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This paper proposes dual-band dual linearly-polarized (LP)/circularly-polarized (CP) antennas using combined metasurface and patch for ultra high frequency (UHF) and 2.45-GHz RF identification (RFID) readers. A metasurface and two stacked patches are employed for the lower and upper bands, respectively. The metasurface and patches are incorporated with a common feeding structure which is composed of two orthogonal microstrip lines and a crossed slot to produce the dual-wideband dual-LP radiation. To reduce the back radiation and improve the gain, the antenna is backed by a metallic cavity. Taking advantage of its low-profile structure, the dual-LP antenna is further integrated with a dual-band branch-line coupler to realize dual-CP radiation. Both proposed antennas yield stable radiation performances across dual operating bands which completely cover the assigned RFID spectrum at the UHF ( $860-960$ MHz) and 2.45-GHz ( $2400-2500$ MHz) bands.

A compact dual-band tripolarized antenna with simple structure and high isolation operating at 2.45 GHz and 3.5 GHz bands is presented. The design is composed of a slotted patch and a monopolar patch connected together by four vias. The antenna uses a double differential-fed scheme for x- and y-horizontally polarized broadside radiations and a single-ended port at the center for vertically-polarized omnidirectional radiation. The combination of slotted patch, monopolar patch, and vias yields several interesting features, which are exploited in the design to achieve dual-band tripolarized operation. Thank to the differential feed scheme and structural symmetry, the proposed antenna achieves a very high isolation among all ports. For verification, the final design is fabricated and measured. The double differential-fed scheme are realized by using two wideband out-of-phase power dividers, whose operational bandwidth covers both 2.45 and 3.5 GHz bands. The antenna with profile of $0.09\lambda _{2.45-{\mathrm { GHz}}}$ yields a measured 10-dB return loss bandwidth of $2.43-2$ .49 GHz and $3.23-3$ .66 GHz and isolation of $\ge $ 35 dB among all ports. Tripolarized radiation is verified with far-field measurement, showing highly symmetrical pattern and low cross-polarization in all three operational modes. The proposed design is a good candidate for dual-band communication systems which require polarization and pattern diversity antennas.

This paper describes a design of dual-polarized composite patch-monopole antenna with reconfigurable radiation pattern. The design consists of a double differential-feed patch loaded with four vertical monopoles symmetrically. The monopoles are connected to or disconnected from the ground plane (GND) by changing the ON/OFF states of p-i-n diodes. By adjusting the connections between the monopoles and GND, the pattern reconfigurability (i.e., one widebeam and three narrow-beam modes) can be achieved for each polarization. The proposed antenna can switch the beam in two dimensions separately $(xz-$ and $yz-$ plane). The proposed antenna is characterized computationally and validated experimentally. The measurements result in an overlapping 10-dB return-loss bandwidth of $2.40 - 2.51$ GHz and a port-to-port isolation ≥20 dB in all the modes. In the far field, the prototype can deploy switched beam flexibility. Its radiation characteristics are confirmed by the far-field measurements, which shows the reconfigurable patterns for both polarizations.

A dual-polarized filtering Fabry–Perot antenna (FPA) with high selectivity and high isolation is proposed for in-band full-duplex (IBFD) applications. The proposed antenna utilizes a square patch as the feeding element, which is fed by a double differential-fed scheme for dual-polarized radiation with high isolation. The patch is loaded with a symmetrical cross-slot and four shorting pins for a broad passband filtering feature. To enhance broadside gain across a wide frequency range, the patch is incorporated with a partially reflecting surface (PRS), which is composed of two complementary cross-slot and patch arrays. Moreover, the frequency selectivity of PRS is exploited to improve the filtering characteristic. The double differential feeds are realized based on out-of-phase power dividers, which are combined with simple low-pass filters to further improve the out-of-band suppression. The final design was fabricated and measured. The measurement results show excellent results with a 10-dB return loss bandwidth of 21.5% (4.91–6.09 GHz), isolation of greater than 40 dB, peak gain of 13.7 dBi, out-of-band suppression level of better than 27 dB, and a cross-polarization level of less than −27 dB.

