Excitation Of Leaky Waves Research Articles

Printed Leaky-Wave Antenna With Aperture Control Using Width-Modulated Microstrip Lines and TM Surface-Wave Feeding by SIW Technology

This letter presents a width-modulated microstrip line leaky-wave antenna (LWA) with substrate-integrated waveguide and microstrip feeding. In particular, the planar antenna system consists of an integrated surface-wave launcher and three identical rows of quasi-periodic width-modulated microstrip lines for TM leaky-wave excitation, which produces a tailored binomial-like aperture distribution on the guiding surface. The behavior of the antenna when changing the width-modulated lines for different aperture distributions is also analyzed and presented. The measured LWA demonstrates a fan beam pattern in the far field with realized gain values greater than 10 dBi and with a beam direction of about $-$ 20 $^\circ$ from broadside at 23 GHz. Also, far-field measurements and near-field data indicate that the half-power beamwidth is below 10 $^\circ$ , and the position of the main beam maximum is relatively stable, i.e., ranging from about $-$ 23 $^\circ$ to $-$ 15 $^\circ$ between 23 and 24 GHz. The measured prototype is also well matched over these frequencies and $|S_{11}| $-$ 20 dB at 23.5 GHz.

Dc to ac field conversion due to leaky-wave excitation in a plasma slab behind an ionization front

We present a way for generating coherent tunable electromagnetic radiation through dc to ac field conversion by an ionization front. The conversion is caused by the excitation of leaky waves behind the transversely limited ionization front propagating in a uniform electrostatic field. This differs significantly from the well-known dc-to-ac-radiation-converter models which consider Doppler-like frequency conversion by a transversely unlimited ionization front propagating in a spatially periodic electric field. We explore the dispersion properties and excitation of these leaky waves radiated through the transverse plasma boundary at the Cherenkov angle to the direction of propagation of a superluminal ionization front as dependent on the parameters of the plasma produced and on the speed of the ionization front. It is shown that not only the center frequency but also the duration and waveform of the generated pulse may significantly depend on the speed of the ionization front. The results indicate the possibility of using such converters based on planar photoconductive antennas to create sources of microwave and terahertz radiation with controllable waveforms that are transformed from video to radio pulse when the angle of incident ionizing radiation is tuned.

Planar Leaky-Wave Antenna Designs Offering Conical-Sector Beam Scanning and Broadside Radiation Using Surface-Wave Launchers

Two planar antenna designs that utilize surface-waves for leaky-wave excitation are investigated. Specifically, a surface-wave launcher is implemented to excite cylindrical surface-waves, which are bound to a grounded dielectric slab. By the addition of circular metallic strip gratings, a partially reflecting surface is realized, providing appropriate conditions for 2-D leaky-wave radiation. In particular, two designs are investigated: a continuous circular strip and a segmented circular strip grating. Results illustrate conical-sector beam scanning for the continuous circular strip grating between 2022 GHz, while broadside radiation is observed at 21.2 GHz by the segmented circular strip design.

Leaky wave phenomena of a conductor‐back dielectric slab covered with a two‐dimensionally electromagnetic band gap superstrate

This paper presents the leaky wave phenomena associated with a waveguide structure that consists of a conductor‐back dielectric slab covered with a superstrate made up of a two‐dimensionally (2‐D) electromagnetic band gap (EBG) structure. A guided wave has its energy bounced back and forth between the metallic ground plane and the EBG structure that is taken as a frequency‐selective reflection mirror. Because of the finite thickness of the 2‐D EBG superstrate, the guided wave will leak or radiate some of its energy into the air region above the waveguide structure to become a leaky wave. By the mode‐matching method and the transverse resonance technique, the overall waveguide structure is formulated as a rigorous electromagnetic boundary value problem to yield an exact dispersion relation of the waveguide so that the complex propagation constant of a guided mode can be accurately determined, including the phase and attenuation constants. Additionally, the electric field distribution inside the waveguide and the far‐field radiation pattern were also calculated to demonstrate the leaky wave phenomena of this waveguide from a microscopic point of view. On the basis of the excitation of leaky waves, the phenomenology concerning a class of directive antennas with EBG structure as superstrate was clarified in this research.

Leaky wave excitation on concave surfaces and hyperbolic Fresnel zones

Leaky waves are waves guided by elastic structures having phase velocities exceeding the speed of sound in the surrounding fluid. When analyzing the excitation of leaky waves on smoothly curved elastic structures it is often helpful to determine the Fermat paths. Those paths satisfy Fermat’s principle for the combined acoustic propagation in the fluid and guided propagation on the structure. It is also helpful to determine the shape of the Fresnel zones associated with deviations from Fermat paths. That information is directly useful for calculating guided leaky wave amplitudes [P. L. Marston, ‘‘Spatial approximation of leaky wave surface amplitudes for three-dimensional high-frequency scattering: Fresnel patches and applications to edge-excited and regular helical waves on cylinders,’’ J. Acoust. Soc. Am. 102, 1628–1638 (1997)]. The Fresnel zones associated with the external excitation of helical leaky waves on submerged circular pipes or cylinders were previously shown to be elliptical. The resulting leaky wave amplitudes are in agreement with amplitudes derivable by other methods and with experiments. In the present work, the analysis is generalized to excitation of leaky waves by ultrasound incident from the concave side of cylindrical shells or pipes. Even though the resulting Fresnel zones are approximately hyperbolic in shape, ray approximations for the amplitude are applicable when expressed using the Hessian of the phase deviation. [Supported by the Office of Naval Research.]

