Current waves on an infinite cylindrical antenna in a warm plasma

The current waves on the outside wall of an infinite cylindrical antenna, fed by a delta-function generator and immersed in a warm isotropic plasma, which is in direct contact with the antenna, is separated into its components: the familiar e.m. wave, the `transverse-plasma? wave and the slow surface wave.     

Current distribution on a dipole antenna in a warm plasma

A cylindrical dipole antenna having finite dimensions, and surrounded by a homogeneous warm plasma, has been treated as a boundary-value problem. The current on the antenna has been determined and has been found to possess an oscillatory component, the fluctuations of which have approximately the wavelength of the surface wave on a corresponding infinite cylinder immersed in a warm plasma.     

Current distribution and input admittance of an infinite cylindrical antenna in anisotropic plasma

The current distribution and input admittance of an infinitely long cylindrical antenna driven by a slice generator and immersed in an anisotropic plasma are investigated. The applied dc magnetic field is along the axis of the antenna. By superimposing the characteristic waves guided along the antenna, the current solution is obtained in the form of a one-dimensional integral, which is examined both analytically and numerically. WhenK_{perp} > 0, the magnitude of the current decays slowly with the distance from the source, and its phase is nearly linear with a "propagation constant" equal tosqrt{K_{perp}}k_{0}for an antenna with very small radius. WhenK_{perp} < 0, the magnitude of the current decreases rapidly away from the source, and its phase is no longer linear. The input admittance decreases with the radius of the antenna.     

The coaxial collinear antenna: Current distribution from the cylindrical antenna equation

A theoretical model and computer simulations of the current on the coaxial collinear (COCO) antenna are presented. Comparisons between numerical results and measurements are made for 24- and 26-element COCO antennas at 50 MHz and for six- and eight-element COCO antennas at 915 MHz. The results show reasonable agreement between first-order theory and measurements.     

Current distribution on a receiving dipole antenna

The current distribution on a receiving dipole antenna is dependent on its feed-point load impedance. Calculations based on the modified King-Middleton approximation have been made of the distribution for antenna half lengths of 0.1, 0.2, 0.25, and0.3 lambda. Representative values of the antenna load impedance, resistive, capacitive, and inductive, are used to show their effects on the antenna current distribution.     

Radiation from a center-fed cylindrical antenna surrounded by a plasma sheath

The radiation field of a plasma-clad cylindrical antenna fed across an infinitesimal gap has been derived. The limitations on the validity of the far-field pattern have been stated. In the vicinity of the antenna, but in the region far from the gap, an expression is derived for the surface wave contribution which is the major source of field strength in this region. Analysis of the contour integration and discussion of the location of the various types of poles of the integrand on their appropriate Riemann sheets are presented. Numerical computations of far-field patterns and surface wave fields are presented graphically.     

Theoretical and experimental study of a finite cylindrical antenna in a plasma column

A numerical technique has been used to solve the integral equation determining the current distribution on a finite cylindrical antenna in a long plasma column; a "cold" isotropic plasma model with a relative dielectric constantepsilon_{r} = 1 - omega_{p}^{2}/omega^{2}is used in the analysis. Collision losses and inhomogeneities in the plasma have been ignored for the sake of mathematical simplicity. It is found that when0 < epsilon_{r} < 1, the plasma column tends to behave like a dielectric and reduces the effective electrical length of the antenna. The current distribution is still somewhat sinusoidal but with a longer wavelength. In the range-1 < epsilon_{r} < 0, the antenna characteristics undergo a drastic change; the current decays very rapidly along the antenna and, for antennas longer than a quarter wavelength, the input admittance remains almost constant and independent of the length. Extensive experimental measurements have been made to verify the theory. The plasma sheath surrounding the antenna was produced by an active, hot.cathode, helium discharge; the plasma diagnostic measurements were made by using Langmuir probes and cavity perturbation techniques. The theoretical and experimental results show a significant degree of qualitative agreement.     

Waves along a cylindrical "Grid-like" antenna in an anisotropic compressible plasma

In an anisotropic compressible homogeneous plasma, the dispersion equation of waves along an infinitely long cylindrical "grid-like" antenna is obtained analytically. Numerical solutions of this equation show that an external static magnetic field has great influence upon the anomalously dispersive region of the dispersion relation around the plasma frequency.     

The admittance of the infinite cylindrical antenna immersed in a lossy, compressible plasma

Some numerical results obtained from an analysis of the admittance of an infinite cylindrical antenna excited at a circumferential gap of finite thickness and immersed in a lossy, compressible (warm) plasma are given. The linearized hydrodynamic equations are used for the electrons (ion motion is neglected). A free-space layer, or vacuum sheath, is used to approximate the ion sheath which forms about an object at floating potential in a nonzero temperature plasma. Values for the antenna admittance are obtained by a direct numerical integration of the Fourier integral for the current, and are presented as a function of frequency for plasma parameter values typical of theEregion of the ionosphere. The admittance exhibits a maximum below the plasma frequency unless the electron temperature and sheath thickness are both zero; however, above the plasma frequency, the sheath and electron temperature have relatively little effect on the antenna admittance. The nonzero plasma temperature considerably enhances the antenna conductance below the plasma frequency compared with the zero-temperature case while at the same time reducing the dependence of the conductance on the electron collision frequency. A susceptance zero the location of which is not sensitive to the vacuum sheath thickness occurs near the plasma frequency.     

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.     

An experimental and theoretical study on the circuit properties of a cylindrical antenna in a hot lossy plasma

Experimental results on the input impedances and the current distributions of cylindrical antennas of various lengths in a large volume of hot lossy plasma are presented. A relatively simple theory based on an integral equation and a grid antenna model is developed. The theoretical results are in good qualitative agreement with the experiments. The theory is also compared to theories of some other researchers. Because of its simplicity, the present theory may prove useful in practical applications.     

Current distribution in arbitrarily oriented receiving and scattering antenna

An expression is derived for the distribution of current along a parasitic antenna when immersed in an arbitrarily directed and polarized incident plane electromagnetic wave. The formula is general, quite accurate, and involves only simple trigonometric terms with complex coefficients. It consists of both the even and the odd components of current. The former includes contributions from an arbitrary load at the center.     

Measurement of antenna current distribution in an anisotropic plasma

The current distribution along a cylindrical antenna immersed in a magnetized plasma has been measured. It has been observed that the standing wavelength along the antenna wire is substantially shortened to a greater degree with increasing plasma frequency under the presence of the dc magnetic field. The gyroresonance has also been observed in the current distribution.     

Simple formula for current on a cylindrical receiving antenna

An approximate formula is presented for the induced current on a loaded (or unloaded) thin cylindrical receiving antenna which is illuminated by an incident plane wave from an arbitrary direction. This formula involves only elementary functions and therefore can be calculated easily. Its derivation is based on the multiple reflection approach originated by Weinstein, Shen, and others. Currents calculated by the present formula are found to be in good agreement with experimental data.