This paper presents a novel design of a Fabry-Perot (FP) resonator antenna with a wide gain bandwidth in X band. The bandwidth enhancement of the antenna is attributed to the positive reflection phase gradient of an electromagnetic band gap (EBG) structure, which is constructed by the combination of two complementary frequency selective surfaces (FSSs). To explain well the design procedure and approach, the EBG structure is modeled as an equivalent circuit and analyzed using the Smith Chart. Experimental results show that the antenna possesses a relative 3 dB gain bandwidth of 28%, from 8.6 GHz to 11.4 GHz, with a peak gain of 13.8 dBi. Moreover, the gain bandwidth can be well covered by the impedance bandwidth for the reflection coefficient ( ${\rm S} _{11}$ ) below $-10~{\rm dB}$ from 8.6 GHz to 11.2 GHz.

Wideband Fabry-Perot Resonator Antenna With Two Complementary FSS Layers

Based on a transformation optics method, a flat compact dual-polarized Luneburg lens antenna is proposed and implemented using the printed-circuit-board-stacked gradient-index metamaterials for beamscanning and multibeam applications at X-bands. The transformed material properties of the planar Luneburg lens are designed with 17-layered permittivity distribution of polynomials. Each layer is discretized into $41 \times 41$ pixels made of broadband and less polarization-dependent unit cells responsible for desired index distributions. The effects of transformation, approximation, and discretization on the lens performance are analyzed comprehensively. Also, to validate the implementation method, a flat Luneburg lens with a thickness of 14.1 mm, a focal length of 28 mm, and an aperture size of $98.9 \times 98.9$ mm2 is designed and tested. A stacked aperture-coupled patch antenna operating at 10 GHz is applied as a feeder. The measured results show that the proposed antenna can operate over a bandwidth of ~20% with an antenna efficiency of 32%, a cross-polarization level of <−17.1 dB, as well as the maximum gain of 15.9/16.35 dBi and a scanning angle of ±32°/±35° for two orthogonal polarizations, respectively. The presented flat Luneburg lens antenna featuring broad bandwidth, high gain, wide scanning angle, and easy fabrication has a high potential in 5G wireless communication, imaging, and remote sensing applications.

A Flat Dual-Polarized Transformation-Optics Beamscanning Luneburg Lens Antenna Using PCB-Stacked Gradient Index Metamaterials

This communication presents the design and experimental verification of a substrate integrated waveguide fed lens antenna for 79 GHz automotive radar applications. The proposed integrated lens antenna consists of a six layer cylindrical Luneburg lens illuminated by 17 source elements of substrate integrated waveguide fed by a planar log periodic dipole antenna array. The Luneburg lens is developed by using a unique foam material AirexR82 ( $\varepsilon _{r} = 1.12$ ), which is drilled and pressed to achieve the different dielectric constant needed to follow the index law inside the lens. The return loss and the radiation characteristics of the proposed integrated lens antenna are investigated and validated by measurements. A good agreement between the measured and simulated results is observed. The measured results confirm the beam scanning capability of the proposed integrated lens antenna for a wide scan angle of ±85° in azimuth plane having a maximum antenna gain of 15 dBi at 82.5 GHz and gain drop of less than 2.75 dB for the edge feeds. The estimated radiation efficiency of the antenna is found to be 67% at 82.5 GHz.

Lens Antenna for Wide Angle Beam Scanning at 79 GHz for Automotive Short Range Radar Applications

Multi-beam antennas are critical components in future terrestrial and non-terrestrial wireless communications networks. The multiple beams produced by these antennas will enable dynamic interconnection of various terrestrial, airborne and space-borne network nodes. As the operating frequency increases to the high millimeter wave (mmWave) and terahertz (THz) bands for beyond 5G (B5G) and sixth-generation (6G) systems, quasi-optical techniques are expected to become dominant in the design of high gain multi-beam antennas. This paper presents a timely overview of the mainstream quasi-optical techniques employed in current and future multi-beam antennas. Their operating principles and design techniques along with those of various quasi-optical beamformers are presented. These include both conventional and advanced lens and reflector based configurations to realize high gain multiple beams at low cost and in small form factors. New research challenges and industry trends in the field, such as planar lenses based on transformation optics and metasurface-based transmitarrays, are discussed to foster further innovations in the microwave and antenna research community.

