Single-Layer, Dual-Band, Circularly Polarized, Proximity-Fed Meshed Patch Antenna

This paper proposes a new method to design a single-layer dual-band circularly polarized (CP) patch antenna with a small frequency ratio. The design consists of one or several pairs of rectangular patches proximity-fed by a 50-<inline-formula> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> microstrip line with an open-circuit termination. By exploiting both capacitive and inductive coupling mechanisms, and both orthogonal radiating modes of these patches, the antenna can be designed to operate at two close resonance frequencies. Due to its simple and single-layer structure, the antenna can be easily adapted with meshed configuration, which is suitable for transparent devices. For verification, a dual-band CP meshed patch antenna with a frequency ratio of 1.12 and two pairs of patch is designed, fabricated, and tested. The measurements show that the antenna prototype provides a <inline-formula> <tex-math notation="LaTeX">$|S_{11}| < -10$ </tex-math></inline-formula>-dB bandwidth of 4.82–5.03 GHz (210 MHz) and 5.49–5.78 GHz (290 MHz), axial ratio < 3-dB bandwidth of 4.88–4.93 GHz (50 MHz) and 5.50–5.57 GHz (70 MHz), and broadside realized gains of 9.0 dBic and 8.6 dBic for the lower and upper bands, respectively.

are one of the most preferred choices for integration with 28 The associate editor coordinating the review of this manuscript and approving it for publication was Binit Lukose . solar cells. Nevertheless, the performance of most of these 29 antennas are still limited with single-band operation. 30 Along with the higher demands in quality of service, 31 data-rate and reliability, many modern wireless communica-32 tion systems employ different frequency bands. Moreover, 33 some of these systems favorably employ circularly polar-34 ized (CP) antennas [14] to mitigate multipath interference, 35 polarization mismatch, and Faraday's effects. Thus, there has 36 been a demand on multi-band CP antennas, which, in some 37 cases, are more reliable than the broadband antennas due 38 to the reduction of out-of-band interference. Furthermore, 39 broadband CP antennas typically require a higher profile 40 or larger planar size. A common approach for designing 41 dual-band CP antennas is to use dual resonant elements 42 printed on multi-layer substrate, i.e., stacked patch configu-43 rations [15], [16], [17], [18], [19], [20]. These designs, how-44 ever, are not suitable for transparent devices as multi-layer 45 structures would reduce the transparency. Some notable 46 experimentally. 77 For completion, we first present the theory of a resonant-78 type proximity-fed patch antenna. The proximity-fed method 79 was first presented by Itoh and Ohtsuka in 1982 [28] to 80 avoid the multi-layer structure and exploit the advantages of 81 non-contacting coupling feeds. It should be noted the princi-82 ple is different from the traveling-wave structures as in [26], 83 [29], and [30]. 84 The basic structure is shown in Fig. 1

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where n is an integer, the patch is aligned at V max .

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If the patch length L p ≈ λ eff1 /2, the patch resonates at  2) At a different frequency f 2 with guided-waveglenth λ g2 , 100 if where n is also an integer, the patch is aligned with I max . 103 If the patch width W p ≈ λ eff2 /2, the patch resonates in 104 TM 01 mode with horizontal polarization through induc-105 tive coupling.

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Therefore, the structure works as a dual-band linear-107 polarized (LP) antenna where the polarization in each band is 108 orthogonal to each other. Furthermore, the level of coupling, 109 which affects the impedance matching, can be controlled by 110 the gap between the patches and the feed lines. The principle 111 demonstrated here is a generalization of what shown in [7], 112 which will be exploited to design a dual-band CP antenna in 113 the next section.   are inherently 90 • phase different, this is naturally achieved 120 and will be exploited in the design. Finally, as mentioned before, the standing-wave voltage 141 and current have 90 • phase difference, thus, CP radiation at 142 both resonance frequencies can be achieved. Fig. 3 illustrates 143 the standing-wave voltage and current along the microstrip 144 line, and how CP is achieved at both lower and upper bands. 145 By tuning the patch dimensions as well as the positions y 1 and 146 y 2 , it will be shown later that satisfactory axial ratio (AR) and 147 broadside radiation can still be achieved.

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For demonstration, a design with a frequency ratio of 1.12 149 (operating at 5.0 GHz and 5.6 GHz) is characterized. Its 150 design parameters are given in Table 1. The simulation results 151 will be shown in the next section. To confirm the analysis 152 shown in Fig Based on the proposed method, dual-band CP antennas with 168 gain enhancement can be realized by adding more radiating 169 elements, as shown in Fig. 5. For the second and third pairs 170 of patches, it needs to resonate with the same phase as the 171 first pair. Strictly speaking, this cannot be satisfied at both 172 frequencies f 1 and f 2 at the same time. However, a compro-173 mise can be made when f 1 and f 2 are very close to each other 174 (so that λ g2 is close to λ g1 ). Here we can choose y 3 ≈ y 4 ≈ 175 0.5(λ g2 + λ g1 ). After the optimization, y 3 = 33.8 mm and 176   Table 1.     Table 2.  [31], [32], [33], and [34], 236 which also showed a minor additional losses (depending on 237 the transparency).  Fig. 9 shows 243 a prototype of the fabricated antenna. The antenna is fab-244 ricated using the standard printed circuit board technology. 245 The prototype employs a SubMiniature version A (SMA) 246 connector as coaxial-to-microstripline transformer. The sim-247 ulation and measurement performances of the antenna pro-248 totype are compared in Fig. 10. There is a good agreement 249 between the simulated and measured results. From Fig. 10(a), 250 the antenna prototype yields a measured −10-dB impedance 251 bandwidth of 210 MHz/290 MHz (about 4.3% and 5.1% at 252 the lower and upper band, respectively). In Fig. 10(b), the 253 measurements result in a AR bandwidth (AR < 3 dB) of 254 about 50 MHz/70 MHz, i.e. corresponding to 1.0% and 1.3% 255 at the the lower and upper band, respectively. As shown in 256 Fig. 10(c), the fabricated mesh prototype achieves a good 257 broadside gain at both frequency bands, which agrees well 258 with the simulations. The meshed patch antenna yields mea-259 sured gains of about 9.0 dBic at 4.9 GHz and 8.6 dBic at 260 5.5 GHz. Finally, simulations of the meshed design indicate 261 radiation efficiencies (RE) of 70% and 76% at the lower and 262 upper bands, respectively. Similar to any meshed antennas in 263 literature, there is a tradeoff between the transparency and 264 radiation efficiency [5]. Although the measured RE is not 265 is >20 dB larger than that of LHCP for the both bands.  antenna in [8] provides a dual-band CP radiation, however, 286 it uses metamaterial components, which makes it not trans-287 parent in a large feeding region. It is also more compli-288 cated with multi-layered structure. To achieve higher gain, the 289 antenna in [8] would require more complex feeding network 290 with array configuration, as compared to the proposed design. 291

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A method of designing a simple dual-band CP patch antenna 293 with tight frequency ratio has been described. The antenna 294 consists of one/several pair of rectangle patches, which are 295 excited by an open-ended 50-microstrip line through prox-296 imity coupling. Based on the capacitive and inductive cou-297 plings, the dimensions and arrangement of these patches are 298 designed to generate a dual-band CP radiation. Due to its 299 simplicity, the design is well suited for applications requir-300 ing transparent radiators. For demonstration, a dual-band 301 CP meshed patch antenna with a frequency ratio of 1.12 at 302 C-band has been designed, fabricated, and tested. It is empha-303 sized again that although non-transparent substrate has been 304 VOLUME 10, 2022