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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 2, FEBRUARY 2013 939 A Coplanar Waveguide Fed Two Arm Archimedean SpiralSlot 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 an-tenna design is presented in this communication. The proposed antennaconsists of two Archimedean spiralslots, loaded with two chip resistors andfed by a coplanar waveguide (CPW) transmission line. The antenna is sit-uated in one plane and fed without using an external balun or a matchingnetwork. The antenna has wideband input impedance bandwidth with cir-cular polarization (CP), and broad beamwidth radiation pattern over 1.6:1bandwidth. 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), spiralantenna, ultra wideband. I. I NTRODUCTION With the rapid progress in wireless technology the demand for com- pact, planar and wideband antennas that covers many communicationservices is becoming more attractive. Furthermore, many radio ser-vices, such as broadband satellite communication services, mobile sys-tems, ground based and airborne direction fi nding systems require an-tennas that are compact, wideband, and circularly polarized. The con-ventional spiral antennas are used widely in many applications that re-quire broadband bandwidth and circularly polarized pattern.However, the conventional feeding structure for planar spiral an-tennasissituatedinthecenterofthespiralwiththeneedforabalunandan impedance matching network and extends into the third dimension[1].The majordisadvantage ofthis methodis thatthe feedingincreasesthe antenna’s size and introduces additionaldesignconstraints which isincompatible with the modern compact communication devices. Thisfeeding method had been studied intensively in the past [2], [3].Feeding the spiral from the border is used as a technique to designa complete planar spiral antenna in the cost of limited bandwidth [1],[5]–[7]. The complete planar spiral antenna proposed in [1] is fed fromthe borderbyMarchandbalunwhichis dif fi culttodesignandincreasesthe antenna size. The balun is needed because of the balanced struc-ture of the two arm spiral antenna and the unbalanced structure of thecoaxial cable.Recently, (CPW)-fed slot antenna has received considerable atten-tion owing to its preferable characteristics, easy fabrication and inte-gration with monolithic microwave integrated circuits (MMIC), a sim- pli fi ed con fi guration with a single metallic layer low radiation loss andthe less dispersion in comparison to a microstrip feed [4].A CPW-fed 2-arm spiral slot antenna is proposed in [5], [6] withtwo different feeding ways. In spite of the advantages of a completely Manuscript received March, 8, 2011; revised October 25, 2011; acceptedSeptember 22, 2012. Date of publication October 23, 2012; date of current ver-sion 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, Fac-ulty of Electrical Engineering, Universiti Teknologi Malaysia, Malaysia(e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]. S. Hall is with the Department of Electronic, Electrical and Computer En-gineering, University of Birmingham, U.K. (e-mail:
[email protected]).Color versions of one or more of the fi gures in this communication are avail-able online at http://ieeexplore.ieee.org.Digital Object Identi fi er 10.1109/TAP.2012.2224831Fig. 1. The proposed spiral slot antenna geometry diagram. planar structure and the absence of the balun and the impedancematching network in the feeding structure, the CP bandwidth is notclearly presented. Hence, it is dif fi cult to evaluate the total perfor-mance of the antennas in compared to the conventional spiral antenna.A novel 3-arm CPW-fed spiral antenna with more detailed study onthe CP bandwidth is presented in [7]. However, the size of the antennais twice that of the conventional one and the spacing between the armsis too small which puts more constraints in the fabrication process.In this communication, the proposed antenna combines and modi- fi es the two models reported in [5], [7] in order to optimize the per-formances in terms of axial ratio (AR) and radiation ef fi ciency. Theantenna is fed from the outer ends by a coplanar waveguide wavetransmission line (CPW-T.L) without the need for an impedance trans-former network or a balun, which allows a completely planar structurewith a size close to the conventional one. Hence, the proposed antennaimproves circular polarization bandwidth and with a reduced size.The performance of proposed antenna is studied in terms of re fl ec-tion coef fi cient, AR, radiation pattern and ef fi ciency. A prototype isimplemented on a low cost FR-4 (Flame Resistance) substrate. The an-tenna has the advantages of ultra wide band impedance bandwidth andwideband