A high-gain filtering Fabry-Perot antenna (FPA) with radar cross Section (RCS) reduction is proposed. The design utilizes a partially reflecting surface (PRS) which is integrated with a chessboard arranged metasurface (CAMS) in a single-layered substrate. The CAMS is composed of periodic strip arrays to reduce monostatic RCS based on the phase cancellation principle. The PRS structure is a cross-slot array which is designed to work with both orthogonal orientations of the CAMS elements while yielding a high reflection magnitude. Accordingly, the proposed FPA achieves a high-gain broadside radiation and wideband RCS reduction. The filtering functionality is achieved from a T-shaped feed patch, which is, interestingly, enhanced by the integration of CAMS in the PRS. For validation, a prototype operating at 6.0 GHz has been fabricated and measured. The antenna with an overall size of $3.2\lambda _{{0}} \times 3.2\lambda _{{0}} \times 0.57\lambda _{{0}}$ at 6.0 GHz exhibits an impedance matching bandwidth (BW) of 5.73 – 6.23 GHz, a 3-dB gain BW of 5.76 – 6.17 GHz with the peak gain of 14.63 dBi, and an out-of-band suppression of ≥20 dB. Meanwhile, it achieves an RCS-reduction bandwidth of about 4.0 to 9.0 GHz (76.9%).

This paper presents a low-profile, high-isolation, dual-polarized antenna with broadband and widebeam characteristics. The antenna consists of a square patch fed by double differential-fed scheme for the high-isolation dual-polarized radiation. The patch is loaded with four top-hat monopoles to not only broaden the beamwidth, but also generate an extra resonance, which is utilized to improve the operational bandwidth. For verification, an antenna prototype with $0.096\lambda _{0}$ -height at the center frequency of 2.5 GHz has been fabricated and measured. The measurements result in a 10-dB return loss bandwidth of 13.8% (2.36 - 2.71 GHz), an isolation of ≥ 43 dB, and excellent widebeam dual-polarized radiation, i.e., at 2.4 - 2.6 GHz, its half-power beamwidths are ≥ 180° and ≥ 120° in the E- and H-planes, respectively.

This paper presents a dual-circularly polarized (dual-CP) antenna with simple configuration, low-profile, and wide bandwidth. Its primary radiating element is an aperture-coupled patch loaded with a stacked patch for a wide operational bandwidth. A single-section hybrid coupler is utilized as a feeding structure of the antenna to generate dual-CP radiation. Different from the priors normally requiring both wideband radiator and wideband feeding structure, this work demonstrates that the wideband dual-CP radiation can be obtained by combining a wideband radiator and a conventional hybrid coupler. A design operating at the center frequency of 5.5 GHz has been fabricated and tested. The measurements result in a −10-dB reflection coefficient bandwidth of 30.0% (4.66 – 6.31 GHz), a 10-dB isolation bandwidth of 30.9% (4.66 – 6.35 GHz), a 3-dB axial ratio (AR) bandwidth of 29.1% (4.70 – 6.30 GHz), and a realized broadside gain of 5.5 – 7.05 dBic. For a more robust dual-CP radiation, a low-sidelobe 4 $\times $ 4 element array of the proposed antenna has been designed, fabricated, and measured. Compared to the single element design, the array prototype yields similar impedance and 3-dB AR bandwidths, but a higher isolation. The measurements on the array prototype result in an isolation of $\geq 14$ dB, a peak gain of 16.3 dBic, a side-lobe level of $\leq -20$ dB, and a cross-polarization of $\leq -16$ dB across the frequency range of 4.7 – 6.3 GHz (29.1%).

