Research Article  Open Access
DualBand Operation of a Circularly Polarized FourArm Curl Antenna with Asymmetric Arm Length
Abstract
This paper presents dualband operation of a singlefeed composite cavitybacked fourarm curl antenna. Dualband operation is achieved with the presence of the asymmetrical arm structure. A pair of vacantquarter printed rings is used in the feed structure to produce a good circular polarization (CP) at both bands. The cavitybacked reflector is employed to improve the CP radiation characteristics in terms of the 3dB axial ratio beamwidth and broadside gain. The proposed antenna is widely applicable in dualband communication systems that have a small frequency ratio. Examples of such a system are global positioning systems.
1. Introduction
Curl antennas, which are formed by short singlearm Archimedean or rectangular spirals, have been developed as small, lowprofile, circularly polarized (CP) radiation elements [1–12]. Antennas of this kind are mechanically simple and produce CP radiation with a singlefeed without the need for an external circuit. A conventional curl antenna, which is placed approximately a quarterwavelength above a large ground plane, can provide good impedance matching and good CP radiation over a bandwidth of approximately 10% [1–8]. However, these antennas have several drawbacks, such as a slight tilt in the broadside radiation pattern due to their singlearm configuration, the need for a large ground plane, and their limitation to singleband operation. Recently, singlefeed fourarm curl antennas have been reported for CP radiation to improve the performance of conventional curl antennas [13, 14]. These curl antennas yield singleband CP operation with the same arm length. A dualband operation can be achieved by dividing the antenna’s arms into two branches of different lengths [15]. However, this approach leads to very complicated antenna configuration in the design process.
This paper describes a simple method to design dualband operation of a fourarm curl antenna. Dualband operation is obtained by creating different arm lengths in the fourarm curl structure. The antenna is optimized for global positioning system (GPS) L1 and L2 bands. The resulting antenna was first characterized with the ANSYS HighFrequency Structure Simulator (HFSS) and then confirmed by experiments. The final design, with an overall size of 90 × 90 × 30 mm^{3} ( at the GPS L2 frequency), yields dB bandwidths of 1.217–1.234 GHz and 1.495–1.595 GHz and AR < 3 dB bandwidths of 1.220–1.230 GHz and 1.565–1.600 GHz. Additionally, the antenna radiates a widebeam righthand circular polarization (RHCP) with high radiation efficiency at both bands. In comparison with the dualband multiarm curl GPS antenna [15], the proposed antenna has a much simpler structure, smaller size, and comparable and AR bandwidths.
2. Antenna Geometry
Figure 1 shows the geometry of the proposed antenna that is composed of a fourarm curl radiator, a coaxial line, and a reflector. The curl radiator, which is a fourarm Archimedean spiral antenna with a small number of turns, was built on both sides of a circular Rogers RO4003 substrate with a radius of , a relative permittivity of 3.38, a loss tangent of 0.0027, and a thickness of . Two arms (orange color) were placed on the top of the substrate whereas the other two arms (grey color) were on the bottom of the substrate. The curl arm was characterized by the Archimedean spiral drawing function of the HFSS software [16], which is defined by a center position of (0, 0, 0), a direction vector (0, 0, 1), a starting point of , a radius change of , a number of turns of , and a stripline width of . The radiator is fed by a vacantquarter printed ring, which helps the antenna to be directly matched to a 50 Ω coaxial line over a broad bandwidth and produces the CP radiation. The reflector is a rectangular box with base dimensions of and a height of . To achieve the dualband operation, the asymmetrical curl arm structure was created by choosing a different number of turns for different arms.
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3. Mechanism of DualBand Operation
As mentioned above, the dualband operation of the proposed antenna is obtained by choosing a different number of turns for different arms. To demonstrate this, the input impedances () of an ideal fourarm curl antenna [13] are examined for symmetric and asymmetric configurations via the HFSS simulation. Figure 2 shows the HFSS simulation modeling of the ideal fourarm curl antenna. The arms were placed on both sides of a 40 mm radius Rogers RO4003 substrate with a thickness of 0.508 mm. The orange arms (#1 and #3) were on the top of the substrate and the grey arms (#2 and #4) were on the bottom of the substrate. Referring to Figure 1(b), the arm was formed by a center position of (0, 0, 0), a direction vector (0, 0, 1), mm, mm, , and mm. The antenna was simulated by assuming two excitation sources (port1 for the orange arms and port2 for the grey arms) and their input impedances ( or ) were calculated for three configurations. Case #1: , case #2: and , and case #3: and . The results were given in Figures 3–5. Since the value is the same as and expresses the input impedance of the ideal fourarm curl antenna, and Figures 3–5 only show the real and imaginary parts of values for the three cases.
