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Time-varying guiding structures introduce an additional degree of freedom, besides spatial variation, that enables better control over the guided wave in a device. Periodically, time-modulated structures which are usually considered enable wave control over narrowband signals. However, for ultrawideband short-pulse signals, time variation in the form of temporal discontinuities is required. Such a setup has recently been proposed as a means to overcome the Bode–Fano bound on impedance matching. While hard (abrupt) temporal discontinuities are relatively simple to analyze by employing continuity of magnetic flux and electric charge, soft (gradual) temporal switching of the guiding structure parameters is more challenging. This article explores the case of a short-pulse dynamics in a 1-D, metamaterial transmission line (TL), medium with general smooth time variation of its parameters. In this time-varying TL, wave-field solutions are obtained by a Wentzel-Kramers-Brillouin (WKB) approach which is more common in the context of gradual spatial variations. Using this methodology, a leading order transmitted and reflected waves due to time variation are derived, followed by a discussion of the energy balance in such switched media. A canonical example of capacitor discharge into a long time-varied TL is given. These results may be used as analysis/synthesis tools for time-varying wave devices in electromagnetics and acoustics. [ABSTRACT FROM AUTHOR]

A complete time-domain network representation of a transmit–receive two-port perfect electric conducting antenna system in an arbitrary distance is presented. We utilize a scattering matrix setting, which, as the impedance matrix formulation, is a proper setting for the time-domain formulation in antenna system representations. The formulation is derived directly in the time domain using both the convolution-type and the correlation-type reciprocity relations. We derive the general expressions for the matrix, in particular the transfer coupled coefficients between the antennas (transfer coupled response), followed by closed-form example of parallel staggered dipole antennas that provides a clear and transparent physical interpretation for the time-domain phenomenology. [ABSTRACT FROM PUBLISHER]

We present a beam-based imaging scheme for targets in homogeneous medium whereby the fields of the source and receiver arrays are expanded using special phase-space bases of collimated beam fields, thus converting the physical data into a beam-domain data describing the scattering amplitudes seen (synthetically) by the receiver beams due to excitation by the source beams. The image is then formed by correlating the backpropagated beam data with the incident beams. The formulation utilizes the ultra-wideband phase-space beam-summation method where the beam bases consist of Gaussian beams that emerge from a discrete set of points and directions in the source and the receiver domains. An important feature of this method is that the beam-sets are frequency independent and hence are calculated once and then used for all frequencies. A closed form expression for the data-transformation matrix from the physical domain to the beam domain is derived, leading to sparse beam-domain data. The beam approach enables local imaging of any sub-domain of interest by retaining only the subset of source and receiver beams that pass through that domain, thus reducing the overall computation complexity. The method properties are explored via numerical simulations in a noisy environment. [ABSTRACT FROM PUBLISHER]

The transient properties of short-pulsed elliptically/circularly polarized antennas, namely, the transmitting and receiving effective heights, are introduced. Due to the pulsed temporal dependence of the radiation pattern, an instantaneous axial ratio is introduced as a quality measure of the polarization performance. For these antennas, it is shown that the trace of the tip of the electric-field polarization vector in time depends on the pulse-width (bandwidth) regime of the excitation signal. Only for quasi-monochromatic or narrow-band excitations is a "pure" elliptical/circular trace achieved (i.e., axial ratio approaching unity for circular polarization). For short-pulsed excitations, a deformation of the elliptical/circular polarization vector trace is obtained due to a Hilbert-transform dependence in the antenna's effective heights and the finite duration pulsed-envelope. A remedy for that deformation, for circular polarization, is obtained by using an array of sequentially rotated circularly polarized antenna elements, which extends the temporal duration for which an instantaneous axial ratio lower than some prescribed value is obtained beyond that of a single antenna. [ABSTRACT FROM AUTHOR]

A fast multilevel algorithm with reduced memory requirements for the evaluation of transient near-field to far-field transforms is presented. The computational scheme is based on a hierarchical decomposition of an arbitrary shaped enclosing surface over which the near-fields of an antenna or a scatterer are given. For surface subdomains at the highest decomposition level, the angular-temporal far-field patterns are calculated directly from the known near fields over a sparse angular grid of directions and a short temporal duration. The multilevel computation comprises angular and temporal interpolations thus increasing angular resolution and temporal duration of radiation patterns while aggregating the subdomain contributions between successive decomposition levels. These steps are repeated until obtaining the transient far-field response of the whole enclosing surface. The computational complexity of the proposed algorithm is substantially lower than that of the direct evaluation. Reduction in memory requirements is obtained by formulating the algorithm as a marching-on-in-time windowed scheme. This approach allows for embedding of the accelerated transforms within existing near-field modeling tools. [ABSTRACT FROM AUTHOR]

