Differential-Fed Dual-Polarized Filtering Fabry-Perot Antenna With High Isolation

In this paper, a dual-polarized filtering Fabry-Perot antenna (FPA) with high-isolation is proposed. It consists of a feed element of differential-fed dual-polarized square patch and a partially reflecting surface (PRS). The patch is capacitively coupled with T-shaped resonators and shorting-vias to obtain broadband characteristic and radiation nulls. The PRS structure is composed of two complementary metasurface layers, which are selected for achieving a positive reflection phase gradient within the broad frequency range. More interestingly, thanks to the frequency selectivity feature, the PRS significantly enhances the filtering characteristic of the FPA. For verification, a prototype of the proposed antenna operating at the 5.5-GHz center frequency has been fabricated and measured. The prototype with an overall size of <inline-formula> <tex-math notation="LaTeX">$\sim 2.0\lambda _{\text {min}} \times 2.0\lambda _{\text {min}} \times 0.49\lambda _{\text {min}}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$\lambda _{\text {min}}$ </tex-math></inline-formula> is the free-space wavelength referring to the lowest operational frequency) result in an impedance bandwidth of 17.1% (<inline-formula> <tex-math notation="LaTeX">$5.02-5.96$ </tex-math></inline-formula> GHz) for 10-dB return loss and a high isolation of <inline-formula> <tex-math notation="LaTeX">$\ge45$ </tex-math></inline-formula> dB. Moreover, the far-field measurements result in a good dual-polarized radiation with the peak gain of 13.0 dBi, cross-polarization level of <inline-formula> <tex-math notation="LaTeX">$\le $ </tex-math></inline-formula>–25 dB within the passband, and out-of-band suppression level of <inline-formula> <tex-math notation="LaTeX">$\ge20$ </tex-math></inline-formula> dB.

those techniques, for instance, include the implementation of 23 feeding structures incorporated with band-pass filters, the uti- 24 lization of defected ground planes, coupling resonators, sub- 25 strate integrated waveguides, and multi-layered structures. 26 In response to the recent rapid change of the wireless com- 27 munications, especially with the 5G and beyond-5G cellular 28 The associate editor coordinating the review of this manuscript and approving it for publication was Hussein Attia . systems, highly-performing filtering antennas with additional 29 incorporated functionalities, such as multiple polarizations 30 and high isolation, become demanding. Dual-polarization 31 is necessary for base station in cellular systems because 32 it mitigates multipath fading effects and increases channel 33 capacity. Accordingly, dual-polarized filtering antennas [2], 34 [3], [ 35 have received much attention in recent years. Furthermore, 36 in order to directly connect to differential RF/microwave 37 circuits without additional balun, differential-feed scheme 38 has applied to the dual-polarized filtering antennas [15], [16], 39 [17], [18], [19], [20], [21]. Interestingly, with perfectly sym-40 metrical structure, these antennas yield a theoretical infinite 41 isolation and zero cross-polarization level. 42 To achieve higher gain, the traditional method of creat-43 ing antenna arrays has been widely utilized in the existing 44 filtering antennas, e.g., [22], [23], [24], [25], [26], [27]. How-45 ever, these arrays require complicated feeding networks and 46 antenna configurations. One effective method to overcome this is to use Fabry-Perot antenna (FPA) [28]. Its basic design 48 is illustrated in Fig. 1(a (1) 55 where D PRS is the increase in directivity caused by the PRS 56 and given by [29]: where γ is the reflection magnitude of the PRS. Note that the 59 equations (1) and (2) are accurate only when the resonant con-60 dition in (3) is satisfied and for a very large PRS. As analyzed 61 in [28], the resonance frequency of an FPA is where c is the speed of light in free space, H c is the cavity 64 height, φ PRS is the reflection phase of the PRS, and φ GND is 65 the reflection phase of the GND given as As the reflection phase of a typical PRS decreases with 68 frequency, the resonance condition (3) is satisfied only for a 69 narrow bandwidth, as illustrated in Fig. 1(b). For broadband 70 operation, a positive reflection phase gradient is required as 71 illustrated in Fig. 1 due to its inherent frequency selective characteristics. Nev-78 ertheless, to the best of our knowledge, this feature has not 79 been comprehensively investigated or exploited to achieve a 80 highly-performing filtering FPA in the literature.

