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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 30 Nov 2023 19:50:46 +0000Surface Currents on a Conducting Sphere Excited by a Dipole
https://resolver.caltech.edu/CaltechAUTHORS:20170810-141443562
Authors: Papas, C. H.; King, Ronold
Year: 1948
DOI: 10.1063/1.1698212
This paper treats the problem of determining the current distribution on the surface of a perfectly conducting sphere when driven by a dipole antenna erected on its surface. Curves of the real and imaginary parts of the surface currents are given for the case of a half‐wave dipole and various radii of the sphere.https://authors.library.caltech.edu/records/c89s8-7hj22The Radiation Resistance of End-Fire and Collinear Arrays
https://resolver.caltech.edu/CaltechAUTHORS:20190307-105334981
Authors: Papas, Charles H.; King, Ronold
Year: 1948
DOI: 10.1109/JRPROC.1948.230279
Expressions for the radiation resistances of end-fire and collinear arrays of half-wave dipoles are obtained in terms of circular functions in a form convenient for computation. No mathematical approximations except for a Fourier representation of the field of a single half-wave dipole are used. The first integral theorem of Sonine and an integral representation of the Bessel function due to Hansen are involved in the integration of the normal component of Poynting's vector. Results computed from the new formula for the radiation resistance of an n-element parallel array in which the spacings and successive phasings of the dipole elements are 180 degrees (bilateral end-fire) agree closely with those of Pistolkors, who used Brillouin's e.m.f. method; they are a little less than the figures of Bontsch-Bruewitsch, who numerically integrated Poynting's vector. Calculations for the radiation resistance of an n-element collinear array using the new formula are compared with those of Bontsch-Bruewitsch, with which they are in satisfactory agreement. The new formula is also used to compute the radiation resistance of an n-element unilateral end-fire array (i.e., an n-element parallel array in which the spacings and successive phasings of the dipole elements are 90 degrees).https://authors.library.caltech.edu/records/p5cxm-ve770On the Infinitely Long Cylindrical Antenna
https://resolver.caltech.edu/CaltechAUTHORS:20170810-143753275
Authors: Papas, Charles H.
Year: 1949
DOI: 10.1063/1.1698401
Using the method of steepest descents the far field and the asymptotic form of the current distribution is obtained for an infinitely long, perfectly conducting cylindrical antenna excited by a localized electromotive force. The low frequency value of the radiation conductance is determined by integrating the radiated energy flux over a large sphere.https://authors.library.caltech.edu/records/sa0p9-w2e06Input Impedance of Wide-Angle Conical Antennas Fed by a Coaxial Line
https://resolver.caltech.edu/CaltechAUTHORS:20190306-124646101
Authors: Papas, Charles H.; King, Ronold
Year: 1949
DOI: 10.1109/JRPROC.1949.234607
The input impedances for conical antennas fed by a coaxial line have been computed for several flare angles. A graph of the auxiliary functions ς_n(x)is included to facilitate impedance calculation for any large flare angle.https://authors.library.caltech.edu/records/nfq32-hpz68Diffraction by a Cylindrical Obstacle
https://resolver.caltech.edu/CaltechAUTHORS:20170810-144930394
Authors: Papas, Charles H.
Year: 1950
DOI: 10.1063/1.1699662
The diffraction of a plane electromagnetic wave by an infinitely long, perfectly conducting cylinder has been treated by a variational method (see the two papers by H. Levine and J. Schwinger). The incident field is assumed to be polarized in the direction of the cylinder axis, and thus the entire field is of two‐dimensional nature. This formulation yields an expression for the diffracted cylindrical wave amplitude, at distances from the cylinder large compared to its transverse dimension and the wave‐length, which is stationary relative to small independent variations of the surface currents arising from plane‐wave excitation along a pair of directions in space; furthermore, the stationary form of the diffracted amplitude is independent of the scale of the surface currents. In accordance with a theorem of Levine and Schwinger, the total plane‐wave scattering cross section is simply related to the diffracted cylindrical wave amplitude in the direction of incidence. To examine the high frequency behavior of the cross section, the surface current induced by a plane wave is taken different from zero only on the illuminated part of the cylinder, where its value is derived from the tangential component of the incident magnetic field. The resulting cross section is obtained and is shown to approach 4ɑ when kɑ approaches infinity (k=2π÷wave‐length, a equals the radius of cylinder).https://authors.library.caltech.edu/records/5f2bc-cwa10Radiation from Wide-Angle Conical Antennas Fed by a Coaxial Line
https://resolver.caltech.edu/CaltechAUTHORS:20190304-140832889
Authors: Papas, Charles H.; King, Ronold
Year: 1951
DOI: 10.1109/JRPROC.1951.230420
An approximate expression for the radiation from spherically capped conical antennas is derived by the Fourier-Laméeigen-function method. Radiation patterns have been calculated for antennas with flare angle of π/6 and various lengths.https://authors.library.caltech.edu/records/440ae-28h43Theory of the Circular Diffraction Antenna
https://resolver.caltech.edu/CaltechAUTHORS:20170810-150359315
Authors: Levine, Harold; Papas, Charles H.
