Attieh Shahvarpour

Radiation Efficiency Issues in Planar Antennas on Electrically Thick Substrates and Solutions

The paper addresses the problem of the radiation efficiency of planar antennas on electrically thick substrates. First, the non-monotonic dependency of the radiation efficiency of an infinitesimal horizontal electric dipole on grounded and ungrounded substrates versus the substrate electrical thickness is analyzed. Next, the phenomenology of the observed radiation efficiency is explained with the help of a novel substrate dipole approach, which reduces the actual structure to an equivalent source dipole composed of the original dipole and the substrate dipole radiating into free space. Finally, two solutions for enhancing the efficiency at electrical thicknesses where the efficiency is minimal are studied.

Analysis of the radiation efficiency of a horizontal electric dipole on a grounded dielectric slab

The radiation efficiency of a horizontal infinitesimal electric dipole on a grounded dielectric slab as a function of its electrical thickness is investigated thoroughly. It is shown that, in contrast to a common belief, the efficiency does not decay monotonically with increasing electrical thickness (or, equivalently, with increasing frequency for a given slab), but oscillates in correlation with the onset of surface waves. Specifically, the efficiency maxima and minima are shown to occur at the cutoff frequencies of the TE and TM surface-wave modes, respectively (excluding the cutoff-less TM0 mode), using a spectral transmission line analysis in conjunction with a simple auxiliary dipole collocated with the source. From this auxiliary dipole, the efficiency maxima and minima are essentially explained in terms of equivalent PMC and PEC walls at the position of the source. To the best of our knowledge, this is the first complete explanation of the oscillatory behavior of the efficiency of a dipole on a grounded substrate. The finding has a fundamental implication for electrically thick planar antennas, which include most millimeter-wave antennas: such antennas may be specifically designed to exhibit their operation frequency at a local maximum of the efficiency even if the substrate is electrically very thick.

Bandwidth enhancement and beam squint reduction of leaky modes in a uniaxially anisotropic meta-substrate

Meta-substrates [1, 2] are artificial dielectric substrates with unusual properties. In [2] an anisotropic meta-substrate was proposed with the same fully-space scanning capability as leaky-wave antennas based on composite right/left-handed (CRLH) patterned transmission line (TL) structures. This meta-substrate is a mushroom type structure [3] with uniaxially anisotropic properties, where the wires perpendicular to the air-substrate interface exhibit a Drude permittivity and the loops between the adjacent mushrooms exhibit a Lorentz permeability. This paper presents a rigorous spectral TL approach analysis [4] of this meta-substrate, which is constituted of the anisotropic medium of Fig.1a, with the constitutive parameter tensors.

Schrödinger solitons in left-handed SiO2–Ag–SiO2 and Ag–SiO2–Ag plasmonic waveguides calculated with a nonlinear transmission line approach

The existence of solitons in SiO2–Ag–SiO2 and Ag–SiO2–Ag plasmonic waveguides (PWGs) is investigated using a nonlinear transmission line approach. Both the SiO2–Ag–SiO2 and the Ag–SiO2–Ag, the former in its TM even mode and the latter in its odd mode, are shown to support bright solitons as solutions to the nonlinear Schrodinger equation. The SiO2–Ag–SiO2 even mode has low left-handed (LH) loss but is undesirably mixed with a right-handed mode, while the Ag–SiO2–Ag odd mode exhibits low-loss pure LH characteristics and is therefore the most suited mode for soliton propagation. Soliton PWGs may find applications in compact switching, pulse shaping, and pulse compressing devices.

Grounded ferrite perfect magnetic conductor and application to waveguide miniaturization

A grounded ferrite PMC, based on Faraday rotation combined with ground reflection in a grounded ferrite slab, is introduced and demonstrated both theoretically and experimentally. This grounded ferrite PMC is shown to enable a TEM rectangular waveguide whose cross-sectional dimensions are independent of frequency, thereby allowing waveguide miniaturization or aperture enhancement. In contrast to previously reported artificial PMC structures (typically EBGs), the proposed grounded ferrite PMC is perfectly homogeneous, and therefore acts as an ideal PMC boundary not only in the far-field but also for sources as close as desired to it. The grounded ferrite PMC operation frequency may be tuned by the applied bias field.

Double-Band Tunable Magnetic Conductor Realized by a Grounded Ferrite Slab Covered With Metal Strip Grating

The property of a perfect magnetic conductor (PMC) boundary is achieved at two frequency bands by using Faraday rotation in a grounded ferrite slab covered with a metal strip grating. The PMC boundary condition is obtained for the wave polarized perpendicularly to the strips. The two frequency bands correspond to 45° and 90° Faraday rotation across the slab, respectively. In the former case, the strip grating traps the incident rays inside the slab after one round trip in the slab. As a result, the incident ray performs two round trips inside the slab before exiting it. This provides the maximum possible Faraday rotation with the smallest possible slab thickness. The concept is demonstrated experimentally by placing the PMC structure along the sidewalls of a rectangular waveguide so as to produce a TEM wave.

Radiation efficiency enhancement of a horizontal dipole on an electrically thick substrate by a PMC ground plane

A method for the enhancement of the radiation efficiency of horizontal electric radiators on grounded substrate at low-efficiency substrate thicknesses, corresponding to TM surface-wave cutoff frequencies, is presented. This method consists in placing a thin (high permittivity) dielectric slab between the initial slab and the ground plane so as to generate an equivalent perfect magnetic conductor (PMC) condition at the bottom of the main substrate. Full-wave simulation results have demonstrated an efficiency enhancement to 40% from a non-radiating dipole at the cutoff frequency of the second TM surface-wave mode. This method can be used to provide more design flexibility in high-efficiency planar antennas in the millimeter-wave regime, where the substrates are typically electrically very thick.

Bandwidth enhancement of a patch antenna using a wire-ferrite substrate

A wire-ferrite substrate, constituted of a 2D array of dielectrically coated metallic wires embedded in a host ferrite substrate is used for enhancing the bandwidth of a patch antenna. Enhancement is achieved as a result of the increased permeability over permittivity ratio, provided by the wire-ferrite structure by the high permeability of the ferrite material close to its resonance and the low permittivity of the Drude permittivity of the wire medium close to its plasma frequency. By using this substrate, a bandwidth enhancement of about 70% has been achieved with respect to the case of a dielectric substrate of same refractive index.

Spectral Transmission Line Analysis of a Composite Right/Left-Handed Uniaxially Anisotropic Meta-substrate

The TM spectral transmission line model of a grounded meta-substrate with uniaxially anisotropic properties, corresponding to a composite right/left-handed sub-wavelength mushroom-type structure, is computed. The dispersion relation is found by the transverse resonance technique. It is analyzed and compared with the particular case of an isotropic grounded slab. The study is limited to the left-handed region of the metasubstrate, which corresponds to the frequency band where the permittivity and permeability are negative and most dispersive. The work considers only TM modes, for which the effects of dispersion and anisotropy are most significant. The leaky-modes in the anisotropic left-handed meta-substrate are shown to exhibit a leakage factor which may be conveniently designed to very low values for super highly directive leaky-wave antennas.