circularly polarized radiation pattern bandwidth with goodradiation ef fi ciency. It should also be noted that there are other ways toachieve wideband CP bandwidth. For example in [8], a much wider CP bandwidth was achieved, although its feeding structure is morecomplex.II. A NTENNA D ESIGN Fig.1.showsthegeometryoftheproposedantennawhichcomposedof two Archimedean spiral slots. One of the spiral slots is extended byhalf 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 theattachment 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 slotrespectively,“ ”isthegrowthrate,while“ ”and“ ”arethedistancefrom the spiral slot start point to the srcin. The width of the slot, isgiven by “ .”The second slot is rotated 180 degrees, and its trace is given by(2) 0018-926X/$31.00 © 2012 IEEE 940 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 2, FEBRUARY 2013 TABLE IU NITS FOR M AGNETIC P ROPERTIES The width of the feed line depends on the growth rate, which isgiven by(3)The number of turns “N” is approximated by(4)( : the maximum distance from the center to the border of thespiral; “g”: Slot width)The antenna characteristic impedance is designed to match with 50devices, which is compatible with many standards. However, by re-ferring to Table I, the size of the antenna and the number of turns thatthe slots are wound is in fl uenced by the factors that de fi ne the antennacharacteristic impedance and the lowest operating frequency.Thus, the lowest operating frequency can be approximated by usingthe equation used for the conventional spiral antenna [3](5)where c: the velocity of light.III. R ADIATION M ECHANISM Theradiationcharacteristicoftheconventionalspiralantennasisex- plained through the band, which states that the spiral antenna radiatesin the active region, or regions, where the currents in adjacent and op- posite arms are in phase [2].However, Wood [9] found out that the radiation mechanism of themicrostrip spiral antenna is completely different and less effective thanthat of the open planar wire spiral antenna [10]. The same mechanismis applied to explain the operating principle of the proposed spiral slotantenna.By referring to Fig. 2, the two opposite magnetic currents balanceeach other, and no radiation occurs along the length (L_Feed). Yet,when the line is curved, the balance is disturbed and radiation occurs.Thisisbecause thecurvature gives a fi nite difference betweenthe outer radius (d1) and the inner radius (d2). This radiation is directly propor-tionaltothecircumferentiallengthandslotwidthsoftheCPW,whereasthe former decreases with increasing radius of curvature.The loss due to radiation is small at very low frequencies, conse-quently making the re fl ected 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 antennaand the power re fl ection decreases.At the lower operating frequency, “ ,” there is a strong radiationfrom the outer ring. Since the radius is of order of one wavelength, the phase does not cancel the radiated fi eld but rather it reinforces it. It hasthe right phase for generating approximate CP pattern, which nearlycorresponds 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 Fig. 2. Antenna radiating and non-radiating sections.Fig. 3. Return loss of proposed spiral antenna . CPW line. However, the phases of the radiation will also change ac-cording to the frequencies and this will result to radiation cancellationdue to phase offsets. Note that the radiation is proportional to both lineand slot widths of the proposed CPW.Comparatively, at the middle frequencies, the middle rings radiateapproximately CP, due to their circumference being one wavelengthand the phase progression almost radians. However, the inner ringsmay also radiate because their radius is very tight, and unless the atten-uation of the wave going down the CPW line is high enough, there will be a signi fi cant amount of energy left to radiate from the inner ringsand further degrades the axial ratio.IV. S IMULATION R ESULTS The antenna performance is investigated using CST-microwave stu-dios simulation software. A circular slot with radius of 2 mm wasslotted at the center of the proposed antenna as shown in Fig. 2. Ini-tially, a spiral antenna with number of turn, , is simulated todetermine the effect of the circular slot adapted at the center of theantenna. The corresponding parameters for the simulated model are:, , and with FR-4substrate 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 re fl ection coef fi cient isslightly affected by the presence of the circular slot where the lowestoperating frequency is slightly reduced to 2.1 GHz, almost identical tothe theoretical calculated value using (5).In order to improve the radiation ef fi ciency, the two spiral arms aretapered [7] by gradually increasing the slots width as shown in Fig. 4.This improvement is demonstrated in Fig. 5 where the radiation ef- fi ciency 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 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 2, FEBRUARY 2013 941 Fig. 4. Two different tapered spiral designs. (a) 0.3 mm/turn (b) 0.6 mm/turn.Fig. 5. Simulated radiation ef fi ciencies of two different TR values and nontapered one. The radiation ef fi ciency was measured at the boresight direction.