Year: 1951
DOI: 10.1063/1.1699816
The circular diffraction antenna consists of a coaxial wave guide fitted with an infinite‐plane conducting baffle, and open to free space. An equivalent circuit description, appropriate to principal‐mode propagation in the coaxial region, is investigated theoretically. Variational expressions for the circuit parameters are derived, and used for accurate numerical evaluation.https://authors.library.caltech.edu/records/60jfp-rwd42A new type of propagation
https://resolver.caltech.edu/CaltechAUTHORS:20170822-174306750
Authors: Papas, Charles H.; Salisbury, W. W.
Year: 1952
DOI: 10.1109/TPGAP.1952.237339
H. Alfven and others have shown that when a conducting fluid is placed in a magnetic field the interaction of the electromagnetic and hydrodynamic forces generates a new type of wave, the nagneto-hydrodynamic wave. Using a description of these waves in an ideal fluid as a prototype, we briefly discuss their properties for various types of conducting fluids. A list of references is given.https://authors.library.caltech.edu/records/858ry-t5g49On Diffraction by a Strip
https://resolver.caltech.edu/CaltechAUTHORS:20190906-141039522
Authors: Erdélyi, A.; Papas, C. H.
Year: 1954
DOI: 10.1073/pnas.40.2.128
PMCID: PMC527954
The problem of diffraction by an infinite strip or slit has been the subject of several investigations. There are at least two "exact" methods for attacking this problem. One of these is the integral equation method, the other the Fourier-Lame method. The integral equation obtained for this problem cannot be solved in closed form: expansion of the solution in powers of the ratio (strip width/wavelength) leads to useful formulas for low frequencies. In the Fourier-Lame method the wave equation is separated in coordinates of the elliptic cylinder, the solution appears as an infinite series of Mathieu functions, and the usefulness of the result is limited by the convergence of these infinite series, and by the available tabulation of Mathieu functions.
The variational technique developed by Levine and Schwinger avoids some of the difficulties of the above-mentioned methods and, at least in principle, is capable of furnishing good approximations for all frequency-ranges. The scattered field may be represented as the effect of the current induced in the strip, and it has been proved by Levine and Schwinger that it is possible to represent the amplitude of the far-zone scattered field in terms of the induced current in a form which is stationary with respect to small variations of the current about the true current. Substitution, in this representation, of a rough approximation for the current may give a remarkably good approximation of the far-zone scattered field amplitude. In this note we assume a normally incident field polarized parallel to the generators of the strip. As a rough approximation, we take a uniform density of the current induced in the strip. Since the incident magnetic field is constant over the strip, Fock's theory may be cited in support of the uniformity of the current distribution, except near the edges where the behaviour of the field indicates an infinite current density. A more detailed analysis of the current, by Moullin and Phillips, is available but was not used here.
Once the (approximate) amplitude of the far-zone field has been obtained, the scattering cross-section may be found by the application of the scattering theorem which relates this cross-section to the imaginary part of the amplitude of the far-zone scattered field along the central line of the umbral region. In spite of the crude approximation adopted for the induced current, the scattering cross-section shows a fair agreement with other available results.https://authors.library.caltech.edu/records/444dy-dv135An Application of Sommerfeld's Complex Order Wave Functions to Antenna Theory
https://resolver.caltech.edu/CaltechAUTHORS:20190909-151527524
Authors: Papas, Charles H.
Year: 1954
DOI: 10.1002/sapm1954331269
In the past wave functions of integral order have been used quite advantageously in the solution of certain antenna and boundary-value problems. However, in some instances these wave functions are completely alien to the problem and introduce difficulties which, indeed, can be resolved but only at the expense of logical simplicity. To place in evidence the usefulness and "naturalness" of complex order wave functions for the solution of certain problems, we examine theoretically the input admittance of a boss antenna with the aid of these functions.https://authors.library.caltech.edu/records/wgg6r-te016Thermodynamic Consideration of Electromagnetic Cavity Resonators
https://resolver.caltech.edu/CaltechAUTHORS:20170810-101146477
Authors: Papas, Charles H.
Year: 1954
DOI: 10.1063/1.1702386
If a cavity resonator has a "simple shape" and is filled with a homogeneous, isotropic medium, to calculate its resonant frequencies and mode functions is a straightforward task in principle. However, it is sometimes of interest to determine the oscillatory properties of a cavity that differs by a small amount in one or more of its physical characteristics from a cavity the oscillatory properties of which are known. For example, the calculation of resonant frequency shift accompanying the introduction
of a small foreign body into the cavity's volume is necessary in certain measurement techniques.https://authors.library.caltech.edu/records/hmcjk-jtx65Electromagnetic Radiation in the Presence of Moving Simple Media
https://resolver.caltech.edu/CaltechAUTHORS:20170810-140533895
Authors: Lee, K. S. H.; Papas, C. H.
Year: 1964
DOI: 10.1063/1.1704088
The radiation pattern of an arbitrary source immersed in a moving simple medium is calculated by deducing the differential equation for the potential 4‐vector in the rest frame of the source and then solving the equation in terms of a Green's function. As an illustrative example, the case where the source is an oscillating dipole is worked out in detail.https://authors.library.caltech.edu/records/xq1st-sjn08Radiation resistance of an oscillating dipole in a moving medium
https://resolver.caltech.edu/CaltechAUTHORS:20190311-155522273
Authors: Daly, P.; Lee, K. S. H.; Papas, C. H.
