IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010
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High-Isolation X-Polar Antenna Mohsen Kaboli, Seyed Abdollah Mirtaheri, and Mohammad Sadegh Abrishamian
Abstract—Antennas with dual linear slant polarization are necessary for modern personal communication base station applications. X-Polar antenna stands for an antenna with dual linear slant polarization. The most significant feature of the proposed antenna is high isolation between two different polarizations. The structure used here includes a reflector, a U-shaped strip, a cross-slot, a patch, and a director. Simulation and measurement results show minimum 30 dB isolation between the two polarizations. This structure yields half-power beamwidth (HPBW) of 65 , a cross-polarization level of less than 15 dB, and a voltage standing wave ratio (VSWR) of better than 1.5. This structure is useful for dual-band and dual-polarized applications such as the Global System for Mobile communications (GSM) and the Universal Mobile Telecommunications System (UMTS) frequency range. Index Terms—Cross-slot, dual linear slant polarization, isolation, U-shaped strip.
I. INTRODUCTION N MODERN communication base stations operating in the Global System for Mobile communications (GSM) frequency band, dual linear slant 45 polarization is widely used to combat multipath propagation effects [1]. To achieve a 23% impedance bandwidth, a voltage standing wave ratio (VSWR) less than 1.5, and isolation more than 30 dB between orthogonal polarizations, the U-shaped strip feed network of the antenna is mounted on the bottom side of a printed circuit board (PCB). Moreover, cross-slots are etched into the top side of the ground plane. The patch and the director placed above the PCB are excited by electromagnetic coupling through the slots. An approximate amount of 65 is desired for half-power beamwidth (HPBW) in horizontal plane. Since the intrinsic HPBW of a patch antenna is greater than 65 [2], the stated desired value is achievable through use of a patch, director, and an aluminum (AL) frame as a reflector. To decrease backward radiation, an electrical wall was used as a reflector. This modified antenna structure is illustrated in Fig. 1. As shown by Fusco et al. in [1], the slant linear 45 polarization was a result of combining two elements. Thus, it may be inferred that four elements are necessary for a dual slant polarized antenna. However, the structure introduced in this letter uses a single element instead of four ones. This topology provides a small and low-cost structure that is especially appropriate for array applications.
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Fig. 1. 2D view of antenna structure. TABLE I PARAMETERS AND VALUES
General dimensioning formulas for such an antenna are well known and can be found in the literature [3], [4]. This letter is organized as follows. Section II describes the antenna structure and design parameters. The simulation and measurement results are presented in Section III. Finally, conclusions are given in Section IV. II. ANTENNA STRUCTURE AND DESIGN PARAMETERS
Manuscript received March 13, 2010; revised April 06, 2010; accepted April 19, 2010. Date of publication May 03, 2010; date of current version May 17, 2010. The authors are with Electrical Engineering Department, K. N. Toosi University of Technology, Tehran 16314, Iran (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.2010.2049557
The proposed antenna consists of 10 major components: the PCB, U-shaped strips, cross-slots, reflector, AL frame, patch, director, radome, spacers, and connectors. The PCB is RF-35 , dissipation factor with relative dielectric constant , and dielectric thickness mm. Design parameters of the antenna and their values are shown in Table I. The length, width, and height are denoted as L, W, and
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010
Fig. 2. Cross-slots and U-shaped strips dimensions on RF-35 substrate.
Fig. 3. Perspective view of proposed antenna, (left) top and (right) bottom.
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Fig. 5. Copolar and cross-polar radiation patterns of port 45 from measurement and simulation results in (a) vertical plane and (b) horizontal plane.
Fig. 4. Measured and simulated return loss and isolation.
H, respectively. Indices F, p, d, and s stand for frame, patch, director, and slot, respectively. is a symbol of the distance between the cross-slot and the patch. The distance between the . Finally, is the dispatch and the director is shown by tance between director and radome (cover). In addition to the aforementioned parameters, there are five tuning parameters in impedance matching and coupling magnitude shown in Fig. 2.
The length, width, and depth of the reflector are 100, 100, and 11 mm, respectively. -shaped air bridge of copper wire with 1-mm diamThe eter is used in the crossover point. The air bridge dimensions are 3.6 mm in width and 1.7 mm in height. As shown in Fig. 3, tuning stubs are used by both the patch and director for the purpose of return loss and isolation improvement. Characteristic impedance and length of microstrip lines in the feed network are as follows: mm mm mm mm mm mm
mm mm mm mm
KABOLI et al.: HIGH-ISOLATION X-POLAR ANTENNA
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TABLE II MEASUREMENT CHARACTERISTICS OF PROPOSED ANTENNA
TABLE III SIMULATED AND MEASURED RESULTS OF PROPOSED ANTENNA
Fig. 6. Copolar and cross-polar radiation patterns of port ment results in (a) vertical plane and (b) horizontal plane.
045 from measure-
Connection to the antenna element is made using semirigid UT-141 coaxial cable with SMA connectors. The other end of each coaxial cable is soldered to the microstrip line through an aluminium connector. In more detail, the coaxial cable shield and its inner conductor connect to the PCB’s ground plane and strip, respectively. III. SIMULATION AND MEASUREMENT RESULTS The proposed antenna, shown in Fig. 3, is not covered by a plastic cover. As shown by simulation, the plastic cover located about 35 mm above the director affects the antenna return loss since it makes the radiator electrically larger. The proposed antenna is simulated by using HFSS software. The measurement and simulation parameters including return loss and isolation are shown in Fig. 4. As can be seen, the an-
tenna return loss is less than 14 dB, and the isolation between two ports is more than 30 dB. The copolar and cross-polar radiation patterns of port 45 in vertical and horizontal planes at frequencies 870, 915, and 960 MHz are plotted in Fig. 5. The copolar and cross-polar radiation patterns of port 45 in vertical and horizontal planes are also shown in Fig. 6. Table II summarizes the properties of the proposed antenna. HPBW in the horizontal plane (H-plane) is less than 65 . It will be improved by reducing the height of the AL frame’s wall. The measured directivity is approximated as [5]
Table III shows simulated and measured results. A single element has a directivity of about 9 dB. For a dual-band application instance of GSM and DCS/UMTS, the structure of the second band may be positioned above the antenna’s director of the first band, whereas the antenna dimensions of the second band are smaller than those of the first band. IV. CONCLUSION The design parameters of a dual linear slant polarized antenna for the GSM frequency range are presented in this study. The
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results of simulations and measurements for this structure yield HPBW of 65 , a cross-polarization level of less than 15 dB in H-plane, an isolation of more than 30 dB, front-to-back ratio of less than 23 dB, and a VSWR of better than 1.5. Also, a single element has a directivity of order 8.5 dB. This proposed antenna element is applicable for dual-band and dual-polarized applications.
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010
REFERENCES [1] V. F. Fusco and P. H. Rao, “Wide-band dual slant linearly polarized antenna,” IEEE, Trans. Antennas Propag., vol. 51, no. 8, pp. 2014–2019, Aug. 2003. [2] T. Biedermann, “A dual-polarized patch antenna with high decoupling,” in Proc. INICA, 2007, pp. 166–177. [3] R. Garg, P. Bhartia, I. Bahl, and A. Ittipiboon, Microstrip Antenna Design Handbook. Norwood, MA: Artech House, 2001. [4] G. Kumar and K. P. Ray, Broadband Microstrip Antennas. Norwood, MA: Artech House, 2003. [5] C. Balanis, Antenna Theory, Analysis and Design, 3rd ed. New York: Wiley, 2005, ch. 6.