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 improvedthe axial ratio. The bandwidths of the axial ratio at the fi rst and secondresonantfrequency,for are 0.5GHz and 0.2 GHzrespectively.Two factors that affect the axial ratio and ef fi ciency of an antennasigni fi cantly are the re fl ected power at the end of the spiral arms andradiation from the inner rings. Therefore, a matched 75 chip resistor is connected across each line to absorb the incident wave and to mini-mize the re fl ections, while the number of turns, N is reduced from 4 to Fig. 7. Simulated ef fi ciencies of loaded and unloaded tapered spiral antenna atrespective N and TR values.Fig. 8. Simulated axial ratio for loaded and unloaded tapered spiral antenna atrespective N and TR values. 2.5 to minimize the radiations from the inner rings. Fig. 7 shows the ef- fi ciency of the antenna after the resistor is added. It is noted that the ra-diationef fi ciencyishigherwithoutresistiveloadingintheband2.0–2.8GHz, while it starts to reduce from 2.8 GHz and above. This dramatic behavior is mainly due to mismatch and ohmic losses, which are in-signi fi cant. A higher impedance mismatch for the unloaded spiral isthe reason for its lower radiation ef fi ciency at certain frequency range.The higher impedance mismatch will increases the re fl ected power andreduces the ef fi ciency of the spiral antenna.Fig. 8 shows the effects of tapering and adding resistive loading tothe proposed spiral antenna. (Note: For , the number of turns, with tapering rate, . For , the number of turn, N is reduced to 2.5 with tapering rate,as shown in Table I). The axial ratio is signi fi -cantly 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 theantenna is as same as the spiral winding direction from outer to inner arm. Therefore, the polarization sense of the antenna is left handed cir-cularly polarized when and right handed circularly po-larized in the opposite direction when where theta is 942 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 2, FEBRUARY 2013 Fig. 9. Fabricated prototype.Fig. 10. Comparison between simulated and measured re fl ection coef fi cient. de fi ned as angle from z to axis in a spherical coordinate system for three dimensionalspace.Asmentionedearlier,radiationfrom 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 signi fi cantly im- proved the axial ratio.V. M EASUREMENTS Fig. 9 shows a fabricated prototype of the proposed antenna on arectangular FR-4 printed circuit board (PCB), (74 mm 62 mm 1.6mm). The designed parameters of the antenna are: ,, , and taperedwith . Fig. 10 shows a comparison between thesimulated and the measured re fl ection coef fi cient. It illustrates goodagreement between the simulated and the measured re fl ection coef fi -cients. Furthermore, the antenna possesses a wide impedance band-width with re fl ection coef fi cient less than , with an initial op-erating frequency equals to 2.2 GHz.Radiation pattern for the prototype antenna was measured in an ane-choic chamber. Figs. 11 and 12 show comparison between simulatedand measured results of normalized LHCP and RHCP at 2.3 GHz and3.3 GHz, respectively for both E-plane and H-plane. These fi guresshow that the proposed antenna design are radiating in both LHCP andRHCP, thus proving its circular polarization properties. The simulatedand measured results are in good agreement. The proposed spiral an-tenna has bi-directional radiation pattern with a slight angle shiftingin its main direction. At E-plane, the antenna radiations are shifted by Fig. 11. Comparison between simulated and measured results of normalizedLHCP and RHCP at 2.3 GHz for (a) E-plane and(b) H-plane.Fig. 12. Comparison between simulated and measured results of normalizedLHCP and RHCP at 3.3 GHz for (a) E-plane and(b) H-plane.