Year: 1965
DOI: 10.1109/TAP.1965.1138489
The radiation resistance of an oscillating electric dipole immersed in a moving medium is calculated by Brillouin's "EMF method." Two cases are studied in detail: in case 1) the moving medium is simple, and in case 2) the moving medium is an ionized gas (plasma). It is found that in case 1), the motion of the medium tends to increase the radiation resistance, whereas in case 2), the motion of the medium tends to decrease the radiation resistance. A comparison is made between Frank's well-known theory of the radiation from an oscillating dipole moving through a material medium and the results obtained in this paper.https://authors.library.caltech.edu/records/k2r6h-xzm52Radiation resistance and irreversible power of antennas in gyroelectric media
https://resolver.caltech.edu/CaltechAUTHORS:20170815-152144050
Authors: Lee, K. S. H.; Papas, C. H.
Year: 1965
DOI: 10.1109/TAP.1965.1138522
In recent years, many investigators have been working on the problem of calculating the radiation resistance of
a dipole antenna immersed in an anisotropic medium [1].
The central difficulty of their method of calculation
is that it yields an infinite value for the radiation resistance. They hare attributed this infinity to the
infinitesimal size of the source, and have suggested that
if the source were of finite spatial extent the difficulty would not arise. It is our contention that the
difficulty is of a more basic nature and is not due to the size of the source but to the method of calculation. The purpose of this letter is to show that if the radiation resistance is calculated with proper conformity to the thermodynamical laws of reversibility and irreversibility, the value of the radiation resistance
will turn out to be finite. Clearly, radiation resistance
is on the same footing as ordinary circuit resistance in the sense that they both are measures of irreversible power, and hence in calculating radiation resistance it is necessary that only the irreversible part of the
power be used. Accordingly, we shall construct an
expression for the irreversible part of the power
emitted by a source, and show that the expression so
constructed is finite and hence leads to a finite value for the radiation resistance. To construct the
required expression, we recall that in the case of
an accelerating point electron in vacuum, the combination
of half the retarded minus half the advanced field is
free from singularity [2] and corresponds to the irreversible power radiated by the electron [3].
We shall extend this idea of taking a combination field
to the case of a monochro matic source Re(Je^(iWt))
radiating into a lossless anisotropic medium.https://authors.library.caltech.edu/records/ctj1w-qhs06Antenna radiation in a moving dispersive medium
https://resolver.caltech.edu/CaltechAUTHORS:20170815-153459396
Authors: Lee, K. S. H.; Papas, C. H.
Year: 1965
DOI: 10.1109/TAP.1965.1138535
The general problem of calculating the radiation field of an antenna immersed in a moving dispersive medium is formulated as an algebraic equation in wave-vector frequency space for the potential 4-vector in the rest frame of the antenna, and is solved in terms of a Green's function having the form of a one-dimensional integral. The special case where the moving medium is a homogeneous ionized gas (plasma) and the antenna is an oscillating dipole is studied in detail. It is found that the far-zone field is not transverse and the Poynting vector is not purely radial.https://authors.library.caltech.edu/records/ztfpm-d4r57On the attenuation of transient fields by imperfectly conducting spherical shells
https://resolver.caltech.edu/CaltechAUTHORS:20190305-104559757
Authors: Harrison, C. W., Jr.; Papas, C. H.
Year: 1965
DOI: 10.1109/TAP.1965.1138552
Exact formulas for the electric and magnetic fields at any arbitrary point within a cavity region completely enclosed by a conducting spherical shell of arbitrary size are derived under the assumption that the exciting electromagnetic field is a linearly polarized, monochromatic, plane wave falling on the external surface of the shell. It is shown that the polarization of the electromagnetic field at the center of the cavity is the same as the polarization of the incident wave. From a knowledge of this steady-state solution, the time history of the electromagnetic field at the center of the cavity is calculated for the case where the incident wave is a Gaussian pulse. Numerical information on the effectiveness of the aluminum and copper shields under steady-state and transient conditions is provided for several pulse durations, shield sizes, and wall thicknesses.https://authors.library.caltech.edu/records/2qzbv-2dj35Electromagnetic Field and Wave Propagation in Gravitation
https://resolver.caltech.edu/CaltechAUTHORS:20170810-101614131
Authors: Mo, Tse Chin; Papas, C. H.
Year: 1971
DOI: 10.1103/PhysRevD.3.1708
From the physical three-vector Maxwell equations for an electromagnetic (E.M.) field in static gravitation, we examine the artifice of replacing the gravitation by an equivalent medium and we find modified Debye potentials for an E.M. wave in a simple, angularly homogeneous, material medium in a Schwarzs-child gravitational field. The fact that these potentials do not obey the generalized scalar wave equation implies that gravitation scatters the vector E.M. wave and a scalar wave differently. Also, we obtain and solve by perturbation the amplitude and eikonal equations for a high-frequency wave in a weak spherical gravitational field. To the order M/r, the state of transverse polarization does not change along a ray path whereas the transverse-field amplitudes are modified by the factor e^(M/r) which strengthens the field near the mass. The longitudinal-field amplitude, on the other hand, is modified by e^(−M/r). These effects, in principle, may provide a further test of classical E.M. theory and general relativity.https://authors.library.caltech.edu/records/057m5-mnd80New Equation of Motion for Classical Charged Particles
https://resolver.caltech.edu/CaltechAUTHORS:20170814-120616863
Authors: Mo, Tse Chin; Papas, C. H.