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Quasi-Optical Multi-Beam Antenna Technologies for B5G and 6G mmWave and THz Networks: A Review

We present a broadband metasurface Luneburg lens for full-angle operation. The design of the Luneburg lens is based on an inverted substrate parallel-plate-waveguide structure with nonuniform circular holes etched on the upper metallic plate. We show that this structure is superior to existing designs as it allows significant simplification of the fabrication process. To achieve a smooth variation of the effective refractive index profile as well as circular symmetry for the Luneburg lens, size-variable meta-atoms are used for synthesizing the metasurface. The effective refractive index profile can be tuned by varying just the hole diameters. By introducing tapered microstrip ports that are evenly distributed on the circumference of the Luneburg lens, full-angle operation of the lens can be achieved. A prototype with a diameter of 400 mm is fabricated and experimentally evaluated at operating frequencies ranging from 6 to 9 GHz. We show good conceptual agreement between the simulated and experimental results.

Design of a Broadband Metasurface Luneburg Lens for Full-Angle Operation

This paper presents a novel design of cylindrical modified Luneberg lens antenna at millimeter-wave (mm-wave) frequencies in which no dielectric is needed as lens material. The cylindrical modified Luneberg lens consists of two air-filled, almost-parallel plates whose spacing continuously varies with the radius to simulate the general Luneberg's Law. A planar antipodal linearly-tapered slot antenna (ALTSA) is placed between the parallel plates at the focal position of the lens as a feed antenna. A combined ray-optics/diffraction method and CST-MWS are used to analyze and design this lens antenna. Measured results of a fabricated cylindrical modified Luneberg lens with a diameter of 100 mm show good agreement with theoretical predictions. At the design frequency of 30 GHz, the measured 3-dB E- and H-plane beamwidths are 8.6° and 68°, respectively. The first sidelobe level in the E-plane is -20 dB, and the cross-polarization is -28 dB below peak. The measured aperture efficiency is 68% at 30 GHz, and varies between 50% and 71% over the tested frequency band of 29-32 GHz. Due to its rotational symmetry, this lens can be used to launch multiple beams by implementing an arc array of planar ALTSA elements at the periphery of the lens. A 21-element antenna array with a -3-D dB beam crossover and a scan angle of 180° is demonstrated. The measured overall scan coverage is up to ±80° with gain drop less than -3 dB.

Air-Filled Parallel-Plate Cylindrical Modified Luneberg Lens Antenna for Multiple-Beam Scanning at Millimeter-Wave Frequencies

This paper presents dual-band equal/unequal Wilkinson power dividers based on a coupled-line section with short-circuited stub (called as the \coupled-line section" for short), which consists of a pair of parallel coupled lines and a short-circuited stub. With the analyses of the phase shift and equivalent characteristic impedance, the coupled-line section is used to replace the quarter-wavelength branch line in the conventional equal/unequal Wilkinson power divider to obtain excellent dual-band operation. The closed-form equations and design procedures of dual-band Wilkinson power divider are given, where one degree of design freedom is obtained and design ∞exibility is shown. As two examples, a dual-band equal Wilkinson power divider with the frequency ratio of 1:8 : 1 and an unequal one with the high power dividing ratio of 7 : 1 and frequency ratio of 1:8 : 1 are designed, fabricated and measured. The measurements are in good agreement with the simulations. It is shown that the proposed power dividers have simple topologies, and can be easily fabricated with small frequency ratios and high power dividing ratios.

