Leaky-wave Structure Research Articles

2-D Fixed-Frequency Terahertz Beam Steering With Microactuated Leaky-Wave Structure

2-D Fixed-Frequency Terahertz Beam Steering With Microactuated Leaky-Wave Structure

Analytical formulation of spatiotemporal modulated graphene-based waveguides using Floquet-Bloch theory

In this work, an analytical model to study graphene-based spatiotemporal modulated structures is developed and verified through comparison with full wave numerical simulations. Graphene is an ideal material for realizing spatiotemporal modulated structures at high frequencies of THz and optics. In this analysis, the electromagnetic response of studied structures is expressed in terms of weighted Floquet-Bloch modes supported by the structure, while graphene is modeled by a spatiotemporal modulated surface current that imposes certain boundary conditions on the modes. The developed analytical technique is a comprehensive tool and can be used for accurate modeling of different kinds of spatiotemporal devices including lossy, guided, and leaky wave structures. To demonstrate the accuracy of the model, two plasmonic waveguides with space and time modulated graphene conductivity are analyzed and their interband and intraband transition between modes are thoroughly investigated. Using the developed analytical model, spatiotemporal modulation phenomena such as mode conversion, wave amplification and nonreciprocal response are explored and discussed for the studied structures.

Design of Leaky Wave Antenna for V2V Communications

In this article, a low-profile, microstrip-based, periodic leaky wave antenna has been proposed for vehicle-to-vehicle (V2V) communications. The leaky-wave structure, loaded periodically with stepped impedance resonator stubs operates in the 5.4–6.2 GHz frequency band, which includes the WAVE (Wireless Access in Vehicular Environments) band (5.85–5.925 GHz) required for automated vehicular communications. The S11 obtained in the spectrum is below −12.68 dB. While the majority of the antennas designed for V2V communications produced omnidirectional patterns, the structure proposed in this work yields a directional pattern, preventing unnecessary outages of signals. The periodic structure gives an added advantage of beam scanning, resulting in a wider, yet directive coverage in the desired frequency band. A backward beam scan from −13˚ to −31˚ is obtained in the frequency range of 5.4–6.2 GHz with a gain variation of around ±0.6 dB only.

A compact star-shaped leaky-wave antenna for half-space steering

ABSTRACT In this paper, a compact leaky-wave planer antenna is proposed. The proposed antenna fabricated by the printed circuit board technology can scan a large part of its upper half-space. The proposed antenna consists of a metallic six-pointed star on a regular hexagon. The back of the hexagon is wholly covered with a conductor. The structure’s detailed design and analysis have been done using the dispersion diagram. In the proposed antenna, changing the frequency can steer the main beam in elevation at six-step angles of azimuth. The simulation and measurement results confirm the theory well. The leaky wave structure is an attractive and economical method to scan half-space.

A Circular-Polarized Vortex Beams Generation With Orbital Angular Momentum Based on a Leaky-Wave Antenna

A circular-polarized vortex beams generation with orbital angular momentum (OAM) based on a leaky-wave antenna with substrate integrated waveguide (SIW) is proposed in this letter. The loop leaky-wave antenna is designed to transmit vortex beams with various OAM modes controlled by the operating frequency. With the frequency change from 11GHz to 12GHz, the OAM modes change accordingly. The OAM antenna realizes a low profile by use of the leaky-wave structure. Meanwhile, two transverse slots and one longitudinal slot are utilized to realize circular polarization to improve the mode purity of OAM. Notably, the measured results demonstrate that the gain reaches 7.18 dBi with vortex beams generated by four OAM modes (+2, +1, 0, -1). And the axial ratio (AR) is good from 11GHz to 12GHz, indicating that circular polarization is achieved.

Leaky-Vivaldi antenna covered with metasurface with leaky wave radiation and aperture radiation.

