Many wireless communication applications such as satellite communications use circularly polarized (CP) signals, with the requirement for easy switching of the polarization sense between uplink and downlink. Specifically, in satellite communications, the trend is also to move to higher frequencies and integrate the receiving and transmitting antennas in one dual-band terminal. However, these simultaneous demands make the design and fabrication of the composing parts very challenging. We propose, here, a dual-band dual-linear polarization (LP)-to-CP converter that works in the transmission mode. The working principle of this polarizer is explained through an example for Ka-band satellite communications at 19.7–20.2 and 29.5–30 GHz. The LP-to-CP converter is a single panel composed of identical unit cells with a thickness of only 1.05 mm and a size of 5.3 mm $\times5.3$ mm. Due to its operation in the transmission mode, the polarizer can be combined with a simple dual-band dual-LP antenna to obtain the desired dual-band dual-CP single antenna. However, the unique property of this polarizer is yet the fact that it converts a given LP wave, e.g., x-polarization, to orthogonal CP waves at the two nonadjacent frequency bands, e.g., left-handed CP at lower band and right-handed CP at higher band. The polarizer is tested both with 20 and 30 GHz LP rectangular horns to illuminate a dual-band transmit array (TA) to obtain wide-angle steering of CP beams. The performance of the polarizer and its association with the TA is evaluated through simulation and measurements. We also present design guidelines for this type of polarizer.

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Dual-Band Dual-Linear-to-Circular Polarization Converter in Transmission Mode Application to $K/Ka$ -Band Satellite Communications

A dual-band linear-to-circular polarization converter (LCPC) based on a single-layer dielectric substrate is proposed. The element of the converter consists of two identical metallic layers with a combination of a connected Jerusalem cross (JC) and an “I”-type dipole for each layer. The proposed converter is designed by using an equivalent circuit model (ECM). Left-handed circularly polarized (LHCP) and right-handed circularly polarized (RHCP) beams can be, respectively, generated at ${K}$ -band and Ka -band excited by a linearly polarized (LP) wave tilted 45° relative to the ${x}$ - and ${y}$ -directions of the converter. In addition, the converter covers two operation bands for ${K}$ -/ Ka -band satellite communications with high conversion efficiency and low polarization extinction ratio (PER). After full-wave optimization, the proposed converter is fabricated and measured. The measured results show a good agreement with the simulated ones. Even though there exists a tradeoff between the angular stability and the bandwidth of the dual-band LCPCs, the measured axial ratio (AR) remains stable in the lower operation band and a slight fluctuation in the higher band with the incident angle of 20°.

Single-Layer Dual-Band Linear-to-Circular Polarization Converter With Wide Axial Ratio Bandwidth and Different Polarization Modes

Transmit array design is more challenging for dual-band operation than for single band, due to the independent 360° phase wrapping jumps needed at each band when large electrical length compensation is involved. This happens when aiming at large gains, typically above 25 dBi with beam scanning and $F/D \le 1$ . No such designs have been reported in the literature. A general method is presented here to reduce the complexity of dual-band transmit array design, valid for arbitrarily large phase error compensation and any band ratio, using a finite number of different unit cells. The procedure is demonstrated for two offset transmit array implementations operating in circular polarization at 20 GHz(Rx) and 30 GHz(Tx) for Ka-band satellite-on-the-move terminals with mechanical beam-steering. An appropriate set of 30 dual-band unit cells is developed with transmission coefficient greater than −0.9 dB. The full-size transmit array is characterized by full-wave simulation enabling elevation beam scanning over 0°–50° with gains reaching 26 dBi at 20 GHz and 29 dBi at 30 GHz. A smaller prototype was fabricated and measured, showing a measured gain of 24 dBi at 20 GHz and 27 dBi at 30 GHz. In both cases, the beam pointing direction is coincident over the two frequency bands, and thus confirming the proposed design procedure.

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High Gain Dual-Band Beam-Steering Transmit Array for Satcom Terminals at Ka-Band

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

This communication presents the design, optimization, fabrication, and characterization of an electronically steerable transmitarray (TA) with 2 bits of phase quantization per unit cell. The proposed TA operates in linear polarization at Ka -band and is composed of $14\times14$ reconfigurable unit-cells. Four p-i-n diodes are integrated on each unit-cell to control the radiated field phase distribution across the TA aperture. The prototype demonstrates experimentally pencil beam scanning over a $120^{\circ } \times 120^{\circ }$ window, a maximum gain at broadside of 19.8 dBi, and a 3 dB fractional bandwidth of 16.2%.