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 at3.3 GHz. For H-plane, the radiations is shifted by approximately neg-ative 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 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 2, FEBRUARY 2013 943 Fig. 14. Comparison between simulated and measured radiation ef fi ciency of the proposed antenna . illustrated that the tapering introduced to the antenna design has signif-icantly increased the bandwidths and improved the CP of the antennaover the frequency band (2–3.4 GHz). Adding the resistive loading hassigni fi cantly minimized the re fl ected powers and improved the axialratio 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 ef fi ciency of the proposed antenna. The measured result is in good agreement withthe simulated result. The overall radiation ef fi ciency of the antenna isabove 60%. The ef fi ciency was signi fi cantly improved after the intro-duction of tapering and resistive loading.VI. C ONCLUSION In this communication,a CPW-fed two-arm Archimedeanspiralslotantenna,whichisloadedwithchipresistors,has beeninvestigated.The proposed antenna combines and modi fi es two existing techniques inordertooptimizeaxialratioandradiationef fi ciency.Theantennastruc-ture is completely planar and it does not require an external matchingnetwork or a balun which eases design and fabrication process. A pro-totypewasfabricatedoninexpensiveFR4substrateviasinglesidemet-allization. The axial ratio of the untapered spiral antenna has low CP properties,astheaxialratiosarehigherthan3dBovertheband.Addingresistive loading to the spiral arms minimize the re fl ected power and producebetteraxialratio.Increasingthetaperingratefrom0.6mm/turnto 1.2 mm/turn has signi fi cantly improved the axial ratio. Measuredaxial ratio of the tapered spiral antenna at is pre-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 proposedantennaisproventoexhibitsCPasitradiatesinbothLHCPandRHCP.The radiation ef fi ciency of the proposed antenna was signi fi cantly im- proved to above 60% after the introduction of tapering and resistiveloading. R EFERENCES[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. An-tennas Propag. , vol. 8, no. 3, pp. 312–323, May 1960.[3] P.C.WerntzandW.L.Stutzman,“Design,analysisandconstructionof an Archimedean spiralantenna and feed structure,” Proc. IEEE Energyand Information Technologies in the Southeast , vol. 1, pp. 308–313,Apr. 1989.[4] C. Canhui and E. Yung, “Dual-band dual-sense circularly-polarizedCPW-fed slot antenna with two spiral slots loaded,” IEEE Trans. An-tennas Propag. , vol. 57, no. 6, pp. 1829–1833, Jun. 2009.[5] W. Chien-Jen and H. De-Fu, “Studies of the novel CPW-fed spiralslot antenna,” IEEE Antennas Wireless Propag. Lett. , vol. 3, no. 1, pp.186–188, Dec. 2004.[6] W.Chien-JenandW.Jin-Wei,“CPW-fedtwo-armspiralslotantenna,”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 spiralantenna,” 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 polarizedslotantenna,” IET Microw., Antennas Propag. , vol. 2, no. 5, pp. 497–502,2008.[9] C. Wood, “Curved microstrip lines as compact wideband circularly po-larized 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 Antennafor Base Stations XuLin Quan and RongLin Li Abstract— A broadband vertically/horizontally dual-polarized omnidi-rectional antenna is proposed for mobile communications. The dual-po-larized antenna is a combination of a modi fi ed low-pro fi le monopole forvertical polarization (VP) and a circular planar loop for horizontal polar-ization (HP). The modi fi ed low-pro fi le monopole is a circular folded patchshorted by four tubes while the circular loop consists of four half-wave-length arc dipoles. The dual-polarized omnidirectional antenna achieves abandwidth of (1.7–2.2 GHz) with an isolation of around 40 dB.The gain variations in the horizontal plane are less than 2.5 dB for VP and1.5 dB for HP. An eight-element dual-polarized antenna array is developedfor base station applications. The antenna gains of the array for both VPand HP are with a difference of less than 1 dB. The beamwidthsin the vertical plane are for VP and for HP. The cross-po-larization levels in the horizontal plane for both VP and HP are lower than. IndexTerms— Basestation,broadband antenna,dual-polarizedantenna,high isolation, omnidirectional antenna. I. I NTRODUCTION Nowadays polarization diversity that combines pairs of antennaswith orthogonal polarizations has been widely used in mobile commu-nications [1]–[5]. For a 360 full coverage vertical/horizontal polar-ization diversity scheme, vertically/horizontally dual-polarized omni-directional 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. Thiswork was supported in part by the National Natural Science Foundation of China (60871061), in part by the Guangdong Province Natural Science Foun-dation (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 En-gineering, South China University of Technology, Guangzhou 510641, China(e-mail:
[email protected]).Color versions of one or more of the fi gures in this communication are avail-able online at http://ieeexplore.ieee.org.Digital Object Identi fi er 10.1109/TAP.2012.22234500018-926X/$31.00 © 2012 IEEE