A leaky-Vivaldi antenna covered with metasurface (LVAM) is proposed in this paper. The traditional Vivaldi antenna covered with metasurface realizes backward frequency beam-scanning from -41∘ to 0∘ in the high-frequency operating band (HFOB) and retains aperture radiation in the low-frequency operating band (LFOB). In the LFOB, the metasurface can be regarded as a transmission line to realize a slow-wave transmission. In the HFOB, the metasurface can be considered a 2D periodic leaky-wave structure to realize a fast-wave transmission. The simulated results show that LVAM has the -10 dB return loss bandwidths of 46.5% and 40.0%, and the realized gain of 8.8-9.6 dBi and 11.8-15.2 dBi cover the 5 G Sub-6 GHz band (3.3-5.3 GHz) and the X band (8.0-12.0 GHz), respectively. The test results are in good agreement with the simulated results. As a dual-band antenna covering the 5 G Sub-6 GHz communication band and military radar band, the proposed antenna can guide the future integrated design of communication and radar antenna systems.

Compact EH0-Mode Microstrip Leaky-Wave Antenna With Enhanced Gain in Broadside

This article presents an EH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -mode microstrip leaky-wave antenna (LWA) with enhanced gain in broadside by bidirectionally exciting the inductive-loaded EH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -mode microstrip line (MSL). In this way, enhanced gain is successfully attained in broadside at the desired frequency. To theoretically investigate such an attractive property, an equivalent circuit model is first established to illustrate that the working frequency as well as the phase and leakage constants can be independently controlled by altering the shunt inductance and resistance, and further properly adjusting their quantities. Then, a row of shorting pins is appropriately loaded at one side of the MSL, so as to realize the inductive-loaded periodical leaky-wave structure for broadside radiation. After that, a more effective and practical transmission line model is derived to fully investigate the side pin-loaded MSL. Based on this transmission line model, an efficient method is proposed to design the working frequency and the values of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta /k_{0}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha /k_{0}$ </tex-math></inline-formula> in a controllable way. Furthermore, a gain enhancement of such an LWA is carried out with resorting to the directivity and radiation efficiency, aiming to acquire the maximum antenna gain. In final, the proposed EH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -mode LWA with broadside radiation is simulated, fabricated, and measured. Good agreement can be found between the simulated far-field radiation patterns and the tested ones, which both evidently indicate that a high-gain broadside radiation beam is indeed achieved at the desired frequency.

Drifting Electrons: Nonreciprocal Plasmonics and Thermal Photonics

Light propagates symmetrically in opposite directions in most materials and structures. This fact -- a consequence of the Lorentz reciprocity principle -- has tremendous implications for science and technology across the electromagnetic spectrum. Here, we investigate an emerging approach to break reciprocity that does not rely on magneto-optical effects or spacetime modulations, but is instead based on biasing a plasmonic material with a direct electric current. Using a 3D Green function formalism and microscopic considerations, we elucidate the propagation properties of surface plasmon-polaritons (SPPs) supported by a generic nonreciprocal platform of this type, revealing some previously overlooked, anomalous, wave-propagation effects. We show that SPPs can propagate in the form of steerable, slow-light, unidirectional beams associated with inflexion points in the modal dispersion. We also clarify the impact of dissipation (due to collisions and Landau damping) on nonreciprocal effects and shed light on the connections between inflexion points, exceptional points at band edges, and modal transitions in leaky-wave structures. We then apply these concepts to the important area of thermal photonics, and provide the first theoretical demonstration of drift-induced nonreciprocal radiative heat transfer between two planar bodies. Our findings may open new opportunities toward the development of nonreciprocal magnet-free devices that combine the benefits of plasmonics and nonreciprocal photonics for wave-guiding and energy applications.

Directivity enhancement and characteristics of space‐wave, leaky‐wave and creeping‐waves for an impedance cylinder coated with dielectric

The dielectric-coated impedance cylinder configuration is investigated thoroughly with respect to the space-wave, leaky-wave and creeping-wave excitations for all source polarizations. A very high directivity is achieved for the space- and leaky-wave radiation for the antenna structure compared to that from a dielectric cylindrical rod investigated earlier. It is also observed that such high directivity for the dominant-mode leaky-wave is accompanied by higher-order azimuthal harmonics with low attenuation constant contributing to the leaky-wave pole, instead of the n = 0 harmonic for the previous configuration. A large enhancement in the space-wave directivity is also achieved, contributed by higher space-wave harmonics compared to lower-order azimuthal harmonics for the cylindrical dielectric rod antenna. The space-wave and leaky-wave directivities are examined with variation in the design parameters and azimuthal harmonics. The phase and attenuation characteristics of the leaky-wave are used to evaluate the optimized leaky-wave structure parameters and appropriate azimuthal harmonics that result in high directivity for the configuration. The creeping-wave characteristics for the configuration are also investigated, with a relatively stronger creeping-wave excitation observed for an inductive surface-impedance.