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As shown in Figure 3, the symmetrical fourarm curl antenna behaves as a resonant antenna in the lowfrequency range, whereas the antenna behaves as a traveling wave antenna in the highfrequency region with an almost stable input impedance value. The resonance was defined as the frequency at which the imaginary value of is zero. With the chosen parameters, the antenna yielded two resonances in the interest frequency range. The first one was 1.58 GHz with an input resistance of approximately 50 Ω while the second was 1.79 GHz with an input resistance of approximately 290 Ω. In the highfrequency region, the real part of was nearly constant with a value of 60 Ω, while the imaginary part of was nearly constant with a value of −90 Ω. For convenience, resonance with ~50 Ω of resistance is called usable resonance, while resonance with a high resistance is called nonusable resonance. This result indicates that the antenna can be well matched to a 50 Ω source without the need for an external circuit at its first resonance.
To create the asymmetrical fourarm curl antennas, the number of turns for arms #1 and #2 was varied while the other arms (#3 and #4) were fixed. Figure 4 shows the values of an asymmetrical fourarm curl antenna with and . The antenna yielded more resonances compared to the symmetrical one. Two resonances at 1.605 and 1.995 GHz can be used because the real values of nearly equal 50 Ω. The values of the other asymmetrical curl antenna ( and ) are shown in Figure 5. The antenna yielded two resonances at 1.375 GHz and 1.705 GHz with the real value of approximately 50 Ω. These results indicate that both asymmetrical configurations of the fourarm curl antenna yielded a dualfrequency operation in the frequency band of interest. To have a CP wave, the two sources of excitation in Figure 2(b) must have equal amplitudes and a 90° phase difference. Consequently, the antenna requires a onetotwo power combiner/splitter and a 90° phase shifter in the feeding structure. This approach complicates the antenna design and fabrication.
To generate the CP radiation with a singlefeed, the asymmetrical fourarm curl antennas are incorporated with a pair of vacantquarter printed rings, as shown in Figure 6. The rings were designed with a 5.4 mm radius and a 1 mm width. Due to the presence of the rings, the antenna geometry was slightly changed compared to the ideal configurations; that is, arms #1 and #4 with a number of were placed on the top of the substrate whereas arms #2 and #3 with a number of were placed on the bottom of the substrate. The other design parameters of the antenna were the same as those of the ideal structure in Figure 2. To reconfirm the dualband operation of the asymmetrical configuration, the and AR values of the antenna were calculated for different and given in Figure 7. For the symmetrical case (), the antenna yielded only the operational band near the GPS L1 frequency. For the asymmetrical case ( and ), the antenna yielded a dualband operation near 1.7 GHz and 2.1 GHz. For the other asymmetrical case ( and ), the antenna yielded a dualfrequency occupied at 1.42 GHz and 1.79 GHz. For both asymmetrical cases, the upper bands were divided by an undesired notch, which could be attributed to the trapping electromagnetic energy between the adjacent arms. This notch can be eliminated by adjusting the width of the curl arm. Furthermore, the identical behaviors of the dualband operation were obtained when is fixed and is varied, and, therefore, their results are not shown.
