230 research outputs found
Exact Solution for the Protected TEM edge mode in a PTD-Symmetric Parallel-Plate Waveguide
A Parity Time-reversal Dual (PTD) symmetric structure constituted by a
Perfectly-Electric-Perfectly magnetic (PEC-PMC) parallel plate waveguide (PPW)
is analyzed. This waveguide supports unimodal transverse electromagnetic (TEM)
edge mode propagation protected against back-scattering from a certain class of
deformations and defects. The TEM solution is found in analytical form by using
three different methods, namely conformal mapping, mode-matching, and
Fourier-transform methods. It is shown through numerical simulations that the
mode propagation is robust with respect to deformations such as 90{\deg} bends
and discontinuity such as transition to free-space. Implementation of the PMC
boundary conditions via both a bed of nails and a mushroom structure is also
successfully investigated
Generation of complex source point expansions from radiation integrals
This paper discusses methods for expanding fields radiated by arbitrary sources enclosed by a certain minimum sphere in terms of Complex Source Point (CSP) beams. Two different approaches are reviewed; the first one is based on a spectral radiation integral, where the Fourier-spectrum is obtained by far field matching. The second approach consists of two steps: first, the equivalence principle is applied to a sphere enclosing the real sources, and a continuous equivalent electric current distribution is obtained in terms of spherical waves; then, the continuous current is extended to complex space and its SW components are properly filtered and sampled to generate the discrete set of CSPs. In both cases, the final result is a compact finite series representation with a number of terms that matches the degrees of freedom of arbitrary radiated fields; it is particularly efficient when the fields are highly directional and the observation domain is limited to a given angular sector. The fact that the CSPs rigorously respect Maxwell's equations ensures the validity of the expansion from near to far zone and allows one to incorporate the CSP representation in a generalized admittance matrix formalism for the analysis of complex problems
Reduction of truncation errors in planar near-field aperture antenna measurements using the method of alternating orthogonal projections
A simple and effective procedure for the reduction of truncation error in planar near-field to far-field transformations is presented. The starting point is the consideration that the actual scan plane truncation implies a reliability of the reconstructed plane wave spectrum of the field radiated by the antenna only within a certain region inside the visible range. Then, the truncation error is reduced by a Maxwellian continuation of the reliable portion of the spectrum: after back propagating the measured field to the antenna plane, a condition of spatial concentration of the primary field is exploited to define a convergent iterative process which is also stable against moderately noisy data. Far-field patterns reconstructed from both simulated and measured near-field data demonstrate the effectiveness of the proposed procedure
Complete Open-Stopband Suppression using Sinusoidally Modulated Anisotropic Metasurfaces
A novel and general approach is presented for the complete suppression of the open-stopband effects in circularly polarized one-dimensional periodic leaky-wave antennas using anisotropic modulated metasurfaces. A theoretical justification of this behaviour is found through the rigorous treatment of the canonical problem of an infinite homogenized impedance surface sinusoidally modulated along the propagation direction. By deriving a closed-form solution of this problem at broadside scan it is shown that, while the sinusoidally modulated isotropic impedance exhibits a null of the attenuation constant, the complex propagation constant for the proposed anisotropic modulation has a finite value and a regular behaviour. A closed-form formula for the leakage constant α is also derived, allowing for an accurate design of the aperture field amplitude. The full wave analysis of a patch-based implementation of the metasurface is in excellent agreement with the results based on the homogenized impedance model, thus, demonstrating the practical applicability of the theoretical results. The elimination of the open-stopband behaviour allows for the design of leaky-wave antennas able to scan the beam from backward to forward without any frequency regions of blindnes
Flexible Unidirectional PTD-Symmetric Waveguide
The transmission and reflection characteristics of a bent square transverse electromagnetic
waveguide constituted by two opposite perfect electric
conductor and perfect magnetic conductor walls are
analyzed. This waveguide exhibits a parity, timereversal, duality (PTD) symmetry with respect to both
the diagonal axes. It is found that this property is
maintained even when the structure is bent in a plane
orthogonal to one of the two PTD symmetry axes (PTDbend). As a consequence, the transverse electromagnetic mode propagation is protected against backscattering
by this class of discontinuities. The preservation of the
PTD symmetry in presence of a geometric flexibility of
the bend is also analyzed, thus introducing a new class
of bendable waveguides that are largely immune to
backscattering by bend discontinuities
Q-Bounded Maximum Directivity of Self-Resonant Antennas
The upper limit on the directivity of self-resonant antennas that fit within a minimum sphere is determined for a given quality factor. This formulation is obtained by analytically solving a rigorous convex problem and is expressed as a rapidly converging analytical series. The total quality factor, inverse of the relative frequency bandwidth, is formulated by considering the quality factors of individual spherical waves. From the exact series, approximate closed-form formulas have been derived, which exhibit high accuracy in complementary ranges of the minimum circumscribed sphere’s radius. These ranges encompass small antennas as well as intermediate to large antennas. Special emphasis is given to small antennas, where the solution is interpreted as combination of dipolar and quadrupolar Huygens’ source contributions with appropriate closed form coefficients. The solution in this range provides continuity to the maximum directivity between 3 and 8 maintaining a constant Q
Perfect conversion of a TM surface-wave into a TM leaky-wave by an isotropic periodic metasurface printed on a grounded dielectric slab
This paper presents an exact solution for a perfect conversion of a
TM-polarized surface wave (SW) into a TM-polarized leaky-wave (LW) using a
reciprocal and lossless penetrable metasurface (MTS) characterized by a scalar
sheet impedance, located on a grounded slab. In contrast to known realizations
of leaky-wave antennas, the optimal surface reactance modulation which is found
here ensures the absence of evanescent higher-order modes of the field
Floquet-wave expansion near the radiating surface. Thus, all the energy carried
by the surface wave is used for launching the single inhomogeneous plane wave
into space without accumulation of reactive energy in the higher-order modes.
It is shown that the resulting penetrable MTS exhibits variation from an
inductive to a capacitive reactance passing through a resonance. The present
formulation complements a previous paper of the authors in which a perfect
conversion from TM-polarized SW to TE-polarized LW was found for impenetrable
boundary conditions. Here, the solution takes into account the grounded slab
dispersion and it is convenient for practical implementation.Comment: 10 pages, 10 figure
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