81
In this paper, we propose a dual-polarized filtering FPA 82 with very high isolation, which can also be suitable for 83 in-band full duplex (IBFD) applications [34]. The antenna 84 employs a differential-fed dual-polarized patch, which is 85 loaded with T-shaped resonators through a capacitively prox-86 imity coupling scheme to achieve the broadband-pass filter-87 ing feature. In order to further enhance the broadside gain and 88 frequency selectivity, the patch antenna is incorporated with 89 a PRS structure of two complementary metasurfaces.  Fig. 2 shows the structure of the proposed FPA, which is 93 composed of a patch, PRS, and GND. To provide band-94 pass filtering feature, the patch is capacitively coupled with 95 T-shaped resonator and shorting via [35]. Distinguished from 96 the design in [35], which used a single-ended port with 97 asymmetrical structure, the proposed antenna employs a sym-98 metrical double differential-feed to achieve highly-isolated 99 dual-polarized radiation. The patch and T-shaped resonators 100 are printed on the top-side of Sub. 1 (Rogers RO4003, 101 ε r = 3.38 and tan δ = 0.0027) with dimensions of 50 mm 102 × 50 mm, which is suspended on the GND via a foam 103 (Rohacell, ε r = 1.07, and tan δ = 0.0006). Four 50-104 coaxial lines are used as inputs of the double differential 105 feed. The PRS is placed at a distance of H c above the GND. 106 To allow a broadband operation, the PRS is composed of two 107 complimentary metasurfaces with 10 × 10 unit-cells, which 108 are built on both sides of the Sub. 2 (Roger Duroid RT5880, 109 VOLUME 10, 2022   The optimized design parameters are listed in Table 1.  of 5.5 GHz. These reflection properties ensure that the PRS 130 structure is proper to be employed in the broadband FPAs.

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To demonstrate the effectiveness of the technique, Fig. 4 133 shows design steps for the proposed FPA. First, the design 134 procedure starts by considering a simple directly-fed patch as 135 the feed element (Ant. 1). Since a conventional patch antenna 136 suffers from a narrow bandwidth, it is capacitively coupled 137 with T-shaped resonator and shorting via [35] to create Ant. 2 138 for the broadband and filtering characteristics. To generate 139 a dual-polarized radiation, double T-shaped resonators are 140 applied to the patch (creating Ant. 3). In the next step, 141 to improve port-to-port isolation, double differential-feed is 142 applied to Ant. 3 creating the feed element of the proposed 143 FPA (Ant. 4). Finally, to enhance the gain and keep the high 144 isolation, the PRS structure is added to Ant. 4 for establishing 145 the proposed design. For demonstration, all antenna configu-146 rations have same design parameters as the final FPA.
Due to the perfectly symmetrical structure, i.e., S 31 = S 32 For realizing the broadband differential-feeds (i.e., same 185 magnitude and 180 • phase difference with perfect isolation), 186 the feeding network of the proposed FPA, as shown in 6(a), 187 is adapted from a wideband planar balun in [37]. The two 188 baluns are designed for the double differential-feeds and each 189 balun is composed of a two-way equal Wilkinson power 190 divider and a wideband 180 • phase shifter. The simulated per-191 formances of the feeding network are illustrated in 6(b). The 192 two baluns yield good performance across a broad frequency 193 range; i.e., at 4.5 -7.0 GHz, the reflection coefficients at the 194 inputs (|S 11 | and |S 22 |) are < −15 dB, nearly equal divided 195 powers at the outputs, and phase differences of 180 • ± 1.5 • . 196 These results indicate that the feeding network just provides 197 the differential signals and does not contribute the filtering 198 characteristic at the desired frequency range.

200
For verification, the proposed dual-polarized FPA is fabri-201 cated and measured. Fig. 7 shows a fabricated prototype of 202 the proposed FPA, which has an overall size of 120 × 120 × 203 29.3 mm 3 (∼2.0λ min × 2.0λ min × 0.49λ min ). The antenna is 204 VOLUME 10, 2022     is isolated with the radiating elements by the GND, these 226 undesired radiations do not affect the antenna performances. 227 The simulation and measurement broadside realized gains 228 of the FPA prototype are illustrated in Fig. 9. The mea-229 surements resulted in a 3-dB gain bandwidth of 21.82% 230 (4.9 -6.1 GHz) with the peak gain of 13.0 dBi, while 231 the simulations result in a 3-dB gain bandwidth of 21.98% 232 (4.94 -6.16 GHz) with the peak gain of 13.5 dBi. Due 233 to the undesired losses caused by the feeding network, the 234 realized prototype yields a slightly smaller gain as compared 235 to the antenna excited by four ports. Also, both simula-236 tion and measurement yield an out-of-band suppression of 237 about 20 dB.    Thirdly, table 2 shows a performance comparison between 265 the proposed FPA and other related works. Relative to most 266 of the high-gain dual-polarized filtering antennas [22], [23], 267 [24], [25], [26], [27], the proposed design yields comparable 268 overall-size, operational-bandwidth, gain, and out-of-band 269 suppression level, but it yields simpler configuration 270 and higher isolation. As compared to most of existing 271 dual-polarized FPAs [38], [39], [40], [41], [42], [43], our 272 design yields several advantages, such as wider bandwidth, 273 higher isolation, and most importantly filtering characteristic. 274  He is currently a Lecturer with The Univer-516 sity of Adelaide. His main research interests 517 include microwave circuits, advanced materials, 518 absorbers, and various types of antennas.