2 Bit Reconfigurable Unit-Cell and Electronically Steerable Transmitarray at $Ka$ -Band

The synthesis of a metasurface exhibiting a specific set of desired scattering properties is a time-consuming and resource-demanding process, which conventionally relies on many cycles of full-wave simulations. It requires an experienced designer to choose the number of metallic layers, the scatterer shapes and dimensions, and the type and the thickness of the separating substrates. In this article, we propose a generative machine learning (ML)-based approach to solve this one-to-many mapping and automate the inverse design of dual- and triple-layer metasurfaces. Using this approach, it is possible to solve optimization problems with single or more constraints by synthesizing thin structures composed of potentially brand-new scatterer designs, in cases where the interlayer coupling between the layers is nonnegligible and synthesis by traditional methods becomes cumbersome. Various examples to provide specific magnitude and phase responses of $x$ - and $y$ -polarized scattering coefficients across a frequency range as well as bounded responses for different metasurface applications are presented to verify the practicality of the proposed method.

A Generative Machine Learning-Based Approach for Inverse Design of Multilayer Metasurfaces

Planar transmit arrays (TAs) have been an attractive solution as gain enhancers for various applications, e.g., satellite communications. The TA performance directly depends on its composing unit-cell characteristics. Planar unit cells can be categorized into two main types: phase-rotation (PR) and phase-delay (PD) cells. There is no hint in the literature about the relative merits of these two types of cells for circular polarization when assessing the final TA performance. This paper offers a systematic comparison between the cells’ working principles and analyzes their impacts on TA performance. Examples of a PR-based TA and a PD-based TA are designed for single-band wide-angle beam steering operating at the satellite Ka-band. They are evaluated by simulation and measurement to quantify performance differences. No previous work employed a PR TA for wide-angle beam steering. This paper shows that PR TA offers a filtering effect toward the cross-polarization component of the source. This leads to better axial ratio and combined 3 dB axial ratio and 3 dB gain bandwidth. However, PD cells are easier to design and insensitive to feed polarization. The analysis in this paper allows a more informed decision when selecting the unit-cell category for any given TA application.

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Phase-Delay Versus Phase-Rotation Cells for Circular Polarization Transmit Arrays—Application to Satellite Ka-Band Beam Steering

A novel antenna-filter-antenna-based asymmetric unit cell suitable for transmit-array (TA) is proposed in this letter. This unit cell converts a clockwise circular-polarized (CP) wave to counterclockwise CP with less than 0.5 dB insertion loss at 20.5 GHz, while its rotation provides up to 360° continuous phase shift in the transmitted wave. Hence, a 14 × 14 array of the proposed unit cell is fabricated and placed in front of a CP feed antenna. Placing the TA leads to a 9.5-dBi increase in the gain of the feed, and it redirects the beam to -15° off broadside. The axial ratio of the whole system is close to 2 dB. Therefore, this transmit-array can be used for fixed and/or steerable high-gain radiations for satellite communication in Ka-band.

Antenna-Filter-Antenna-Based Transmit-Array for Circular-Polarization Application

A wideband circularly polarized antenna with L-shaped monopole slots and rotated parasitic patches has been proposed. The measured results show that the antenna has $ {\rm VSWR}\ ( and axial-ratio ( ${\rm AR} ) bandwidth of 64%. A novel wideband feeding structure has been used to excite the L-shaped slot and match the structure to 50- $\Omega$ impedance. The rotated parasitic patches are devised in the close proximity of slot to further enhance the circular polarization purity. Moreover, a Bazooka (sleeve) balun is used to eliminate the currents on the shield of coaxial feeding line and degrade AR. The antenna is relatively low-profile, low-cost, and easy to fabricate.

Parinaz Naseri

Papers

Dual-Band Dual-Linear-to-Circular Polarization Converter in Transmission Mode Application to $K/Ka$ -Band Satellite Communications

A Generative Machine Learning-Based Approach for Inverse Design of Multilayer Metasurfaces

Phase-Delay Versus Phase-Rotation Cells for Circular Polarization Transmit Arrays—Application to Satellite Ka-Band Beam Steering

Antenna-Filter-Antenna-Based Transmit-Array for Circular-Polarization Application

Circularly Polarized Monopole L-Shaped Slot Antenna With Enhanced Axial-Ratio Bandwidth