Year: 1971
DOI: 10.1103/PhysRevD.4.3566
With the intuitive new ideas that (1) in classical electrodynamics, radiation reaction should be expressible by the external field and the charge's kinematics, (2) a charge experiences, in addition to the Lorentz forces, another "small" external force e_1F^(μλ)̇u_λ proportional to its acceleration, and (3) inertia plus radiation is balanced by these two external forces, we propose the new equation of motion,
ṁu^μ − ((2e^3)/3m)F^(λα)_(ext)̇u_λu_αu^μ = eF^(μλ)_(ext)u_λ + e_1F^(μλ)_(ext)̇u_λ, where mass conservation requires e_1 = (2e^3)/3m. (The particle's spin is not considered in this work.) This equation for a classical charge is free from all the well-known difficulties of the Lorentz-Dirac equation. It conserves energy and momentum in a modified form in which the energy-momentum tensor contains a part t^(μν)(x) made of a new field-charge interaction
φ^μ(x), in addition to the conventional "local" part made of
F^(μν)_(ret)(x) and F^(μν)_(ext)(x) only, and therefore it no longer satisfies the conventional "local" conservation laws. It predicts correct radiation damping, as demonstrated here by applying it to various cases of basic physical importance. Also, it implies that a massless particle follows a null geodesic and cannot interact with the electromagnetic field whether it be charged or not; this implication may add a new degree of freedom to the charge-conservation law.https://authors.library.caltech.edu/records/xep93-b2543Scalar Debye Potentials for Electromagnetic Fields in Spherical Gravity and Spherical Media. I
https://resolver.caltech.edu/CaltechAUTHORS:20170814-114209224
Authors: Mo, Tse Chin; Papas, C. H.
Year: 1972
DOI: 10.1103/PhysRevD.6.2071
Modified scalar Debye potentials for electromagnetic (EM) waves in spherical gravity and spherical media are found. These potentials decompose the EM waves into two completely independent electric and magnetic radial modes and achieve scalarization and boundary fitting. Their equations, being different in a vacuum medium from
Φ^(;μ)_(;μ) = 0 of a scalar field, are reduced to one-dimensional Helmholtz equations under a separability condition, and can have their gravity effect "nullified" by a particular medium. Also the reflection coefficient R_l for an l spherical wave satisfies a Ricatti equation, and the phase shifts δ_l and scattering cross sections are related to R_l. For an incident plane EM wave, the nonforward differential scattering cross section is expressed in terms of the
R_l for the case where the medium and/or gravity tapers off slower than (radius)^(−1) and R_l,δ_l themselves diverge.https://authors.library.caltech.edu/records/h29f3-p4e06Reply to Comment on the "New Equation of Motion for Classical Charged Particles"
https://resolver.caltech.edu/CaltechAUTHORS:20170814-115055733
Authors: Mo, Tse Chin; Papas, C. H.
Year: 1972
DOI: 10.1103/PhysRevD.6.2293
We reply to a criticism made on the new equation of motion proposed by us for classical charged particles.https://authors.library.caltech.edu/records/x2abb-vyq04A new inversion method in electromagnetic wave propagation
https://resolver.caltech.edu/CaltechAUTHORS:20190311-154417844
Authors: Edenhofer, P.; Franklin, J. N.; Papas, C. H.
Year: 1973
DOI: 10.1109/TAP.1973.1140459
A theoretical investigation by an integral equation approach for the determination of the atmospheric refractive index profile from satellite radio tracking data is reported. A numerical solution is constructed by applying a minimum mean-squared estimator. An iterative procedure is suggested and shown to be convergent.https://authors.library.caltech.edu/records/a5m9t-6cy51General scaling method for electromagnetic fields with application to a matching problem
https://resolver.caltech.edu/CaltechAUTHORS:MOTjmp73
Authors: Mo, Tse Chin; Papas, C. H.; Baum, Carl E.
Year: 1973
DOI: 10.1063/1.1666341
A scaling method that reduces an electromagnetic problem described by a complicated geometry in a complicated medium to one described by a simple Cartesian geometry in a simple medium is explored and developed. This method creates and identifies an equivalent class of problems and their solutions from the Cartesian simple medium problem. We illustrate the usefulness of the method by applying it to the design of a reflectionless, distortionless, loaded matching section connecting a cylindrical and a conical coaxial waveguide, with the TEM fields being explicitly found everywhere.https://authors.library.caltech.edu/records/6k9za-2bt97Differential-geometry scaling method for electromagnetic field and its applications to coaxial waveguide junctions
https://resolver.caltech.edu/CaltechAUTHORS:20170809-172052975
Authors: Mo, Tse Chin; Papas, C. H.; Baum, Carl E.
Year: 1973
DOI: 10.1109/APS.1973.1147129
It is well-known that in mechanics and fluid dynamics one can transform or scale one problem and its solution to create a whole class of equivalent problems and their solutions[1]. Different problems and their solution behaviors of one equivalent class may look very different, but among them there are properties they share. The essence of such a scaling is to get appropriate dimensionless parameters that are common to them all.https://authors.library.caltech.edu/records/f79jp-3we27Pulsed antennas
https://resolver.caltech.edu/CaltechAUTHORS:20170808-174432239
Authors: Franceschetti, Giorgio; Papas, Charles H.