Gap acousto-electric waves in structures of arbitrary anisotropy

A theoretical study has been performed of the acousto-electric waves guided by an air gap between two identical piezoelectric crystals of arbitrary anisotropy. The permissible number of such gap waves traveling slower than all bulk modes in the structure has been established. The interdependence between the existence of these "subsonic" gap waves and surface acoustic waves on the mechanically free surface of crystals constituting the structure has been investigated. The occurrence of leaky gap waves also has been discussed. In addition, the specific features of the resonance reflection in gap structures caused by the excitation of leaky waves have been analyzed.

Leaky wave excitation on three‐dimensional via‐fed printed interconnects

Leaky wave excitation on three‐dimensional, via‐fed single and coupled microstrip interconnects is studied. Closed‐form asymptotic expressions for the fields associated with the interconnect are derived and are applied in the traveling/standing wave and leaky wave regimes, both of which lead to radiation. The leaky wave beam angle is found to correspond to the usual two‐dimensional ray optics leakage angle for long interconnects, as expected, and depends on interconnect length, spacing, and excitation for shorter interconnects. Comparisons with full‐wave results are shown for the case of via‐fed coupled interconnects.

Spatial approximation of leaky wave surface amplitudes for three-dimensional high-frequency scattering: Fresnel patches and application to edge-excited and regular helical waves on cylinders

For sonar imaging systems and other situations where scattering amplitudes are resolved spatially (e.g., ultrasonic microscopy and nondestructive testing) approximations of outgoing leaky wave amplitudes are needed as a function of position on the imaged surface. The approach developed here approximates the amplitude of a leaky wave pole contribution to the total scattering as a spatial convolution of the local incident pressure with a spatial response function. Leaky rays to a surface point of interest follow a Fermat path having a stationary phase whereas the pole contribution becomes a surface integral that includes defective paths. Increased curvature of the surface or of the incident wavefront ordinarily cause more rapid dephasing along defective paths and a corresponding reduction in size of the Fresnel coupling patch. Examples given include leaky wave excitation on a partially coated cylinder at normal incidence and regular helical leaky wave excitation on tilted cylinders. A helical wave is found to be excited by diffraction at the edge of an idealized coating truncated along the cylinder’s axial direction. The leaky wave amplitude becomes proportional to a Fresnel integral of complex argument which accounts for the partial blockage of the Fresnel coupling patch.

Direct measurement of substrate refractive indices and determination of layer indices in slab-guiding structures

We present a new method for accurate and nondestructive measurement of the refractive indices of substrates and guiding layers in slab waveguides. This method is based on the excitation of leaky waves in substrates and guided waves in guiding layers owing to the etching of grating couplers on the top of structures. It is particularly applicable to high refractive-index materials and to in situ measurements near the energy band gap of semiconductor waveguides. We present results that were obtained for an InP substrate with an InGaAsP epitaxial layer with regard to their refractive indices and temperature coefficients.

Mode conversion and leaky-wave excitation at open-end coupled-microstrip discontinuities

The method of moments (MoM) is used to study mode conversion and leaky-wave excitation at an asymmetric coupled-microstrip discontinuity. The results show that significant mode conversion can occur at such discontinuities and that dominant leaky-wave modes can be excited strongly. Numerical issues with regard to the MoM analysis of such discontinuities are addressed as well, and for some examples it is shown that inclusion of a complete-domain basis function for the leaky mode improves numerical stability dramatically. >

Comments on the relation between surface wave theory and the theory of reflection

The sextic Stroh formalism, previously extensively used in the analysis of subsonic phenomena, has been used for the analysis of reflection phenomena and leaky surface waves in the first transsonic range of velocities. In particular the behaviour of the reflection problem at the limiting velocity is studied. It is shown that when the condition of free surface can be satisfied without the inhomogeneous partial wave, a situation which would appear to be the natural limiting case of a surface wave of infinite penetration, the body wave alone satisfies the condition of free surface. This result illuminates the Barnett-Lothe existence theorem for subsonic surface waves. The close connection between the reflection problem and the leaky surface wave problem becomes very apparent in the formalism used. It is shown that for a point on a branch of leaky waves where the solution is undamped, the conditions for simple reflection, i.e. reflection only involving the two body waves, are also present. In the vicinity of such a point reflection is accompanied by resonance excitation of leaky waves. The paper concludes with some explicit calculations for transversely isotropic solids.

Electronically Scannable Plasma Leaky Wave Antenna System

Excitation of leaky waves on annular plasma column enclosing dielectric rod having conductor along the axis and excited by magnetic ring source in the dielectric rod is studied. Such a configuratio...