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It is well known that the resonances of the curl antenna can be easily controlled by the number of turns, the spacing between turns, and the width of the arm, as well as the dielectric substrate (permittivity and thickness). Also, CP radiation can easily be achieved by adjusting the ring configurations. Accordingly, the asymmetrical fourarm curl antenna was optimized for good impedance matching and CP radiation at the GPS L1 and L2 frequencies. Note that without the reflector, the fourarm curl antennas radiate a bidirectional electromagnetic wave at their operational frequencies. To achieve a desired broadside pattern at both bands, a cavitybacked reflector with the base dimensions of 90 × 90 mm^{2} ( at the GPS L2 frequency or at the GPS L1 frequency) and a height of 30 mm ( at the GPS L2 frequency or at the GPS L1 frequency) was employed in the antenna. Referring to Figure 1(b), the parameters of the final design are as follows: mm, mm, mm, mm, mm, mm, mm, mm, mm, , , , and . To demonstrate the effects of the cavity on AR performance, the and AR values of the dualband asymmetrical fourarm curl antenna in different configurations, including those in free space and at different heights of the cavity, were calculated and are shown in Figure 8. The cavitybacked antenna yielded a narrower impedance bandwidth, but significantly better CP performance compared to the case in free space. With an increase of , the resonance and CP center frequencies of the antenna decreased at both operating frequencies, and the AR characteristic improved significantly. For mm, the simulations resulted in good impedance matching and CP radiation at both bands. Its impedance bandwidths were 1.220–1.235 GHz and 1.510–1.605 GHz for dB, and its 3dB AR bandwidths were 1.222–1.232 GHz and 1.570–1.605 GHz with two CP center frequencies at 1.227 GHz ( dB) and 1.580 GHz ( dB). As shown in Figure 9, the antenna yielded an excellent unidirectional radiation pattern at both bands. The simulations resulted in a gain of 6.9 dBic and a radiation efficiency of 97% at the GPS L1 frequency but a gain of 6.7 dBic and a radiation efficiency of 91% at the GPS L2 frequency.
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4. Measurements
For verification, the dualband asymmetrical fourarm curl GPS antenna was realized and measured. The radiator was built on both sides of a Rogers RO4003 substrate with a copper thickness of 17 μm via a standard wetetching technology. The reflector was constructed from five copper plates (one 90 × 90 mm^{2} and four 90 × 30 mm^{2}) with a thickness of 0.2 mm. Figure 11 shows the measured and simulated and AR values of the fabricated prototype (Figure 10). The measurements resulted in a dB bandwidth of 1.217–1.234 GHz and 1.495–1.595 GHz and a 3dB AR bandwidth of 1.220–1.230 GHz and 1.565–1.600 GHz with two CP center frequencies at 1.225 GHz ( dB) and 1.590 GHz ( dB), respectively. These measurements agreed quite closely with the HFSS predictions.
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Figure 12 shows a comparison of simulated and measured gain patterns for the fabricated antenna at the GPS L1 and L2 frequencies. The measured results are in good agreement with the HFSS simulation. The antenna yielded an RHCP radiation with wide beam and symmetrical profiles at both bands. At the GPS L1 frequency, the measurements resulted in a gain of 7.0 dBic, a fronttoback ratio of 18 dB, and halfpower beam width (HPBW) of 82° and 84° in the and planes, respectively. At the GPS L2 frequency, the antenna yielded a gain of 6.7 dBic, a fronttoback ratio of 15 dB, and HPBW of 92° and 82° in both and planes, respectively. As shown in Figure 13, both simulations and measurements resulted in a broad 3dB AR beamwidth (>120°) at two frequencies. Additionally, the measured radiation efficiencies were 95.0% and 87.0% at the GPS L1 and L2 frequencies, respectively.
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Table 1 shows the comparison of the measured performance of the proposed antenna with that of the dualband multiarm curl GPS antenna [15]. The proposed antenna yielded a simpler configuration, a smaller size, and comparable operational bandwidths. Table 2 shows a comparison of the measured performance of the proposed antenna and those of previous singlefeed dualband GPS antennas [17–23]. It is observed that the proposed antenna yielded a significantly wider 3dB AR beamwidth at both bands as compared to the other antennas.


5. Conclusion
A composite cavitybacked fourarm curl antenna has been developed for dualband operation by creating an asymmetrical arm structure. The antenna with overall dimensions of mm^{3} ( at the GPS L2 frequency) resulted in a dB bandwidth of 1.217–1.234 GHz and 1.495–1.595 GHz and a 3dB AR bandwidth of 1.220–1.230 GHz and 1.565–1.600 GHz. Furthermore, the antenna radiates widebeam RHCP (>120°) and operates with high radiation efficiency (>85%) at both bands. With many advantages, such as compact size, good CP radiation, widebeam, and high radiation efficiency, this antenna is a good candidate for dualband applications with a small frequency ratio.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This work was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea Government (MSIP) [no. 20090083512].
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Copyright
Copyright © 2016 Son Xuat Ta and Ikmo Park. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.