Year: 1974
DOI: 10.1109/TAP.1974.1140872
The problem of pulsed antennas has two complementary parts: i) analysis of the radiation field when the driving voltage is given and ii) synthesis of the driven voltage when the radiation field is given. In this paper a number of heuristic procedures are presented, relating to the computation of transient radiation from elementary sources, coaxial apertures, infinitely long cylindrical antennas, finite cylindrical antennas, and loop antennas. Comparison with available rigorous solutions and experiments is also provided.https://authors.library.caltech.edu/records/ksgyn-cj603On the contactless suspension of objects by electric and magnetic fields
https://resolver.caltech.edu/CaltechAUTHORS:20221004-680171300.31
Authors: Papas, C. H.
Year: 1977
DOI: 10.1007/bf00882611
In connection with the contactless suspension of objects by electric and magnetic fields, an electrodynamical version of Kapitza's theorem on the stabilizing influence of a rapidly oscillating field is deduced.https://authors.library.caltech.edu/records/5y7m9-fdd49Theory of time-dependent propagation in multimode lightguides
https://resolver.caltech.edu/CaltechAUTHORS:20170811-104945560
Authors: Crosignani, Bruno; Papas, Charles H.; Di Porto, Paolo
Year: 1977
DOI: 10.1364/JOSA.67.001300
A coupled system of equations governing the propagation of a signal in a statistical ensemble of multimode optical fibers is presented. It describes, besides the usual average modal powers, the evolution of the interference terms between the mode amplitudes and of the modal power fluctuations. Our procedure allows us to treat the general nonstationary nonmonochromatic case of an arbitrary signal fed into the lightguide by a source possessing a finite spectral bandwidth. The introduction of modal power fluctuations permits us to establish a theorem connecting the value of the modal power, averaged over the fiber ensemble, with the actual one concerning a single fiber. These two values coincide, in the polychromatic case, for large values of the fiber length, thus providing the main result of the paper, that is the justification of the statistical approach to the problem of propagation. Furthermore, the analysis of the interference terms presents evidence for the difference between the propagation of an amplitude-modulated and a frequency-modulated signal.https://authors.library.caltech.edu/records/2py7q-dbs22On the application of symmetrization to the transmission of electromagnetic waves through small convex apertures of arbitrary shape
https://resolver.caltech.edu/CaltechAUTHORS:20201020-072247308
Authors: Jaggard, D. L.; Papas, C. H.
Year: 1978
DOI: 10.1007/bf00896885
The transmission of an electromagnetic wave through a small aperture in a perfectly conducting screen is examined from the viewpoint of symmetrization.https://authors.library.caltech.edu/records/93jf2-xd015Modal dispersion in lightguides in the presence of strong coupling
https://resolver.caltech.edu/CaltechAUTHORS:20170811-130812146
Authors: Crosignani, Bruno; Papas, Charles H.; Di Porto, Paolo
Year: 1978
DOI: 10.1364/JOSA.68.001586
The effect of strong mode coupling on modal dispersion in optical fibers has been investigated. The pulse dispersion turns out to be qualitatively different from the one relative to the weak-coupling case, while it exhibits a drastic reduction as compared with that of the uncoupled case. The role of the initial pulse length and of the source coherence time has been elucidated.https://authors.library.caltech.edu/records/wv0c0-6ts22Steady state and transient electromagnetic coupling through slabs
https://resolver.caltech.edu/CaltechAUTHORS:20190311-151948345
Authors: Franceschetti, Giorgio; Papas, Charles H.
Year: 1979
DOI: 10.1109/TAP.1979.1142153
The problem of electromagnetic transmission through a slab where transmitting and receiving antennas are at finite distances from the slab is considered. The mathematical formulation of the problem is quite general. A detailed solution is presented for the case of a highly conducting slab exposed to sinusoidal and transient excitations. A discussion is given of the conditions under which measurements with the source and receiver at finite distances are equivalent to the same measurements with plane wave excitation.https://authors.library.caltech.edu/records/azn17-f5852Temporal spreading of a pulse propagating in a two-mode optical Fiber
https://resolver.caltech.edu/CaltechAUTHORS:20170811-132310169
Authors: Crosignani, Bruno; Papas, Charles H.; Di Porto, Paolo
Year: 1979
DOI: 10.1364/JOSA.69.001596
The dependence of the temporal width of the impulse response on the length z of a two-mode optical fiber is examined. This quantity, which is proportional to z in the absence of mode coupling and to z^(1/2) in the presence of weak random coupling among the guided modes, possesses a different dependence in the case of a deterministic resonant-coupling model, appropriate for describing a rather general class of actual situations. The relevant role played by the coherence time of the signal is demonstrated.https://authors.library.caltech.edu/records/zk4av-e9t48On the near-zone inverse Doppler effect
https://resolver.caltech.edu/CaltechAUTHORS:20190311-151618074
Authors: Engheta, Nader; Mickelson, Alan R.; Papas, Charles H.
