This paper presents a two-dimensional Fabry-Pérot cavity antenna (FPCA) based on a bianisotropic Huygens' metasurface (BHMS) acting as an enhanced version of the usual partially reflective surface. The BHMS wavefront transformation capabilities enable achieving broadside pencil-beam radiation without relying on the conventional resonance condition that constraints classic FPCA designs regarding excitation and maximum directivity. By rigorously stipulating the guided and radiated field, the BHMS is able to implement the required transmission phase shift across the radiating aperture while guaranteeing proper wave propagation inside the cavity without impedance mismatches. Hence, the proposed approach allows achieving higher directivity without the guided modes getting closer to cutoff nor the beam becoming conical. This concept is validated through a design example showcasing a directive broadside beam with a preliminarily stable frequency behavior in simulation.

The radiation pattern and efficiency of a leaky-wave antenna is dependent on its aperture field distribution. The required field magnitude is achieved by modulating the leakage factor along the antenna, for which an approximate formula derived in the last century continues to be widely used nowadays. However, this expression assumes the field transverse profile inside the guiding structure does not change along its length. This contribution revisits this well-known formula in order to obtain it from a rigorous approach assuming a leaky rectangular waveguide supporting a TE field configuration. Thus, the origin of the limitation of the maximum leakage factor value is explained for this scenario, its influence is quantified by establishing a numerical criterion, and the validity of the approximate formula is justified. Finally, some analytical examples are discussed, showing how the approximate leakage factor differs from the more accurate numerically computed one in limit cases, and how the radiation patterns are affected.

This paper investigates the influence of the modulation waveform on the frequency translation of time-modulated metasurfaces. Using an analytical Floquet model and FDTD simulations, several temporal waveforms are analyzed. Results show that sinusoidal, square, and triangular modulations produce symmetric multi-harmonic spectra, while the sawtooth waveform enables nearly single-harmonic translation following the Serrodyne principle. The modulation phase controls the translation direction, and the strong correlation between analytical and numerical results confirms the model's accuracy and the potential of waveform engineering for reconfigurable frequency translation.

This paper investigates frequency translation phenomena in transmissive time-modulated metasurfaces using a Floquet-based analytical model validated by time-domain FDTD simulations, enabling direct comparison of the transmitted electric field from a time-varying dielectric slab. The impact of different modulation waveforms on harmonic selectivity and frequency translation is investigated.

Acoustic signals, which have been utilized for decades in the spatial localization of objects, have found applications in fields as diverse as sonar for underwater navigation, communication, and object detection. Traditional methods often rely on arrays of transducers, which necessitate the use of expensive hardware and processing algorithms. An emerging alternative is the Acoustic Leaky Wave Antenna (ALWA), which is inspired by electromagnetic leaky wave antennas. ALWA technology employs a single transducer to emit directional beams that scan angular space by frequency manipulation. Conventional arrays offer cost-effectiveness and simplicity of design, but ALWAs have the advantage of operating on the principle of energy leakage, which is achieved by various mechanisms, such as uniform apertures or slits periodic along the waveguide. This technology, applicable to underwater and airborne communications, offers compact and energy-efficient solutions, which facilitate the development of the “Underwater Internet of Things” and autonomous communication systems for underwater vehicles. This work presents a parametric study of this type of antennas with axisymmetric geometry by means of a numerical solution based on the Finite Element Method. Together with analytical studies, the physical phenomenology underlying this technology will be described, including directivity, transmission and reflection parameters, beam scanning and dispersion curve. Finally, the design is validated through experiments.

Time-varying systems enable advanced electromagnetic devices by surpassing the limitations of conventional designs. This paper presents the influence of the time-modulation shape on an example of a time-varying system. The circuit is a coupled-resonator filter which is already available in the literature and which presents non-reciprocal frequency behaviour due to the time modulation of its capacitances. Different modulation schemes, including sinusoidal and sawtooth signals, are evaluated through MATLAB simulations to assess the effects of non-linear varactor C-V characteristic and other modulation specifications on filter performance. It is verified that the fundamental parameters that control the frequency response are the modulation period and the relative amplitude of the time variation. The specific shape of the modulation (i.e., its higher harmonics) seems to play a secondary role in the studied cases.

