig. 7. From full wave simulation, the radiation patterns of the reflectarray have been calculated. In Fig. 8, the radiation patterns at f0 = 95 GHz is shown. The peak gain at f0 is 31.73 dBi and corresponding aperture efficiency is 66.1% which is higher or comparable to that of other reflectarrays. The side lobe levels are -18.2 dB (E-plane) and -16.7 dB (H-plane), respectively. The half-power beam widths are 3.8° (E-plane) and 3.6° (H-plane), respectively. The gain against frequency is shown in Fig. 9 and the achieved 1-dB gain bandwidth is 15.9%. IV. CONCLUSION In this paper, a broadband microstrip reflectarray has been presented which is designed with the proposed double-cross element at W-band. By analyzing the operating mechanism of the element, the element has been designed to achieve broadband characteristics. The reflectarray has been designed at 95 GHz and 1-dB gain bandwidth of 15.9% has been achieved.
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