Presented here is an ultra-wideband-correlation based scheme for imaging and inversion of an nnknown weak and lossless scatterer embedded in a known background medium. The scheme uses an excitation and reception of ultra wideband/short-pulsed fields by an array of transducers located outside the imaging domain. The scatterer image is formed by cross correlating (in the short-pulsed domain or via spectral integration in the ultra wideband domain) the numerically/ anal ytically back-propagated, measured, and scattered data set with the forward-propagation excitations. It is shown that in the ultra wideband domain, the forward-backward propagation functions form a frame set in a finite Hilbert space. Within the weak scattering assumption (Born approximation) the scatterer's image and object function (velocity profile) are related via the corresponding frame operator. Therefore, an exact inversion scheme of the frame operator is readily available to yield the object function via an iterative scheme or using the dual frame set. Numerical examples that demonstrate the performance of the imaging and inversion schemes for scatterers with various velocity profiles are presented. It is shown that the scatterer image is generally of poor resolution. However, on inversion, a high-quality velocity profile is obtained that captures the scatterer fine details. [ABSTRACT FROM AUTHOR]

A new time domain (TD) representation for radiation from apertures, wherein the field is expanded in a discrete lattice of shifted, tilted, and retarded pulsed beam (PB) propagators is proposed. The PB representation provides a uniformly accurate systematic framework for ray-based tracking of ultra-wide-band transient fields in complex configurations, e.g., slowly or abruptly varying inhomogeneous media. The excitation coefficients of the PBs are extracted from the aperture source distribution via a new "discretized local slant stack transform." The formulation is structured upon a new class of frames, termed windowed Radon transform frames. The sets of expansion and analysis functions are shown to be dual frames, thus, facilitating representation of arbitrary bandlimited source fields. Explicit expressions for the processing windows (the analysis frame set) and for the PB propagators (the expansion set) are provided for the class of isodiffracting PBs. Accuracy of the PB expansion technique is demonstrated for a focusing aperture case, that tends to fail the conventional ray shooting schemes. [ABSTRACT FROM AUTHOR]

The ultrawide-band (UWB) beam summation representation introduced in Part I of this two-part sequence, is extended here to make it more efficient for excitations with band-widths that are larger than one octave. The main features of the basic formulation in Part I were: 1) utilization of over complete windowed Fourier transform (WFT) frames to construct the beam lattice (or skeleton) that is independent of the frequency; 2) the use of isodiffracting Gaussian beams (ID-GBs) provides the snuggest frame representation, and thus stable and localized expansion coefficients, for all frequencies; 3) the ID-GBs can readily be tracked in the ambient medium and, due to the ID property, their propagation parameters are calculated only once and then can be used for all frequencies. Although the basic formulation can accommodate large bandwidths, it becomes increasingly less efficient at the low end of the frequency spectrum where the over completeness increases. The self-consistent multib and beam-summation scheme presented here extends and generalizes the basic formulation of Part I by dividing the excitation band into one-octave sub-bands and applying the UWB beam expansion of Part I In each band. This is done, though, via a novel self-consistent scheme wherein the beam sets at the lower bands are decimated subsets of those at the highest band, so that only the set of beam-propagators at the highest band needs to be traced, while for the lower bands one uses properly decimated subsets. This approach requires less beam elements at the lower frequency bands and thus keeps the overcompleteness (oversampling) in all the bands below a given level. As in Part I, we provide the guidelines for choosing the expansion parameters and then demonstrate the effectiveness of the new scheme via a numerical example of UWB focused aperture whose frequency band spans several octaves. [ABSTRACT FROM AUTHOR]

A new discrete phase space Gaussian beam (GB) summation representation for ultrawide-band (UWB) radiation from an aperture source distribution is presented. The formulation is based on the theory of the windowed Fourier transform (WFT) frames, wherein we introduce a novel relation between the frequency and the frame overcompleteness. With this procedure, the discrete lattice of beams that are emitted by the aperture satisfies the main requirement of being frequency independent, so that only a single set of beams needs to be traced through the medium for all the frequencies in the band. It is also shown that a properly tuned class of iso-diffracting (ID) Gaussian-windows provides the "snuggest" frame representation for all frequencies, thus generating stable and localized expansion coefficients. Furthermore, due to the ID property, the resulting CBs propagators are fully described by frequency independent matrices whose calculation in the ambient environment need to be done only once for all frequencies. Consequently, the theory may also be expressed directly in the time-domain as will be presented elsewhere. The localization implied by the new formulation is demonstrated numerically for an UWB focused aperture. It is shown that the algorithm extracts the local radiation properties of the aperture source and enhances only those beams that conform with these properties, i.e., those residing near the phase space Lagrange manifold. Further localization is due to the fact the algorithm accounts only for beams that pass within a few beam widths vicinity of the observation point. It is thus shown that the total number of beams is much smaller than the Landau Pollak bound on the aperture's degrees of freedom. [ABSTRACT FROM AUTHOR]

Part II. Emphasizes the development of a time-domain (TD) near field analysis of the radiation of short-pulse antennas. Reactive concepts of time-harmonic fields in terms of stored energy; Demonstration of TD concepts; TD Q factor that quantifies radiation efficiency.

Part I. Focuses on the analysis of radiation from time-dependent source distributions directly in the time-domain (TD). Calculation of multipole moment functions; Formulations used; Derivation of bounds of the antenna properties.

Presents the comprehensive characterization of antennas in the time domain (TD) which has been derived entirely in the context of the TD field equations. Characterization of transmitting antennas; System characterization of a receiving antenna; Transmit-receive antenna system; Example of short dipole system.