A Patch Antenna Coupling of Periodic Leak-Wave Structure With Tri-Frequency Capability

A patch antenna coupling of a periodic leak-wave structure with tri-frequency capability is proposed. The periodic offset double-sided parallel stripline (DSPSL) leaky wave structure (LWS) is embedded in the direction of the whole ground of the microstrip patch substrate, then the capability of tri-frequency could be obtained by the different Floquet mode of the propagation constant. According to the design principle, a unified formula was derived to calculate the tri-frequency points at the same time. Though applying of antenna design principle and the simple tri-frequency formula, a tri-frequency patch antenna was designed and tested. Experiment results show that the antenna operated at 1.71, 2.05, and 2.72 GHz. The measured and calculated results show high consistency.

The Transition Between Reactive and Radiative Regimes for Leaky Modes in Planar Waveguides Based on Homogenized Partially Reflecting Surfaces

The gradual transition from a reactive to a radiative regime is studied for leaky modes supported by multilayered planar open waveguides. The so-called leaky cutoff condition, i.e., the frequency for which the leaky phase constant β equals the leaky attenuation constant α, originally introduced for microstrip lines and other printed structures, is investigated here with the aim of providing detailed information on the relative amount of reactive and radiative attenuation for leaky modes excited by finite sources and propagating as cylindrical waves along general planar waveguides. Analytical results are derived on the basis of a lossy parallel-plate-waveguide model and are validated through full-wave numerical simulations of 2-D leaky-wave structures based on grounded slabs covered with lossless or lossy partially reflecting surfaces (including, e.g., graphene layers) that can be treated as homogenized sheets. An analysis of the complex wave impedance of the considered leaky modes is also provided, in order to assess the frequency ranges where a good input matching can be expected for practical sources. In this regard, an ad hoc impedance matching network is designed and full-wave validated for a specific case to show that is indeed possible to achieve a good impedance matching below the cutoff in practical designs.

Leaky Wave-Guide Based Dielectric Resonator Antenna for Millimeter-Wave Applications

In this paper, a leaky wave antenna based on the parallel non-radiative dielectric guide (NRD) has been proposed. This novel design uses two parallel dielectric strip waveguides having a dielectric constant of 2.2 are placed over a substrate of the dielectric constant of 2.55 and both are separated by a ground plane. The upper structure is bounded by deflating conducting strip having distance less than λ0/2. The leakage from this antenna is in the form of surface wave concentrated more in the broadside direction rather than usual endfire or off broadside direction. It has been observed the proposed radiating structure has radiation efficiency of 82%, the peak gain of 5.94 dB with narrow impedance bandwidth, and the resonating frequency at 13.59 GHz, 13.43 GHz, and 13.93 GHz. A comparative analysis is made by increasing the height of the dielectric strip to increase the number of modes. This leaky-wave structure based on a non-radiative dielectric (NRD) guide can be a suitable candidate for the device to device (D2D) communication in the millimeter-wave band.