Year: 1980
DOI: 10.1109/TAP.1980.1142368
Attention is invited to the recently discovered inverse Doppler effect which occurs in the near-zone field of an antenna emitting a continuous wave. On approaching the antenna, the received signal is blue-shifted in the far zone and then red-shifted in the near zone; and on receding from the antenna, the received signal is blue-shifted in the near zone and then red-shifted in the far zone. Calculations are presented for the ease where the antenna is a simple dipole. It is shown that this effect gives not only the vector velocity of the moving receiver but also its range, i.e., its distance from the antenna.https://authors.library.caltech.edu/records/xbfa3-vem80Role of intensity fluctuations in nonlinear pulse propagation
https://resolver.caltech.edu/CaltechAUTHORS:CROol80
Authors: Crosignani, B.; Papas, C. H.; Di Porto, P.
Year: 1980
The effect of intensity fluctuations and the finite coherence time of the field on the propagation of nonlinear optical pulses is discussed. In particular, the statistical properties of the carrier are shown to affect the power level for soliton propagation.https://authors.library.caltech.edu/records/cd7q8-fxp98Coupled-mode theory approach to nonlinear pulse propagation in optical fibers
https://resolver.caltech.edu/CaltechAUTHORS:CROol81
Authors: Crosignani, B.; Papas, C. H.; Di Porto, P.
Year: 1981
The formalism of coupled-mode theory is adopted for describing nonlinear pulse propagation in optical fibers, the coupling being induced by the nonlinear part of the refractive index. This approach describes in a natural way the influence of the waveguide, and in principle allows of the possibility of investigating soliton propagation when more than one mode is excited.https://authors.library.caltech.edu/records/13snf-6d524Nonlinear electromagnetics [book review]
https://resolver.caltech.edu/CaltechAUTHORS:20170731-174447658
Authors: Papas, C. H.
Year: 1981
DOI: 10.1109/MAP.1981.27582
The theory of nonlinear electric circuits is a
highly advanced subject. It has been intensively
developed in the Soviet Union under the direction of
N. M. Kryloff and N. N. Bogoljubov for the past fifty
years and has been an active topic of research in
western countries for about thirty-five. On the other
hand, research on nonlinear electromagnetic waves is just getting under way and has only recently started to receive the serious attention it deserves. Of course, much of this attention is due to the expectation that nonlinear electromagnetic wave phenomena will find use in radically new devices and radically new systems.https://authors.library.caltech.edu/records/26nne-sqa91On the polarization properties of the far-zone radiation fields of primary and secondary electromagnetic sources
https://resolver.caltech.edu/CaltechAUTHORS:20170726-174923692
Authors: Engheta, N.; Papas, C. H.; Elachi, C.
Year: 1984
DOI: 10.1109/APS.1984.1149190
The far-zone field of a transmitting antenna (primary source) resembles the far-zone field by a scattered (secondary source) in that each of the far-zone fields is a TEM wave travelling radially outward from a body of finite spatial extent....https://authors.library.caltech.edu/records/n0r0w-w0t64On the optimum polarizations of incoherently reflected waves
https://resolver.caltech.edu/CaltechAUTHORS:20190311-150106246
Authors: van Zyl, Jakob J.; Papas, Charles H.; Elachi, Charles
Year: 1987
DOI: 10.1109/TAP.1987.1144175
In radar imaging, the scattered waves are usually partially polarized. Accordingly, the concept of optimum polarization must be extended to the case of incoherent scattering where the scattered waves are partially polarized. Here, it will be shown that the Stokes scattering operator is the most suitable characterization of incoherent scattering. The problem of finding the polarization that would yield an optimum amount of power received from the scatterer is solved by assuming a knowledge of the Stokes scattering operator instead of the 2 x 2 scattering matrix with complex elements. The advantage of this method is that it may be used to find the optimum polarizations for the case wherein the scatterers can only be fully characterized by their Stokes scattering operator (incoherent scattering) and the case wherein the scatterer can be fully characterized by the complex 2 x 2 scattering matrix (coherent scattering). In this report, it will he shown that the optimum polarizations reported thus far in the literature, i.e., when the problem is solved by using a knowledge of the 2 x 2 scattering matrix, form the solutions for a subset of a more general class of problems. When the solution of the problem is based on a knowledge of the Stokes scattering operator, it is found that for incoherent scattering six optimum polarizations can exist, whereas when the solution is based on the 2 x 2 scattering matrix, the number of optimum polarizations is necessarily limited to four.https://authors.library.caltech.edu/records/h4qvv-rsf84Electromagnetic wave propagation through a dielectric-chiral interface and through a chiral slab
https://resolver.caltech.edu/CaltechAUTHORS:BASjosaa88
Authors: Bassiri, S.; Papas, C. H.; Engheta, N.
Year: 1988
The reflection from and transmission through a semi-infinite chiral medium are analyzed by obtaining the Fresnel equations in terms of parallel- and perpendicular-polarized modes, and a comparison is made with results reported previously. The chiral medium is described electromagnetically by the constitutive relations D = εE + iγB and H = iγE + (1/μ)B. The constants ε, μ, and γ are real and have values that are fixed by the size, the shape, and the spatial distribution of the elements that collectively compose the medium. The conditions are obtained for the total internal reflection of the incident wave from the interface and for the existence of the Brewster angle. The effects of the chirality on the polarization and the intensity of the reflected wave from the chiral half-space are discussed and illustrated by using the Stokes parameters. The propagation of electromagnetic waves through an infinite slab of chiral medium is formulated for oblique incidence and solved analytically for the case of normal incidence.https://authors.library.caltech.edu/records/a0znb-cma40Power Flow Structures in Two Dimensional Electromagnetic Fields
https://resolver.caltech.edu/CaltechAUTHORS:20190717-101829357
Authors: Rizvi, A. A.; Papas, C. H.