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Dual-band equal/unequal wilkinson power dividers based on coupled-line section with short-circuited stub

A new omnidirectional circularly polarized (CP) cylindrical dielectric resonator (DR) antenna (DRA) is excited by four open-ended logarithmic spiral slots in the ground plane. Radiated E
 θ
 and E
 φ
 components are obtained from the TM
 01δ
 mode of the DR and the slotted ground plane, respectively. Omnidirectional CP fields can be obtained when E
 θ
 and E
 φ
 are equal in magnitude but different in phase by 90°. This CP technique does not require any special shape or treatment of DR, greatly facilitating the design. To demonstrate the idea, a prototype using a glass DR was fabricated. It is a dual-function DRA that also serves as the cover of light sources. The prototype has a measured bandwidth of ~6.2%, covering the entire 2.4-GHz WLAN band.

Omnidirectional circularly polarized dielectric resonator antenna with logarithmic spiral slots in the ground

This article presents a compact absorptive filtering patch antenna. It consists of a filtering patch antenna and a bandstop filter (BSF), with their transfer functions being complementary to each other. A slot is fabricated in each of the patch and ground, giving a total of two radiation nulls for the lower bandedge. By using a dual-stub feed, two radiation nulls are also obtained for the upper bandedge. For the BSF, a $\lambda _{\mathrm {g}}$ /2 defected ground structure (DGS) and a $\lambda _{\mathrm {g}}$ /4 defected microstrip structure (DMS) are used in the design. It is terminated by a chip resistor. Since the filtering patch antenna and BSF have complementary transfer functions, the incident energy can be radiated effectively in the passband but largely absorbed by the resistor in the stopbands. As a result, only little energy will be reflected over a wide frequency range, giving a reflectionless characteristic. To demonstrate this idea, an absorptive filtering antenna operating at 5.8 GHz was designed, fabricated, and tested. Its impedance is matched from 5 to 6.5 GHz, with the measured out-of-band suppression being higher than 17 and 20 dB for the lower and upper stopbands, respectively. The measured peak realized gain is 7.28 dBi.

Compact Absorptive Filtering Patch Antenna

The cylindrical dielectric resonator (DR) antenna (DRA) is excited in its omnidirectional TM $_{01\delta }$ mode by a planar shorted microstrip cross. With this nonintrusive feed, the DRA can be fabricated without the need of drilling a hole in the DR as required in the probe feed method. This DRA is applied to the first omnidirectional circularly polarized (CP) diversity DRA. To generate omnidirectional CP fields, the TM $_{01\delta }$ and TE $_{011+\delta }$ modes are excited simultaneously. The TE $_{011+\delta }$ mode is excited by four microstrip arcs. They provide a pair of equivalent magnetic dipoles that generate fields that are orthogonal to those of the TM $_{01\delta }$ mode. Omnidirectional CP fields can be obtained when the (orthogonal) fields of the TM $_{01\delta }$ and TE $_{011+\delta }$ modes are equal in amplitude but in phase quadrature. In our two-port CP diversity design, phase differences of +90° and −90° are obtained in ports 1 and 2 to generate right- and left-hand CP fields, respectively. Prototypes at ~2.4 GHz were designed, fabricated, and measured for WLAN applications. The S-parameters, radiation patterns, antenna gains, and efficiencies are studied. For the diversity design, the axial ratio, envelope correlation coefficient, and mean effective gain are also obtained. The measured and simulation results are in reasonable agreement.

Nan Yang

Papers

Air-Filled Parallel-Plate Cylindrical Modified Luneberg Lens Antenna for Multiple-Beam Scanning at Millimeter-Wave Frequencies

Dual-band equal/unequal wilkinson power dividers based on coupled-line section with short-circuited stub

Omnidirectional circularly polarized dielectric resonator antenna with logarithmic spiral slots in the ground

Compact Absorptive Filtering Patch Antenna

Omnidirectional Dielectric Resonator Antenna With a Planar Feed for Circular Polarization Diversity Design