Pulse radiation from an insulated antenna: an analog of Cherenkovradiation from a moving charged particle

Cherenkov radiation arises when a charged particle moves with a constant velocity that is greater than the speed of light in the surrounding medium. This radiation has distinctive characteristics. Near the charge, the electric field is most intense along a conical surface with apex at the charge-the Mach cone. In the far field, the radiation occurs predominantly in one direction-at the Cherenkov angle. An insulated antenna consists of a metallic cylindrical conductor covered by a concentric sheath of dielectric. In use, this antenna is embedded in a medium whose permittivity is often much greater than the permittivity of the insulation. When the antenna is excited by a pulse of voltage, a pulse of charge appears to travel along its length. The apparent velocity of this charge is close to the speed of light in the insulation, which, because of the difference in the permittivities, is greater than the speed of light in the surrounding medium. Thus, the radiation from the pulse excited, insulated antenna should be analogous to Cherenkov radiation from the moving charged particle. In this paper, the pulse-excited, traveling-wave insulated linear antenna is accurately analyzed using the finite-difference time-domain (FDTD) method. Results are obtained for the charge on the conductor, the near field, and the far field. These results show the striking similarity of the radiation from this antenna to Cherenkov radiation from the moving charge     

Wire and strip conductors over a dielectric-coated conducting ordielectric half-space

The complex wavenumber and characteristic impedance are determined for a wire or flat strip over a dielectric-coated half-space that can be a conductor or a dielectric with large permittivity. Elevated microstrip is an example of the configuration. The properties of the wire as an antenna or transmission lines are determined from those of the insulated antenna with a two-layer eccentric insulation. The theory is extended to the strip conductor with the help of a comparison of the tubular and strip conductors over a perfectly conducting half-space     

Theory of the terminated insulated antenna in a conducting medium

The general problem of the insulated antenna in an isotropic homogeneous medium of infinite extent is reviewed under the assumption that the medium is sufficiently conducting to permit the application of coaxial line theory. The currents and associated electromagnetic fields for the completely insulated antenna are obtained and the directional properties of the latter explained by comparison with a collinear array. An antenna in which only a central portion is insulated, while the ends are bare, is analyzed in terms of the theories of the insulated and bare antenna, both when the structure is driven by generators at the junctions of the bare and insulated sections and when it is driven by an internal coaxial feeder in the manner of a sleeve dipole. Finally, the center-driven insulated antenna with bare ends is investigated and its field is compared with that of the antenna driven at the junctions of the bare and insulated sections.     

An experimental study of the insulated dipole antenna immersed in a conducting medium

The driving-point admittance and the amplitude and phase distributions of the current referred to the driving-point were measured for an insulated cylindrical antenna immersed in a conducting medium. The ratiosigma/omegaepsilon_{r}epsilon_{0}of the conducting medium was varied fromsigma/omegaepsilon_{r}epsilon_{0}= 0.036to 8.8, a range which includes a variety of media such as poor insulators, the ionosphere, plasmas, dry earth, wet earth, lake water and sea water. The antenna heightbeta hin radians was varied frombeta h=0.1throughbeta h=2piat intervals of 0.1. The thickness of the insulator was varied fromb/a = 1.25tob/a = 12.0whereais the radius of the antenna andbthe radius of the insulator. Measurements have also been made of the admittance, current and phase distributions along an insulated antenna with a conductive top load, that is, one whose tip is in direct contact with the conducting medium. It is found that when the tip of the antenna is in direct contact with the conducting medium, the current increases almost linearly as the end of the antenna is approached. This is quite unlike the decaying sinusoidal distribution on the completely insulated antenna. In a general sense, the experimental results are in fair agreement with an approximate theoretical expression for the admittance of an insulated antenna immersed in a highly conducting solution. The approximate theory is based on the driving-point admittance of a coaxial line whose outer conductor is imperfectly conducting and infinite in extent.     

Insulated antenna with metamaterial coating operating in lossy medium

The problem of an insulated electric dipole antenna that has been coated with a layer of a double negative metamaterial operating in a lossy medium is considered. Theoretical predictions of its input impedance indicate that it may be possible to significantly improve the bandwidth of such an antenna with the proper selection of material parameters.     

A Coaxial Microwave Applicator for Direct Heating of Liquids Filling Chemical Reactors

A coaxial microwave applicator radiating in a liquid medium contained inside a chemical reactor is described. The applicator consists of an insulated asymmetrical dipole antenna that radiates almost isotropically. Hence, it appears well suited to directly heat the medium that fills the vessel, making it unnecessary to use a microwave oven as currently done in microwave assisted chemistry. The electromagnetic (EM) properties of the insulated dipole antenna emitting in high permittivity lossy media are first reviewed to define the applicator design criteria. A 3-D numerical EM solver is then employed to analyze the radiation of the applicator in the surrounding medium taking into account the whole structure of the reactor. Safe operations are always assured by the proper design of the applicator-vessel mechanical connection that drastically reduces unwanted stray radiation. The agreement between the theoretical analysis and the experiments performed is good, thus confirming that the coaxial applicator compares favorably with the traditional methods of activation, which use a single or multimode resonant EM cavity.     

