A Coplanar Waveguide Fed Two Arm Archimedean Spiral Slot ...

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 2, FEBRUARY 2013

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A Coplanar Waveguide Fed Two Arm Archimedean Spiral Slot Antenna With Improved Bandwidth O. Ahmad Mashaal, S. K. A. Rahim, A. Y. Abdulrahman, M. I. Sabran, M. S. A. Rani, and P. S. Hall

Abstract—A compact wideband circularly polarized (CP) printed antenna design is presented in this communication. The proposed antenna consists of two Archimedean spiral slots, loaded with two chip resistors and fed by a coplanar waveguide (CPW) transmission line. The antenna is situated in one plane and fed without using an external balun or a matching network. The antenna has wideband input impedance bandwidth with circular polarization (CP), and broad beamwidth radiation pattern over 1.6:1 bandwidth. The study has shown that the proposed antenna is capable of improving the CP bandwidth and reducing the overall antenna size. Index Terms—Circular polarization, coplanar waveguide (CPW), spiral antenna, ultra wideband.

I. INTRODUCTION With the rapid progress in wireless technology the demand for compact, planar and wideband antennas that covers many communication services is becoming more attractive. Furthermore, many radio services, such as broadband satellite communication services, mobile systems, ground based and airborne direction finding systems require antennas that are compact, wideband, and circularly polarized. The conventional spiral antennas are used widely in many applications that require broadband bandwidth and circularly polarized pattern. However, the conventional feeding structure for planar spiral antennas is situated in the center of the spiral with the need for a balun and an impedance matching network and extends into the third dimension [1]. The major disadvantage of this method is that the feeding increases the antenna’s size and introduces additional design constraints which is incompatible with the modern compact communication devices. This feeding method had been studied intensively in the past [2], [3]. Feeding the spiral from the border is used as a technique to design a complete planar spiral antenna in the cost of limited bandwidth [1], [5]–[7]. The complete planar spiral antenna proposed in [1] is fed from the border by Marchand balun which is difficult to design and increases the antenna size. The balun is needed because of the balanced structure of the two arm spiral antenna and the unbalanced structure of the coaxial cable. Recently, (CPW)-fed slot antenna has received considerable attention owing to its preferable characteristics, easy fabrication and integration with monolithic microwave integrated circuits (MMIC), a simplified configuration with a single metallic layer low radiation loss and the less dispersion in comparison to a microstrip feed [4]. A CPW-fed 2-arm spiral slot antenna is proposed in [5], [6] with two different feeding ways. In spite of the advantages of a completely Manuscript received March, 8, 2011; revised October 25, 2011; accepted September 22, 2012. Date of publication October 23, 2012; date of current version January 30, 2013. O. Ahmad Mashaal, S. K. A. Rahim, A. Y. Abdulrahman, M. I. Sabran, and M. S. A. Rani are with the Wireless Communication Centre, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Malaysia (e-mail: [email protected]; [email protected]; Abdulrahman. [email protected]; [email protected]; [email protected]. P. S. Hall is with the Department of Electronic, Electrical and Computer Engineering, University of Birmingham, U.K. (e-mail: [email protected]). 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.2012.2224831

Fig. 1. The proposed spiral slot antenna geometry diagram.

planar structure and the absence of the balun and the impedance matching network in the feeding structure, the CP bandwidth is not clearly presented. Hence, it is difficult to evaluate the total performance of the antennas in compared to the conventional spiral antenna. A novel 3-arm CPW-fed spiral antenna with more detailed study on the CP bandwidth is presented in [7]. However, the size of the antenna is twice that of the conventional one and the spacing between the arms is too small which puts more constraints in the fabrication process. In this communication, the proposed antenna combines and modifies the two models reported in [5], [7] in order to optimize the performances in terms of axial ratio (AR) and radiation efficiency. The antenna is fed from the outer ends by a coplanar waveguide wave transmission line (CPW-T.L) without the need for an impedance transformer network or a balun, which allows a completely planar structure with a size close to the conventional one. Hence, the proposed antenna improves circular polarization bandwidth and with a reduced size. The performance of proposed antenna is studied in terms of reflection coefficient, AR, radiation pattern and efficiency. A prototype is implemented on a low cost FR-4 (Flame Resistance) substrate. The antenna has the advantages of ultra wide band impedance bandwidth and wideband circularly polarized radiation pattern bandwidth with good radiation efficiency. It should also be noted that there are other ways to achieve wideband CP bandwidth. For example in [8], a much wider CP bandwidth was achieved, although its feeding structure is more complex. II. ANTENNA DESIGN Fig. 1. shows the geometry of the proposed antenna which composed of two Archimedean spiral slots. One of the spiral slots is extended by half a turn to form the CPW feed line with the other one. A circular slot is inserted at the center of the antenna in order to facilitate the attachment of the two chip resistors. One of slots trace is given by [2]

(1) Where and are the inner and the outer radius of the spiral slot respectively, “ ” is the growth rate, while “ ” and “ ” are the distance from the spiral slot start point to the origin. The width of the slot, is .” given by “ The second slot is rotated 180 degrees, and its trace is given by

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(2)

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TABLE I UNITS FOR MAGNETIC PROPERTIES

Fig. 2. Antenna radiating and non-radiating sections.