Omega-type bianisotropic Huygens' metasurfaces (HMSs) offer a novel approach for controlling the aperture field distribution of leaky wave antennas (LWAs). This article presents a methodology utilizing a parallel-plate waveguide with an MS as its top plate. Previous limitations on constant leakage factor are addressed using a slowly varying amplitude approximation (SVAA) in order to satisfy Maxwell's wave equation, enabling the design of the radiation pattern. A semi-analytical algorithm is employed to obtain the required multilayer unit-cell geometries. Here, the interlayer coupling is taken into account, enabling an efficient synthesis of these antennas. Several designs presenting different pointing angles and aperture field distributions are carried out, showing very good agreement between theory and realistic simulations without further full-wave optimization. Finally, the design process is experimentally validated through several prototypes, whose measurements are shown and discussed. These results, demonstrating straightforward semi-analytical synthesis of versatile aperture profiles, would significantly broaden the applicability of such antennas in next-generation wireless systems.

This work presents a reconfigurable leaky-wave antenna based on an Omega-bianisotropic Huygens' metasurface as leaky surface and gap waveguide as guiding structure. The latter is filled by a nematic liquid crystal, which is a tunable dielectric whose particles can be polarized through an external bias voltage. The gap waveguide technology allows for a non-electrical contact feature, which facilitates the connection of the required external bias voltage. The beam radiation angle can be controlled at a fixed frequency by adjusting the relative permittivity of the LC, so that electronical beam steering can be achieved. A design at 20 GHz of a leaky-wave antenna with beam scanning capabilities in the range of approximately [-10 degrees, 10 degrees] with the Merck GT7-29001 LC is accomplished and successfully tested through full-wave simulations.

This contribution proposes a design methodology to realize Fabry-Perot cavity antennas with enhanced aperture efficiency and bandwidth. The proposed antenna employs a tapered Partially Reflective Surface (PRS) and leverages a com-bined approach incorporating principles from the simplified ray model and two-dimensional leaky-wave theory. By employing a transmission line model of the PRS, unit cells are selected that simultaneously satisfy the resonance condition, present a positive reflection phase slope, and synthesize the necessary leakage factor function. A design to obtain a uniform aperture distribution is carried out. The simulation results are compared against the best design from a previous study, thus showing the improvement realized by incorporating leaky-wave theory into the antenna design process.

This article presents a systematic approach to broadband Fabry-P & eacute;rot cavity antenna (FPCA) synthesis, integrating ray-optics and leaky wave perspectives to overcome limitations of traditional heuristic designs. By precisely engineering the partially reflective surface (PRS) properties, desired radial aperture field distributions are achieved, enabling radiation pattern shaping and enhancing aperture efficiency. Simultaneously, a positive-slope phase technique is employed to increase the operational bandwidth. Furthermore, the practical excitation of FPCAs is addressed, considering fundamental feeding limitations and offering guidelines for the primary source selection. The methodology is validated through three different FPCA designs that use single both-side-etched laminate PRSs. The experimental results of the fabricated prototypes are compared to full-wave simulations, demonstrating improved directivity-bandwidth product and successful excitation using a printed dipole feed.

In Doppler radar systems for human respiratory motion measurement, the change in wave polarization after reflection can be critical. In this contribution, a polarization-agile antenna based on a cavity-back slot fed by stripline is proposed to improve the experimental setup to investigate polarimetric effects in such measurements. The antenna consists of a series-fed sequentially-rotated array of four elements terminated in two ports. In this way, it is demonstrated that both right-handed and left-handed circular polarizations can be received and, by combination of both, arbitrary polarization can be obtained.

The use of an equivalent circuit based on eigenstates is proposed for the analysis and modeling of biperiodic cross-strip scatterers. The scatterer, modeled as a four-port network, is first simplified as a two-port network, and its general eigenstate equivalent-circuit topology is shown. By using this topology, the equivalent circuit for the case of a single-strip scatterer is obtained. A multimode expansion procedure is used to calculate the values of the admittance of the model by assuming the surface current on the strip. Then, due to the capability of the equivalent circuit to decompose the eigen-excitations, the extraction of the parameters for the cross is reduced to calculating the ones from two different strips. This results in a very simple model of the structure that provides accurate results. The performance of the model is validated with full-wave electromagnetic simulations.