Year: 2000
DOI: 10.2528/pier99092401
Qualitative behaviour of time average power flow in electromagnetic fields can be studied by observing the critical points of the Poynting vector field, S. In order to analyze the behaviour of the flow lines of a plane Poynting vector field in the neighbourhood of a critical point, the S field is expanded in a Taylor series. Using this expansion, critical points can be classified according to their order and degeneracy. A formula for the index of rotation of the S field at a critical point is derived. The behaviour of the transverse electric or magnetic field component in the neighbourhood of the critical point is also studied. Lowest order critical points are always nondegenerate and they have interesting properties with regards to polarization and energy distribution. Examples involving linearly polarized system of interfering plane and/or cylindrical waves are given to show the critical points. The behaviour of flow lines is illustrated in these examples.https://authors.library.caltech.edu/records/nd55d-bb606Diffraction by a Strip
https://resolver.caltech.edu/CaltechAUTHORS:20190906-135712159
Authors: Erdélyi, A.; Papas, C. H.
Year: 2019
DOI: 10.7907/T5HY-3105
The problem of diffraction by an infinite strip or slit has been the subject of several investigations. There are at least two "exact" methods for attacking this problem. One of these is the integral equation method, the other the Fourier-Lamé method. The integral equation obtained for this problem cannot be solved in closed form; expansion of the solution in powers of the ratio (strip width/wavelength) leads to useful formulas for low frequencies. In the Fourier-Lamé method the wave equation is separated in coordinates of the elliptic cylinder, the solution appears as an infinite series of Mathieu functions, and the usefulness of the result is limited by the convergence of these infinite series, and by the available tabulation of Mathieu functions.
The variational technique developed by Levine and Schwinger avoids some of the difficulties of the above-mentioned methods and, at least in principle, is capable of furnishing good approximations for all frequency-ranges. The scattered field may be represented as the effect of the current induced in the strip, and it has been proved by Levine and Schwinger that it is possible to represent the amplitude of the far-zone scattered field in terms of the induced current in a form which is stationary with respect to small variations of the current about the true current. Substitution, in this representation, of a rough approximation for the current may give a remarkably good approximation of the far-zone scattered field amplitude. In this note we assume a normally incident field polarized parallel to the generators of the strip. As a rough approximation, we take a uniform density of the current induced in the strip. Since the incident magnetic field is constant over the strip, Fock's theory may be cited in support of the uniformity of the current distribution, except near the edges where the behaviour of the field indicates an infinite current density. A more detailed analysis of the current, by Moullin and Phillips, is available but was not used here.
Once the (approximate) amplitude of the far-zone field has been obtained, the scattering cross-section may be found by the application of the scattering theorem which relates this cross-section to the imaginary part of the amplitude of the far-zone scattered field along the central line of the umbral region. In spite of the crude approximation adopted for the induced current, the scattering cross-section shows a fair agreement with other available results.https://authors.library.caltech.edu/records/9f7yz-n0z83An Application of Sommerfeld's Complex Order Wave Functions to Antenna Theory
https://resolver.caltech.edu/CaltechAUTHORS:20190909-152146587
Authors: Papas, C. H.
Year: 2019
DOI: 10.7907/DDVZ-XC57
In the past wave functions of integral order have been used quite advantageously in the solution of certain antenna and boundary-value problems. However, in some instances these wave functions are completely alien to the problem and introduce difficulties which, indeed, can be resolved but only at the expense of logical simplicity. To place in evidence the usefulness and "naturalness" of complex order wave functions for the solution of certain problems, we examine theoretically the input admittance of a boss antenna with the aid of these functions.https://authors.library.caltech.edu/records/547n1-g5781On Perturbation Theory of Electromagnetic Cavity Resonators
https://resolver.caltech.edu/CaltechAUTHORS:20190910-100627820
Authors: Papas, C. H.
Year: 2019
DOI: 10.7907/0AYQ-NK07
In this note the Lagrangian function for the electromagnetic field of a cavity resonator is found. And from this Lagrangian is deduced a perturbation formula which includes Müller's celebrated result as a special case. The same perturbation formula is derived also from the Boltzmann-Ehrenfest adiabatic theorem in a most simple manner.https://authors.library.caltech.edu/records/64rr1-q2b80A Note Concerning a Gyroelectric Medium
https://resolver.caltech.edu/CaltechAUTHORS:20190910-154055354
Authors: Papas, Charles H.
Year: 2019
DOI: 10.7907/S6QJ-RS64
The fact that a homogeneous electron gas when immersed in a uniform magnetostatic field becomes electrically anisotropic, i.e., gyroelectric, is placed in evidence. The permeability of the gas remains equal to that of free space, but its dielectric constant is transformed to a dyadic or tensor upon application of the magnetostatic field. The properties of the dielectric tensor are such that a plane electromagnetic wave propagating through such a medium under- goes a Faraday rotation. This rotation is the dual of the Faraday rotation produced by gyromagnetic media.
The dielectric tensor of the electron gas is deduced and the Faraday rotation constant is calculated.https://authors.library.caltech.edu/records/qkxzp-g7p39On the Application of a Variational Principle to Antenna Theory
https://resolver.caltech.edu/CaltechAUTHORS:20190926-173654623
Authors: Papas, Charles H.