60 GHz Compact Larger Beam Scanning Range PCB Leaky-Wave Antenna Using HMSIW for Millimeter-Wave Applications

A half-mode substrate integrated waveguide (HMSIW), based on printed circuit board (PCB) technology, is employed to construct a novel V-band leaky-wave antenna incorporating composite right-/left-handed (CRLH) media. Including modified half-sinusoidally varied interdigital capacitive slots on the radiating edge of the HMSIW enables the feature of effective CRLH media to produce adequate radiation coverage and bandwidth with improved sidelobe level (SLL) and higher gain. Unit cell characteristics and space harmonic analysis were thoroughly investigated through dispersion and Bloch impedance analysis. By appropriately tuning the physical parameters of the design, the balanced condition was obtained at a broadside frequency of 60 GHz. The highly directed radiated fan beam covered a scanning range of 120° with beam steering from -72° to 48° of the visible space. The proposed antenna operated over 55-65 GHz with a maximum realized gain of 14.5 dBi, maximum beamwidth of 9.15°, better than 15 dB SLL and 20 dB cross-polar radiation. All simulated responses showed reasonably good accordance with experimental data thereby validating the novel design of the proposed leaky-wave structure for millimeter-wave wireless applications.

Electronically controlled beam steering leaky wave antenna in nematic liquid crystal technology

A novel nematic liquid crystal (LC) technology-based electronically controlled leaky wave antenna (LWA) with microstrip-waveguide conversion working mechanism and wide beam steering range is presented in this article. The LWA is a combination of an inverted microstrip structure and rectangular waveguide. According to the characteristics of LC materials in microwave band, a broadband microstrip-waveguide conversion device is proposed. The gradient slot leaky wave structure is combined with the microstrip-waveguide conversion device to form an electronically controlled LWA with continuous tunable beam. Simulation and experiment results show that the LWA proposed in this article has a 32° beam scanning range at 12 GHz and good impedance matching and stable gain, suggesting the great potential of nematic LC materials for extensive applications in microwave band in the future.

A Multi-Frequency Patch Antenna With Double Sided Parallel Strip Line Periodic Structure

A multi-frequency patch antenna with double-sided parallel-strip line (DSPSL) periodic offset structure is proposed. The designed antenna consists of a simple rectangular patch as well as an embedded one-dimensional DSPSL periodic offset structure, which is employed to replace the traditional rectangular microstrip patch antenna. The periodic DSPSL structure is characterized leaky wave structure. After the periodic DSPSL structure is embedded, the resonance frequency of rectangular patch is offset, and the leaky wave characteristics can determine the number and position of new operating frequencies. As an application, a tri-band antenna is manufactured and these three operating frequencies can be calculated by the derived formulas studied in this communication. Experiment results suggest that the antenna can work at 4.28, 4.78, and 6.42 GHz, respectively. The results of simulated, measured and calculated show high consistency.

Design of graphene metasurface based sensitive infrared biosensor

The article investigates graphene metasurface based infrared biosensor. Graphene metasurface perturbations are added in the Si3N4 waveguide to create leaky wave structure. Biomolecules in blood plasma form are placed over the Si3N4 waveguide to observe the sensing characteristics. Numerical results of absorption, reflectance, electric field and sensitivity are presented in this paper. The sensitivity is calculated from the shift in the absorption peak of biosensor. The sensitivity of the proposed biosensor is also compared with the previously published biosensor designs. In addition, the design results in the form of absorption and reflectance is observed for the different physical parameters like waveguide height (S), biomolecules/air layer height(B), SiO2 layer height(C) and graphene perturbations period(G). The corresponding electric field response is also presented for the proposed design at different frequencies to show the leakage of light in the biomolecules layer.

Multibeam Bidirectional Wideband/Wide-Scanning-Angle Holographic Leaky-Wave Antenna

This letter shows experimentally the realization of a high-gain multibeam bidirectional radiation in a wide frequency range via a planar holographic leaky-wave structure where the beams can scan from backward to forward direction as the frequency varies from 12 to 18 GHz. To realize these two key features, i.e., wideband characteristic and bidirectional multiple-beam pattern, we exploit the characteristic of a wideband surface wave launcher and inherent nature of a metasurface supporting TE mode at the same time. In more detail, this antenna benefits from a TE-mode antipodal Vivaldi as a distinguished feed, which has a broadband behavior, on a surface without metallic ground plane and realizes a high-gain bidirectional multibeam pattern. A four-beam linearly polarized antenna, including two backward and broadside beams in front side and two backward and broadside beams in the back side of the antenna is designed, fabricated, and tested at 12 GHz. The measured gain, sidelobe level, cross polarization, efficiency, and 3 dB bandwidth of the gain, are 17.8 dBi, −17 dB, −30 dB, 26.5%, and 43.3%, respectively, which are great achievements. Moreover, the maximum scan angle of $\text{44}^\circ $ , i.e., from $({\boldsymbol{\theta },\boldsymbol{\varphi }}) = ({\text{30}^\circ,180}) $ to $({\boldsymbol{\theta },\boldsymbol{\varphi }}) = ({\text{14}^\circ,\text{0}^\circ})$ , is achieved for the backward beams when the frequency varies from f = 12 GHz to f = 18 GHz.