This paper presents a method for effectively characterizing the dielectric permittivity of nematic liquid crystals across a broad frequency range.These materials show significant potential for reconfigurable devices operating in microwave and millimeterwave frequencies.To achieve this goal, an additive manufacturing technique is used to create a microstrip line that can be filled with liquid that acts as its substrate.The liquid crystal is then biased to modulate its permittivity.After manufacturing, a time-gating approach is used to extract the permittivity, eliminating the need for TRL calibration.Finally, the approach is validated through simulations and experimental results, which closely align with those reported using other methods in the bibliography.

The analytical values of the parameters of an eigenstate-based equivalent circuit are obtained for the analysis and modeling of a simple bi-periodic scatterer. The scatterer geometry consists of a rotated, very narrow slot. Analytical values of this kind of equivalent circuit are obtained for the first time by comparison with a fully-analytical multi-mode equivalent circuit. Analytical results are contrasted with simulated ones, and a very good agreement is found.

A methodology based on omega-type bianisotropic Huygens' metasurfaces is presented to control the aperture field distribution of leaky-wave antennas. The studied structure is a parallel-plate waveguide with the top plate replaced by a metasurface. Previous works achieved independent control of the phase constant and the leakage factor, but they were constrained to be constant. The required theoretical extensions to overcome this limitation are presented in this work, thus enabling the design of arbitrary radiation patterns. A slowly varying amplitude approximation approach is employed to satisfy Maxwell's wave equation and obtain the relation between the horizontal and vertical wavenumbers. In addition, a semianalytical algorithm able to predict near-field coupling effects is applied in the microscopic design of the metasurface unit cells. Two designs are carried out with real unit cells, presenting different aperture configurations. Finally, electromagnetic simulations validate the methodology with an excellent agreement without any further full-wave optimization.

A leaky-wave antenna made of cavity-backed-slot radiating elements with open-stopband suppression is presented. The cavity-backed slots are seriesly fed by a strip. The bandwidth of the unit cell of the array is enhanced by a recently-proposed technique that uses a matching stub. The open-stopband effect is mitigated by misaligning the stub with respect to the slot. An array is designed and fabricated. The array is built using an aluminum set of cavities filled with 3D printed material to support the substrate where the feed and slots are etched. The measurement of the prototype shows the overall good performance of the array. The array is able to scan its beam from -70 degrees to 50 degrees in the 2.8-4.4 GHz frequency range, keeping good matching even at the broadside frequency. The gain is stable over the band.

A novel technique to design Fabry-Pérot antennas with non-homogeneous partially reflective surfaces (PRSs) is described. It uses a transmission line circuit model to efficiently obtain all necessary unit cell designs that satisfy the cavity resonance condition. This method allows an increase in directivity without reducing the bandwidth for a given footprint. Some design examples in the Ku-band are presented, showing the evolution from a simple single-layer PRS to a non-homogeneous two-layer one. The latter achieves an increase of about 3 dB in directivity while maintaining the bandwidth in electromagnetic simulations. This way, the gain-bandwidth product is improved from a value of 5 to almost 9, effectively raising the antenna efficiency.

The use of an eigenstate-based equivalent-circuit topology is proposed for the analysis and modeling of lossless and lossy bi-periodic scatterers. It significantly simplifies the design of this kind of surfaces, since it reduces the number of elements with respect to other general equivalent circuits. It contains at most only two admittances and one complex turns ratio. The real parts of these admittances can be assured to be nonnegative, an interesting aspect in the modeling of lossy surfaces such as those present in absorbers. Moreover, due to the capability of decomposition into the eigenexcitations of the structure, the circuit provides an important physical insight. Different cases of scatterers have been analyzed: symmetric and asymmetric, and lossy and lossless. In all these cases, modeling of the circuit admittances has been successfully achieved with a few positive and frequency-independent RLC elements. In the case of structures with symmetries, the turns ratio directly reflects the physical orientation of the scatterer eigenexcitations. Furthermore, in the case of lossy scatterers without symmetries, the resulting equivalent circuit reveals that their eigenexcitations are not linear polarizations, but elliptic polarizations whose properties are described by a complex turns ratio.