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 $\sim 2.0\lambda _{\text {min}} \times 2.0\lambda _{\text {min}} \times 0.49\lambda _{\text {min}}$ ( $\lambda _{\text {min}}$ is the free-space wavelength referring to the lowest operational frequency) result in an impedance bandwidth of 17.1% ( $5.02-5.96$ GHz) for 10-dB return loss and a high isolation of $\ge45$ 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 $\le $ –25 dB within the passband, and out-of-band suppression level of $\ge20$ dB.

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- $\Omega $ 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 $|S_{11}| < -10$ -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.

In this paper, the design of a wideband high-gain low-profile linearly polarized antenna is presented. The design objectives were achieved by encircling a folded strip around a planar radiating element on the top substrate with corner-cut parasitic patches on the bottom substrate placed above the ground plane. A half-wavelength printed dipole utilized as a radiating element was employed to generate linearly polarized radiation. The contributions of the folded strips and parasitic patches are significant factors in enhancing the bandwidth by producing extra resonance and phase compensation. The experiments and full-wave electromagnetic simulations have provided a platform for the analysis, validation, and verification of the antenna design. The antenna structure has the overall dimensions of 120 mm × 120 mm × 16.3 mm (0.69λo × 0.69λo × 0.094λo at 1.74 GHz), and it demonstrated satisfactory measured performance characteristics with a -10-dB impedance bandwidth of 1.39-2.09 GHz (40.22%), a broadside gain of 6.1-8.2 dBi, and a radiation efficiency >88% within the impedance bandwidth.

In this paper, a low-profile tripolarized antenna with the wideband operation is presented. It includes a monopolar patch element for vertically linear polarization (LP) omnidirectional radiation in the horizontal plane and two orthogonal dipoles for x- and y-direction LP broadside radiations. In order to achieve the wideband, nevertheless keeping the low-profile, the monopolar patch antenna is coupled with an annular ring, while the two orthogonal dipoles are incorporated with a metasurface. Optimized design with a profile of 0.09λmin (λmin is the free space wavelength referring to the lowest operational frequency) has been fabricated and measured. The measurements result in an overlapped bandwidth of 24.45% (2.19-2.80 GHz) for reflection coefficient

A broadband electrically small antenna with directive circularly polarized radiation is presented. It is composed of a compact single-feed crossed dipole driver, which is backed by a near-field resonant parasitic (NFRP) reflector to achieve the directive radiation pattern. The NFRP reflector is designed to generate extra resonances and minimum axial ratio (AR) points in the antenna system. These features are combined with those resulting from the driver to broaden the operational bandwidth. The proposed antenna was fabricated and measured. The antenna prototype, with a low profile (0.066 λo at 1.45 GHz) and an electrically small size (ka = 0.71 at 1.45 GHz), has a measured |S11|

This paper presents low-profile broadband antennas, which are composed of four parasitic patches placed between planar radiators and a perfect electric conductor ground plane. Two types of planar radiators, a conventional dipole and a crossed dipole, are employed to produce linearly polarized (LP) and circularly polarized (CP) radiations, respectively. The radiator and parasitic patches are realized on thin substrates to lower the cost. Owing to the presence of parasitic patches, the antenna performance improves in terms of profile reduction, resonant frequency decrease, and bandwidth enhancement. These improvements are discussed and confirmed computationally and experimentally. The LP design with the overall dimensions of 120 mm × 120 mm × 16.3 mm (0.64λ0 × 0.64λ0 × 0.087λ0 at 1.6 GHz) has a |S11| 96%. The CP design, which has the same physical size as the LP case, has a |S11| 90%.

A cavity-backed angled-dipole antenna is proposed for millimeter-wave wireless applications. The angled-dipole radiator is built on both sides of an RT/Duroid 5880 substrate (εr=2.2) and fed by a parallel-plate transmission line. The cavity-backed reflector is utilized to improve the radiation characteristics of the angled dipole, such as gain, back-radiation, symmetric pattern, and similar 3 dB beamwidth in the E- and H-planes. The design, with a cavity aperture of 0.5λ28-GHz×0.5λ28-GHz, results in a S11

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.