Investigation of Shorting Vias for Suppressing the Open Stopband in an SIW Periodic Leaky-Wave Structure

Shorting vias loading in a substrate integrated waveguide (SIW) periodic leaky-wave structure is investigated for suppressing the open stopband. The SIW periodic leaky-wave structure with transverse slots can have the beam scanned in the backward and in the forward directions, but suffers from a large open stopband in which the reflection coefficient is increased and the gain drops drastically. A set of shorting vias are adopted to load the structure in each unit cell for suppressing the open stopband. The working principle of the technique is explained, with approximate equivalent circuits. When the slot and the shorting vias are spaced by a quarter guided wavelength, the series inductance in the slot can be canceled out by the shunt inductance in the shorting vias, and therefore, the open stopband can be suppressed. An antenna based on the via-loaded periodic structure is designed for validation. Simulation and measured results validate that the open stopband is successfully suppressed with the loading of shorting vias.

Application of metasurfaces in the design of performance-enhanced low-profile antennas

This paper presents a review of metasurface-based antennas conducted at the Microwave Communication Laboratory (MCL) of Ajou University in the Republic of Korea. In this paper, profile miniaturization, bandwidth enhancement, multiband operation, and radiation pattern control of metasurface-based antennas are considered. The paper first presents metasurface-based antennas implemented by placing various radiators on top of the metasurface. It then presents antennas implemented by placing the radiators below the metasurface with and without the ground plane. Metasurface-based antennas are not only able to achieve high efficiency with a low profile but they are also able to generate extra resonances from the metasurface structures, which significantly enhances the overall performance of the antennas. These additional resonances were utilized in multiband and/or wideband operations. In addition, the design of a planar compact wide-gain-bandwidth metasurface-based antenna and its radiation characteristics are presented at a terahertz (THz) frequency range. The THz antennas were designed with metasurfaces and a planar leaky-wave feeding structure. Finally, the outlook on future research at the MCL for antenna-related work and their applications using metasurfaces is provided.

Experimental Validation of a Wideband Metasurface for Wide-Angle Scanning Leaky-Wave Antennas

Beam scanning arrays typically suffer from scan loss; an increasing degradation in gain as the beam is scanned from broadside toward the horizon in any given scan plane. Here, a metasurface is presented that reduces the effects of scan loss for a leaky-wave antenna (LWA). The metasurface is simple, being composed of an ultrathin sheet of subwavelength split-ring resonators. The leaky-wave structure is balanced, scanning from the forward region, through broadside, and into the backward region, and designed to scan in the magnetic plane. The metasurface is effectively invisible at broadside, where balanced LWAs are most sensitive to external loading. It is shown that the introduction of the metasurface results in increased directivity, and hence, gain, as the beam is scanned off broadside, having an increasing effect as the beam is scanned to the horizon. Simulations show that the metasurface improves the effective aperture distribution at higher scan angles, resulting in a more directive main beam, while having a negligible impact on cross-polarization gain. Experimental validation results show that the scan range of the antenna is increased from $-39 {^{\circ }} \leq \theta \leq +32 {^{\circ }}$ to $-64 {^{\circ }} \leq \theta \leq +70 {^{\circ }}$ , when loaded with the metasurface, demonstrating a flattened gain profile over a 135° range centered about broadside. Moreover, this scan range occurs over a frequency band spanning from 9 to 15.5 GHz, demonstrating a relative bandwidth of 53% for the metasurface.