The width of the feed line given by

depends on the growth rate,

which is (3)

The number of turns “N” is approximated by (4) : the maximum distance from the center to the border of the ( spiral; “g”: Slot width) The antenna characteristic impedance is designed to match with 50 devices, which is compatible with many standards. However, by referring to Table I, the size of the antenna and the number of turns that the slots are wound is influenced by the factors that define the antenna characteristic impedance and the lowest operating frequency. Thus, the lowest operating frequency can be approximated by using the equation used for the conventional spiral antenna [3]

Fig. 3. Return loss of proposed spiral antenna

.

(5) where c: the velocity of light. III. RADIATION MECHANISM The radiation characteristic of the conventional spiral antennas is explained through the band, which states that the spiral antenna radiates in the active region, or regions, where the currents in adjacent and opposite arms are in phase [2]. However, Wood [9] found out that the radiation mechanism of the microstrip spiral antenna is completely different and less effective than that of the open planar wire spiral antenna [10]. The same mechanism is applied to explain the operating principle of the proposed spiral slot antenna. By referring to Fig. 2, the two opposite magnetic currents balance each other, and no radiation occurs along the length (L_Feed). Yet, when the line is curved, the balance is disturbed and radiation occurs. This is because the curvature gives a finite difference between the outer radius (d1) and the inner radius (d2). This radiation is directly proportional to the circumferential length and slot widths of the CPW, whereas the former decreases with increasing radius of curvature. The loss due to radiation is small at very low frequencies, consequently making the reflected energy from the center to become high. Moreover, as the frequency increases, radiation increases. As a result, there is a loss of energy as it travels into the centre of the spiral antenna and the power reflection decreases. At the lower operating frequency, “ ,” there is a strong radiation from the outer ring. Since the radius is of order of one wavelength, the phase does not cancel the radiated field but rather it reinforces it. It has the right phase for generating approximate CP pattern, which nearly corresponds to the condition of “ radians” phase progression [10]. Furthermore, as the line and slot width increase at higher frequency, it permits the radiations to occur due to imbalance caused by the curved

CPW line. However, the phases of the radiation will also change according to the frequencies and this will result to radiation cancellation due to phase offsets. Note that the radiation is proportional to both line and slot widths of the proposed CPW. Comparatively, at the middle frequencies, the middle rings radiate approximately CP, due to their circumference being one wavelength radians. However, the inner rings and the phase progression almost may also radiate because their radius is very tight, and unless the attenuation of the wave going down the CPW line is high enough, there will be a significant amount of energy left to radiate from the inner rings and further degrades the axial ratio. IV. SIMULATION RESULTS The antenna performance is investigated using CST-microwave studios simulation software. A circular slot with radius of 2 mm was slotted at the center of the proposed antenna as shown in Fig. 2. Ini, is simulated to tially, a spiral antenna with number of turn, determine the effect of the circular slot adapted at the center of the antenna. The corresponding parameters for the simulated model are: , , and with FR-4 substrate permittivity of 4.6 and 1.6 mm height. The effect of the slot, in term of return loss is shown in Fig. 3. The reflection coefficient is slightly affected by the presence of the circular slot where the lowest operating frequency is slightly reduced to 2.1 GHz, almost identical to the theoretical calculated value using (5). In order to improve the radiation efficiency, the two spiral arms are tapered [7] by gradually increasing the slots width as shown in Fig. 4. This improvement is demonstrated in Fig. 5 where the radiation efficiency is slightly improved to be more than 60% within the range (2–3.7 GHz). The effect of arms tapering on the axial ratio is illustrated in Fig. 6. For this analysis, the number of turns, N is set to 4. Increasing the


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Fig. 4. Two different tapered spiral designs. (a) 0.3 mm/turn (b) 0.6 mm/turn.

Fig. 7. Simulated efficiencies of loaded and unloaded tapered spiral antenna at respective N and TR values.

Fig. 5. Simulated radiation efficiencies of two different TR values and non tapered one. The radiation efficiency was measured at the boresight direction .

Fig. 8. Simulated axial ratio for loaded and unloaded tapered spiral antenna at respective N and TR values.