A uniformly valid loaded antenna theory

A quasi-static layered approximation is used to simplify the layered solution for insulated antennas to the solution of a generalized impedance boundary value problem, whose solution is expressed in terms of an integral. This integral applies to insulated antennas imbedded in a dense medium, insulated antennas imbedded in air (dielectric-coated antennas), and impedance-loaded antennas, all referred to as loaded antennas. The branch cut contribution for large distances is given by the Sommerfeld space wave formula. The physical transition of loaded antennas to bare antennas is investigated through the asymptotic evaluation of this integral. Simple uniform formulas for loaded antenna current are derived and generalized to cover the same range of validity as the integral. The direct calculation of the input admittance is consistent with the derived uniform formula for antenna current. For insulated antennas in a dense medium, the complete transmission line theory describes the antenna current through the transition to bare antennas     

Antennas in material media near boundaries with application to communication and geophysical exploration, Part II: The terminated insulated antenna

The properties of the insulated antenna in a dissipative medium near a boundary are reviewed. The important effects on antennas of being embedded in media with different properties over a wide range of frequencies are treated specifically in terms of three examples. These are: 1) a traveling-wave antenna in the earth below air atf sim 0.1MHz for subsurface communication; 2) an antiresonant antenna on the sea floor atf sim 1kHz and 3) an electrically short antenna on the sea floor atf sim 1Hz--both for geophysical measurements. In each case the current distribution, impedance, input power, effective length, and electric field in the range of interest are determined.     

A new analysis of the sandwich-wire antenna

It is hypothesized that the exact shape of the center conductor of a sandwich-wire antenna, provided it is of fairly simple form, is not important in determining the radiating properties of the antenna. An analysis is therefore made by standard principles of a sandwich-wire antenna having a particularly simple shape for the center conductor. This approach leads to an explicit formula for the structure attenuation constant in terms of its geometrical parameters which may be used for design purposes. Experimental measurements which support the theory and justify the shape-independence hypothesis are presented.     

A 2 1/4-turn spiral antenna for catheter cardiac ablation

To investigate the delivery of microwave energy by a catheter positioned inside the heart for ablating small abnormal regions producing cardiac arrhythmias, a 2 1/4-turn spiral catheter-based microwave antenna applicator has been developed. The antenna consists of the center conductor with continuous insulating material extending from the coaxial feed cable formed into a spiral antenna. The insulator completely isolates the center conductor from tissue. Phantom experiments were performed on homogeneous tissue equivalent medium. The reflection coefficient of the antenna at different frequencies and for different spiral lengths, the time course and temperature profile of an ablation, and the dosimetry of power versus temperature, all indicate that the high-power heating patterns from this antenna are both wider and deeper than with the other microwave antenna systems and radio-frequency electrodes.     

The insulated linear antenna-revisited

In the past, the insulated linear antenna has been analyzed with an approximate transmission-line theory. The range of validity for this theory has not been established. In this paper, the finite-difference time-domain (FDTD) method is used to analyze the insulated monopole antenna. The validity of the FDTD analysis is established by comparison of results with accurate measurements for a variety of antennas. The FDTD analysis is then used to determine the accuracy of the approximate transmission-line theory. Graphs are provided to quantify the errors in the approximate theory as functions of the geometry and the electrical properties of the monopole antenna     

Measured properties of bare and insulated antennas in sand

An experimental system for measuring the electrical properties of antennas embedded in moist sand is described. Measured antenna input admittances for bare linear and circular-loop antennas in sand are compared with theory. Insulated linear and circular-loop antennas with the dielectric constant of the insulation comparable to that of the surrounding medium are considered. Measured input admittances of linear antennas with different insulation sizes and dielectric constants are used to illustrate the effect of the insulation on the admittance. Measured input admittances for the spherically insulated circular-loop antenna are compared with theory and are found to be in good agreement.     