Year: 2019
DOI: 10.7907/T5ZR-NF60
The strong limitations on the applicability of exact methods of analysis to electromagnetic boundary-value problems have encouraged the development of certain approximation techniques. Among the most practical of these techniques is Schwinger's variational method. In this paper Schwinger's variational principle is derived from first principles and its application to antenna theory is critically examined.
La limitation de l'application des méthodes exactes d'analyses aux problèmes électromagnétiques comportant des conditions aux limites a encouragé le développement de certaines méthodes d'approximation. La plus pratique de ces méthodes est la méthode variationelle de Schwinger. Dans cette communication le principe variationnel de Schwinger sera dérivé de principes fondamentaux et on examinera de facon critique son application à la théorie des antennes.https://authors.library.caltech.edu/records/wskpy-nrb63On the High-Frequency Oscillations of the Electronic Plasma
https://resolver.caltech.edu/CaltechAUTHORS:20191001-125537405
Authors: Papas, Charles H.
Year: 2019
DOI: 10.7907/RHS9-WC14
For the purpose of this note an electronic plasma is defined as a gas of classical, non-relativistic electrons immersed in a constant charge-neutralizing background. The plasma is assumed to be spatially limitless and free of externally applied fields.
An exact analysis of the general oscillatory behavior of the plasma is forbiddingly difficult because it requires a detailed knowledge of the collision mechanism and ultimately leads to an intractable integro-differential equation. However, there are two extreme cases that are simple enough to be handled mathematically.
One of these limiting cases occurs when the collisions are so frequent that the electronic distribution is Maxwellian in every volume element and local equilibrium is established. Then the behavior of the plasma is determined by macroscopic hydrodynamical equations which lead to the dispersion relation
ω^2 = ω_(p)^(2) + (5/3)((ℋT)/m) k^2
where ω_p is the plasma frequency given by
ω_(p)^(2) = (ne^2)/(mε_o)
with T denoting the equilibrium temperature, ℋ Boltzmann's constant, k the wave number, m and e the electronic mass and charge respectively, and ε_o the dielectric constant of free space (M.K.S. system). This dispersion relation does not agree with the dispersion relation derived by the Thomsons, by Bailey, by Borgnis, and others. The reasons for this discrepancy have been reported by Van Kampen. In the other limiting case the collisions of the electrons with the ions and with each other are negligible and the collision term of Boltzmann's equation can be set equal to zero. This state is physically approximated when the frequency of oscillation is sufficiently high. Under special circumstances the dispersion relation in this case is approximately given by
ω^2 = ω_(p)^(2) + 3((ℋT)/m) k^2.
In this lecture we shall critically examine the theory of the high-frequency case, placing in evidence the tacit assumptions and hypotheses upon which the theory is based.https://authors.library.caltech.edu/records/9pres-v6038On the Attenuation of Guided Waves in the Limit of High Frequencies
https://resolver.caltech.edu/CaltechAUTHORS:20191001-161617326
Authors: Papas, Charles H.
Year: 2019
DOI: 10.7907/TSY9-2J11
The conventional formulas for the attenuation of waves due to the wall losses in uniform waveguides are based on the two assumptions that the wall currents are the same as the loss-free currents and that the surface resistance of the highly conductive walls is isotropic. In the limit of high frequencies the former assumption remains valid whereas the latter assumption breaks down. As the frequency is increased the surface resistance becomes anisotropic in the sense that it assumes different values depending on whether the wall current is longitudinal or transverse. In this paper new attenuation formulas are derived, which take into account the high-frequency anisotropy of the surface resistance and hence yield accurate results for all frequencies.https://authors.library.caltech.edu/records/8d40j-jwa63The Incoherent Scattering of Electromagnetic Waves by Free Electrons
https://resolver.caltech.edu/CaltechAUTHORS:20200220-111337349
Authors: Papas, C. H.; Lee, K. S. H.
Year: 2020
DOI: 10.7907/2R2B-5P77
In this paper the incoherent scattering of an electromagnetic wave by free electrons is examined theoretically. Under the assumption that the electrons have a Maxwellian velocity distribution, the scattered power and its frequency spectrum are calculated. The applicability of these results to ionospheric and laboratory plasmas is discussed.https://authors.library.caltech.edu/records/njnr4-n0755On the Index of Refraction of Spatially Periodic Plasma
https://resolver.caltech.edu/CaltechAUTHORS:20200219-164336165
Authors: Papas, C. H.
Year: 2020
DOI: 10.7907/YN3X-4B88
A knowledge of the change produced in the index of refraction of a uniform plasma by the spontaneous generation of coagula or inhomogeneities is essential to the use of electromagnetic waves as a diagnostic tool. The general problem is a difficult one to handle, but certain non-trivial cases are mathematically tractable. One of these, which is also of some practical import, occurs when the inhomogeneities are periodically distributed throughout the plasma. Here this special case is analyzed within the framework of the theory of periodic structures. The problem is reduced by virtue of Floquet's theorem to an equivalent problem for the domain of a unit cell with periodic boundary conditions. An approximate solution is obtained by a simplified theory. As a specific application the calculation for a plasma with periodically spaced spherical inhomogeneities is worked out in detail.https://authors.library.caltech.edu/records/2rcsv-40451