Fig. 6. Simulated axial ratios, for two different TR values and non tapered one. . The AR was measured at the boresight direction

Tapering Rate (TR) from 0.3 mm/turn to 0.6 mm/turn has improved the axial ratio. The bandwidths of the axial ratio at the first and second are 0.5 GHz and 0.2 GHz resonant frequency, for respectively. Two factors that affect the axial ratio and efficiency of an antenna significantly are the reflected power at the end of the spiral arms and radiation from the inner rings. Therefore, a matched 75 chip resistor is connected across each line to absorb the incident wave and to minimize the reflections, while the number of turns, N is reduced from 4 to

2.5 to minimize the radiations from the inner rings. Fig. 7 shows the efficiency of the antenna after the resistor is added. It is noted that the radiation efficiency is higher without resistive loading in the band 2.0–2.8 GHz, while it starts to reduce from 2.8 GHz and above. This dramatic behavior is mainly due to mismatch and ohmic losses, which are insignificant. A higher impedance mismatch for the unloaded spiral is the reason for its lower radiation efficiency at certain frequency range. The higher impedance mismatch will increases the reflected power and reduces the efficiency of the spiral antenna. Fig. 8 shows the effects of tapering and adding resistive loading to , the number of the proposed spiral antenna. (Note: For with tapering rate, . For turns, , the number of turn, N is reduced to 2.5 with tapering rate, as shown in Table I). The axial ratio is significantly improved for the loaded antenna to be less than 3 dB over the band (2–3.3 GHz). It is well known that the polarization sense of the antenna is as same as the spiral winding direction from outer to inner arm. Therefore, the polarization sense of the antenna is left handed cirand right handed circularly pocularly polarized when where theta is larized in the opposite direction when

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Fig. 9. Fabricated prototype. Fig. 11. Comparison between simulated and measured results of normalized at 2.3 GHz for (a) E-plane and LHCP and RHCP (b) H-plane.

Fig. 10. Comparison between simulated and measured reflection coefficient.

defined as angle from z to axis in a spherical coordinate system for three dimensional space. As mentioned earlier, radiation from the inner rings at the middle frequencies can degrade the axial ratio. Therefore, reducing the number of turns, N from 4 to 2.5, has significantly improved the axial ratio.

Fig. 12. Comparison between simulated and measured results of normalized at 3.3 GHz for (a) E-plane and LHCP and RHCP (b) H-plane.

V. MEASUREMENTS Fig. 9 shows a fabricated prototype of the proposed antenna on a rectangular FR-4 printed circuit board (PCB), (74 mm 62 mm 1.6 , mm). The designed parameters of the antenna are: , , and tapered with . Fig. 10 shows a comparison between the simulated and the measured reflection coefficient. It illustrates good agreement between the simulated and the measured reflection coefficients. Furthermore, the antenna possesses a wide impedance band, with an initial opwidth with reflection coefficient less than erating frequency equals to 2.2 GHz. Radiation pattern for the prototype antenna was measured in an anechoic chamber. Figs. 11 and 12 show comparison between simulated and measured results of normalized LHCP and RHCP at 2.3 GHz and 3.3 GHz, respectively for both E-plane and H-plane. These figures show that the proposed antenna design are radiating in both LHCP and RHCP, thus proving its circular polarization properties. The simulated and measured results are in good agreement. The proposed spiral antenna has bi-directional radiation pattern with a slight angle shifting in its main direction. At E-plane, the antenna radiations are shifted by

Fig. 13. Comparison between simulated and measured axial ratio of the pro. posed antenna

positive 15 for both RHCP and LHCP at 2.3 GHz and negative 30 at 3.3 GHz. For H-plane, the radiations is shifted by approximately negative 20 at 2.3 GHz and positive 20 at 3.3 GHz. Fig. 13 shows the simulated and measured axial ratio results for the proposed antenna. The compared results are in good agreement. It is


Fig. 14. Comparison between simulated and measured radiation efficiency of . the proposed antenna

illustrated that the tapering introduced to the antenna design has significantly increased the bandwidths and improved the CP of the antenna over the frequency band (2–3.4 GHz). Adding the resistive loading has significantly minimized the reflected powers and improved the axial ratio of the spiral antenna. The measured axial ratio bandwidth of the proposed tapered and loaded spiral antenna is 1.3 GHz. Fig. 14 shows the simulated and measured radiation efficiency of the proposed antenna. The measured result is in good agreement with the simulated result. The overall radiation efficiency of the antenna is above 60%. The efficiency was significantly improved after the introduction of tapering and resistive loading. VI. CONCLUSION In this communication, a CPW-fed two-arm Archimedean spiral slot antenna, which is loaded with chip resistors, has been investigated. The proposed antenna combines and modifies two existing techniques in order to optimize axial ratio and radiation efficiency. The antenna structure is completely planar and it does not require an external matching network or a balun which eases design and fabrication process. A prototype was fabricated on inexpensive FR4 substrate via single side metallization. The axial ratio of the untapered spiral antenna has low CP properties, as the axial ratios are higher than 3 dB over the band. Adding resistive loading to the spiral arms minimize the reflected power and produce better axial ratio. Increasing the tapering rate from 0.6 mm/turn to 1.2 mm/turn has significantly improved the axial ratio. Measured is preaxial ratio of the tapered spiral antenna at sented and it is in good agreement with the simulated results over the band (2–3.4 GHz). The simulated and measured RHCP and LHCP of the proposed tapered spiral antenna are also presented. The proposed antenna is proven to exhibits CP as it radiates in both LHCP and RHCP. The radiation efficiency of the proposed antenna was significantly improved to above 60% after the introduction of tapering and resistive loading.