Electric field sensors in electromagnetic sounding

A staked and an insulated dipole antenna were used to monitor transmissions from a distant very-low-frequency transmitter. The field strength recorded from the staked antenna was found to be much larger than that recorded from the insulated antenna. It is shown that this can be adequately described as magnetic field induction. It is postulated that an insulated antenna is a far better electric field probe for surface impedance investigations including audio magnetotelluric (AMT), controlled source audio magnetotelluric (CSAMT), and VLF sounding techniques     

Input impedance characteristics of coaxial slot antennas forinterstitial microwave hyperthermia

In this paper, a transmission-line type of input impedance model originally developed by King et al. (1983) for the insulated dipole antenna embedded in a homogeneous dissipative medium is extended to the case of insulated coaxial slot antenna. Physical construction of the latter indicates the presence of additional current path(s) inside the feed line, which shall lead to the shortening of its resonance length. This effect is taken into account in the impedance model and verified by experiments. Furthermore, a simple strategy for optimizing the applicator's impedance-matching performance is also described and verified. Excellent agreements observed between theoretical and measured S₁₁ data indicate that these models can be relied upon when designing an efficient applicator for interstitial microwave hyperthermia     

Infinite insulated cylindrical antenna in a simple anisotropic medium

The characteristics of an infinite cylindrical antenna, insulated from the surrounding uniaxially anisotropic plasma medium by a concentric cylindrical sheath of free space, are investigated for the case in which the antenna is excited by means of a delta-function voltage source and the gyroelectric axis is parallel to the axis of the antenna. For sufficiently large thickness of the insulation, guided waves are found to be supported along the antenna and their dispersion and power relations are analyzed. The radiation conductance of the antenna contributed by the space waves is evaluated and is found to be practically independent of the insulation thickness for frequencies above the plasma frequency. For frequencies below the plasma frequency, the radiation conductance, which is infinite in the absence of the sheath, remains finite and decreases rapidly with the increase in the thickness of the insulation. The antenna current and the radiation pattern in the far zone are also examined.     

Fields generated by bare and insulated cables buried in a lossyhalf-space

The propagation constants and resulting fields for the current excited on bare and insulated cables buried in a lossy Earth medium are examined. The fields are formulated in integral form under the thin-wire approximation. The discrete mode propagation constants for the insulated and bare cable types have dramatically different behavior, and subsequently affect the radiated fields. Unlike the insulated cable, it is shown that for the bare conductor the accurate determination of the propagation constant is imperative in the calculation of some of the field quantities at the Earth's surface. Simple near-field expressions are developed from the integral forms and numerical results are presented for various cable structures and burial depths. The study implies that the use of various commonly utilized approximations, such as the uniform axial current assumption, may not be valid for the bare or grounded buried conductors     

The Near Field of an Insulated Dipole in a Dissipative Dielectric Medium (Short Paper)

The near field of uninsulated dipole antenna in a dissipative dielectric medium is important in microwave hyperthermia and geophysical applications. In this paper, the electric field near an insulated dipole has been calculated by the direct numerical evaluation of a surface integral over the insulation. The computed results are compared with those previously obtained by an approximate numerical calculation.     

Impedance of a finite insulated cylindrical antenna in a cold plasma with a longitudinal magnetic field

A variational formulation is developed for the impedance of a finite cylindrical antenna embedded in a dielectric cylinder, which is surrounded by a magnetoionic medium (cold electron plasma) with the static magnetic field impressed in a direction parallel to the antenna axis. Closed form expressions are obtained in the limit of low frequencies, and for short antennas in a uniaxial medium. The impedance of a short antenna is nearly the same as for an assumed triangular current distribution, except that further resonances are observed in the vicinity of the gyro frequency, where the antenna becomes electrically long. These resonances may be shifted to frequencies exceeding the gyro frequency in the presence of an insulating layer around the antenna. For very thin insulating layers the wave number of the variationally approximated current distribution is to the first order equal tosqrt{epsilon_{1}} k_{0}(epsilon_{1}is the leading diagonal element of the permittivity matrix), where the gyro frequency may be both smaller or larger than the plasma frequency. However, this approximation does not apply to current distributions along the insulated antenna. The present calculations are also compared with earlier work on antenna impedances.     

A Galerkin moment method for the analysis of an insulated antennain a dissipative dielectric medium

A Galerkin moment method is employed to solve the problem of a dielectric-coated dipole antenna in a dissipative medium. Piecewise sinusoids are used as basis and testing functions. The dielectric coating is modeled by equivalent-volume polarization currents, which are simply related to the conduction current distribution. No additional unknowns are introduced, and the size of the moment-method matrix is the same as that for bare antennas. Exact and approximate formulas for the near electric field are derived. The computed results exhibit excellent agreement with those previously published for a symmetric, as well as an asymmetric insulated dipole. Compared to its existing competitors, the new method appears to be more general and computationally efficient     

Relationships for Green's function spectral dyadics involvinganisotropic imperfect conductors embedded in layered anisotropic media

The Green's function spectral dyadic that characterizes a perfectly conducting patch antenna or propagating line can be modified to account for noninfinite conductivity. This is done through a relationship for a single planar interface imbedded in an anisotropic medium. A relationship for an arbitrary number of planar interfaces imbedded in an anisotropic medium is also derived. The imperfect conductor is represented by an anisotropic impedance