Research Article | Open Access
Dual-Band Operation of a Circularly Polarized Four-Arm Curl Antenna with Asymmetric Arm Length
This paper presents dual-band operation of a single-feed composite cavity-backed four-arm curl antenna. Dual-band operation is achieved with the presence of the asymmetrical arm structure. A pair of vacant-quarter printed rings is used in the feed structure to produce a good circular polarization (CP) at both bands. The cavity-backed reflector is employed to improve the CP radiation characteristics in terms of the 3-dB axial ratio beamwidth and broadside gain. The proposed antenna is widely applicable in dual-band communication systems that have a small frequency ratio. Examples of such a system are global positioning systems.
Curl antennas, which are formed by short single-arm Archimedean or rectangular spirals, have been developed as small, low-profile, circularly polarized (CP) radiation elements [1–12]. Antennas of this kind are mechanically simple and produce CP radiation with a single-feed without the need for an external circuit. A conventional curl antenna, which is placed approximately a quarter-wavelength 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 single-arm configuration, the need for a large ground plane, and their limitation to single-band operation. Recently, single-feed four-arm curl antennas have been reported for CP radiation to improve the performance of conventional curl antennas [13, 14]. These curl antennas yield single-band CP operation with the same arm length. A dual-band operation can be achieved by dividing the antenna’s arms into two branches of different lengths . However, this approach leads to very complicated antenna configuration in the design process.
This paper describes a simple method to design dual-band operation of a four-arm curl antenna. Dual-band operation is obtained by creating different arm lengths in the four-arm curl structure. The antenna is optimized for global positioning system (GPS) L1 and L2 bands. The resulting antenna was first characterized with the ANSYS High-Frequency Structure Simulator (HFSS) and then confirmed by experiments. The final design, with an overall size of 90 × 90 × 30 mm3 ( 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 wide-beam right-hand circular polarization (RHCP) with high radiation efficiency at both bands. In comparison with the dual-band multiarm curl GPS antenna , 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 four-arm curl radiator, a coaxial line, and a reflector. The curl radiator, which is a four-arm 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 , 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 vacant-quarter 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 dual-band operation, the asymmetrical curl arm structure was created by choosing a different number of turns for different arms.
3. Mechanism of Dual-Band Operation
As mentioned above, the dual-band 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 four-arm curl antenna  are examined for symmetric and asymmetric configurations via the HFSS simulation. Figure 2 shows the HFSS simulation modeling of the ideal four-arm 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 (port-1 for the orange arms and port-2 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 four-arm curl antenna, and Figures 3–5 only show the real and imaginary parts of values for the three cases.
As shown in Figure 3, the symmetrical four-arm curl antenna behaves as a resonant antenna in the low-frequency range, whereas the antenna behaves as a traveling wave antenna in the high-frequency 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 high-frequency 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 four-arm 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 four-arm 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 four-arm curl antenna yielded a dual-frequency 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 one-to-two 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 single-feed, the asymmetrical four-arm curl antennas are incorporated with a pair of vacant-quarter 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 dual-band 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 dual-band operation near 1.7 GHz and 2.1 GHz. For the other asymmetrical case ( and ), the antenna yielded a dual-frequency 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 dual-band operation were obtained when is fixed and is varied, and, therefore, their results are not shown.
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 four-arm curl antenna was optimized for good impedance matching and CP radiation at the GPS L1 and L2 frequencies. Note that without the reflector, the four-arm curl antennas radiate a bidirectional electromagnetic wave at their operational frequencies. To achieve a desired broadside pattern at both bands, a cavity-backed reflector with the base dimensions of 90 × 90 mm2 ( 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 dual-band asymmetrical four-arm 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 cavity-backed 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 3-dB 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.
For verification, the dual-band asymmetrical four-arm 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 wet-etching technology. The reflector was constructed from five copper plates (one 90 × 90 mm2 and four 90 × 30 mm2) 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 3-dB 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.
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 front-to-back ratio of 18 dB, and half-power 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 front-to-back 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 3-dB 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.
Table 1 shows the comparison of the measured performance of the proposed antenna with that of the dual-band multiarm curl GPS antenna . 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 single-feed dual-band GPS antennas [17–23]. It is observed that the proposed antenna yielded a significantly wider 3-dB AR beamwidth at both bands as compared to the other antennas.
A composite cavity-backed four-arm curl antenna has been developed for dual-band operation by creating an asymmetrical arm structure. The antenna with overall dimensions of mm3 ( at the GPS L2 frequency) resulted in a dB bandwidth of 1.217–1.234 GHz and 1.495–1.595 GHz and a 3-dB AR bandwidth of 1.220–1.230 GHz and 1.565–1.600 GHz. Furthermore, the antenna radiates wide-beam RHCP (>120°) and operates with high radiation efficiency (>85%) at both bands. With many advantages, such as compact size, good CP radiation, wide-beam, and high radiation efficiency, this antenna is a good candidate for dual-band 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.
This work was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea Government (MSIP) [no. 2009-0083512].
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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.