REFERENCES [1] E. Gschwendtner, J. Parlebas, and W. Wiesbeck, “Spiral antenna with planar external feeding,” in Proc. 29th Eur. Microwave Conf, 1999, vol. 1, pp. 135–138. [2] J. Kaiser, “The Archimedean two-wire spiral antenna,” IRE Trans. Antennas Propag., vol. 8, no. 3, pp. 312–323, May 1960. [3] P. C. Werntz and W. L. Stutzman, “Design, analysis and construction of an Archimedean spiral antenna and feed structure,” Proc. IEEE Energy and Information Technologies in the Southeast, vol. 1, pp. 308–313, Apr. 1989.

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[4] C. Canhui and E. Yung, “Dual-band dual-sense circularly-polarized CPW-fed slot antenna with two spiral slots loaded,” IEEE Trans. Antennas Propag., vol. 57, no. 6, pp. 1829–1833, Jun. 2009. [5] W. Chien-Jen and H. De-Fu, “Studies of the novel CPW-fed spiral slot antenna,” IEEE Antennas Wireless Propag. Lett., vol. 3, no. 1, pp. 186–188, Dec. 2004. [6] W. Chien-Jen and W. Jin-Wei, “CPW-fed two-arm spiral slot antenna,” in TENCON IEEE Region 10 Conf., Oct.–Nov. 30–2, 2007, pp. 1–4. [7] D. J. Muller and K. Sarabandi, “Design and analysis of a 3-arm spiral antenna,” IEEE Trans. Antennas Propag., vol. 55, no. 2, pp. 258–266, Feb. 2007. [8] L. Bian, Y.-X. Guo, and X.-Q. Shi, “Wideband circularly polarized slot antenna,” IET Microw., Antennas Propag., vol. 2, no. 5, pp. 497–502, 2008. [9] C. Wood, “Curved microstrip lines as compact wideband circularly polarized antennas,” IEE J. Microw. Opt. Acoust., vol. 3, no. 1, pp. 5–5, Jan. 1979. [10] J. R. James, P. S. Hall, and C. Wood, Microstrip Antenna Theory and Design. London: Peter Peregrinus, 1981, pp. 206–210.

A Broadband Dual-Polarized Omnidirectional Antenna for Base Stations XuLin Quan and RongLin Li

Abstract—A broadband vertically/horizontally dual-polarized omnidirectional antenna is proposed for mobile communications. The dual-polarized antenna is a combination of a modified low-profile monopole for vertical polarization (VP) and a circular planar loop for horizontal polarization (HP). The modified low-profile monopole is a circular folded patch shorted by four tubes while the circular loop consists of four half-wavelength arc dipoles. The dual-polarized omnidirectional antenna achieves a (1.7–2.2 GHz) with an isolation of around 40 dB. bandwidth of The gain variations in the horizontal plane are less than 2.5 dB for VP and 1.5 dB for HP. An eight-element dual-polarized antenna array is developed for base station applications. The antenna gains of the array for both VP with a difference of less than 1 dB. The beamwidths and HP are for VP and for HP. The cross-poin the vertical plane are larization levels in the horizontal plane for both VP and HP are lower than . Index Terms—Base station, broadband antenna, dual-polarized antenna, high isolation, omnidirectional antenna.

I. INTRODUCTION Nowadays polarization diversity that combines pairs of antennas with orthogonal polarizations has been widely used in mobile communications [1]–[5]. For a 360 full coverage vertical/horizontal polarization diversity scheme, vertically/horizontally dual-polarized omnidirectional antennas are needed for base stations [6]. In modern mobile Manuscript received July 05, 2012; accepted September 19, 2012. Date of publication October 09, 2012; date of current version January 30, 2013. This work was supported in part by the National Natural Science Foundation of China (60871061), in part by the Guangdong Province Natural Science Foundation (8151064101000085), and in part by the Specialized Research Fund for the Doctoral Program of Higher Education (20080561). X. Quan and R.L. Li are with the School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510641, China (e-mail: [email protected]). 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.2012.2223450

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