Input admittance of a multilayer insulated monopole antenna

An efficient numerical technique based on the modal-expansion method in conjunction with a recursive algorithm is developed for a multilayer insulated monopole antenna fed by a coaxial transmission line. The modal-expansion analysis is facilitated by introducing a perfectly matched boundary (PMB) at a variable height over the ground plane of the monopole. The current distribution and input admittance are computed by finding the expansion coefficients of the electromagnetic field expressions. Numerical results for the input admittance of a dielectric-coated monopole antenna and an air-insulated monopole are compared with experimental ones available in the literature. Good agreement is achieved. Calculated results for the effects of various parameters on the input admittance of an air-insulated monopole antenna are presented and discussed     

Rigorous analysis finite length insulated antenna in air

A Weiner-Hopf-type analysis is used to solve the problem of a dielectric coated dipole antenna in free space. Analytic expressions are derived for the input admittance and the current distribution along a center-fed antenna excited across a gap of nonzero width. Such expressions are represented in terms of a function representing the aperture-gap field, which is considered equal to that at the gap of a similar solid insulated antenna with infinite extension. Results are in very good agreement with reported experimental results at different outer radii and dielectric constants of the dielectric coating for different antenna lengths. The numerical work involves only the determination of certain integrals using standard integration routines     

An experimental and theoretical study of the monopole embedded in a cylinder of anisotropic dielectric

The characteristics of a monopole antenna embedded in an anisotropic cylinder of semi-infinite length were studied. The anisotropic medium was created artificially by thin layers of lossless material whose dielectric constant alternated between two values, 15.0 and 1.06. A theoretical and experimental analysis was carried out for the axially anisotropic (uniaxial) orientation of the dielectric medium with curves shown for input admittance and current distribution. An experimental investigation was also carried out for the radially anisotropic orientation with measurements made for the input admittance, axial charge distribution, and the angular and axial current distributions.     

Feed gap aperture field and input admittance of an infinitely longinsulated antenna

Expressions are derived for the input admittance and aperture electromagnetic fields in the feed gap of a solid insulated antenna of infinite extent. This is done by solving the boundary value problem where the fields are represented by Fourier series built up by superposition of basic sets of φ-independent cylindrical waves. Such waves are obtained from the solution of the Helmholtz equation governing the z-directed electric Hertzian potential. The axial aperture electric field in the gap is in the form of a Fourier-Bessel series and is used to find the input admittance. Results are plotted for the aperture fields and tabulated for the input admittances at different radii, insulating shell dielectric constants, and gap widths     

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.     

Pattern synthesis using reactive loads and transmission linesections for a singly fed array

A feed network design is presented for an array antenna fed by a single source. A specified radiation pattern from the array, with a set of known terminal voltages, can be synthesized by choosing the appropriate characteristic impedance and length of feed transmission-line sections and reactive loads at the terminals of each element. Two configurations, a series feed and a parallel feed, are described. As an example, this method is applied to the feed network design of a singly fed linear dipole array, but the method is general enough to be applied to any type of array which can be characterized by a multiport terminal admittance matrix. Some results are shown for the bandwidth of the radiation pattern and input impedance     

Wiener-Hopf type analysis of dielectric-coated dipole antenna inrelatively dense medium

The problem of dielectric-coated dipole antennas in a relatively dense medium is solved using the Wiener-Hopf technique. The analysis is done for a perfectly conducting dipole antenna coated by a low-loss dielectric and embedded in a lossy medium of a wavenumber larger than that of the lossy coating. Analytic expression are derived for the input admittance and the current distribution along the dipole which is excited at the center across a nonzero width feed gap. Results are compared with available literature data for different antenna lengths embedded in an ambient medium with a loss tangent of σ/ω∈. The numerical work involves only the determination of certain integrals using standard integration routines     

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.     

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.     

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

A variational formulation is developed for the impedance of a finite strip antenna embedded in a planar dielectric slab which is surrounded by a magnetoionic medium (cold electron plasma) with a static magnetic field impressed in a direction perpendicular to the antenna surface. Closed form expressions are obtained in the limit of low frequencies, and for a short antenna in a uniaxial medium. The impedance becomes large at the plasma frequency, near the upper hybrid resonance frequency, and further resonances are observed near the gyro-frequency if the gyro-frequency exceeds the plasma frequency. The reactance of a short antenna is inductive at low frequencies, but becomes capacitive as the thickness of the insulation around the antenna is increased. For very thin insulating layers the wave number of the variationally approximated current distribution exceedssqrt{sqrt{epsilon_{1}} sqrt{epsilon_{3}}} k_{0}(epsilon_{1}andepsilon_{3}are the two diagonal elements of the permittivity matrix), whereepsilon_{1}andepsilon_{3}may have positive or negative real parts. However, this approximation does not apply to current distributions along an insulated antenna. The present calculations are also compared with earlier work on antenna impedances.     

Linear antennas in a warm plasma driven from a coaxial air-filled transmission line

The Green's function is formed for the warm-plasma region surrounding an antenna driven from a coaxial air-filled transmission line, and hence the current distribution and input admittance of three types of antenna having this feed system are computed. Comparison is made between the antenna characteristics when driven in this fashion and by an idealised delta gap.     

Spectral analysis of microstrip antennas with CG-FFT:two-dimensional results

Theoretical expressions for the currents excited on the top and bottom surfaces of a coaxial-probe-fed patch antenna are derived. Exact expressions for the far-zone field in terms of the total current on the patch are found. The analysis is specialized to a two-dimensional case. Detailed results for current, far-zone fields, and radiating-aperture admittance have been computed for this two-dimensional antenna model using the conjugate-gradient-fast Fourier transform (CG-FFT) technique. For a thick-substrate antenna it is found that the upper surface current on the patch is significant. Under this condition the traditional approximation of the total current by the bottom surface current alone may be in considerable error     

Corrugated spherical antenna

The radiation characteristics of a corrugated metallic sphere with an azimuthal slot are studied. For the purpose of analysis, a dielectric coating equivalent to the corrugation on the surface is assumed. Expressions for the equivalent relative permittivity, input admittance, gain and radiated far field, based on the boundary-value approach, are presented. For a spherical antenna of k0a = 4.9087 with corrugation corresponding to an equivalent dielectric coating of relative permittivity 8.36, it is shown that the increase in gain over the uncorrugated antenna is 6.15 dB. The above antenna was fabricated and the experimental pattern obtained is compared with the theoretical result.     

Broadband CPW fed stacked patch antenna

A broadband CPW fed stacked patch antenna well suited for integration with monolithic and optical integrated circuits is presented. The antenna is designed with a bandwidth of 40% on a high dielectric constant feed substrate. The predicted and measured input impedance and far field radiation patterns are given     

A theoretical study of the input impedance of a circular microstrip disk antenna

A theoretical study is presented of the input impedance of a circular microstrip disk antenna excited by a coaxial line. The theory is based on Green's function technique applied to the disk cavity with the boundary admittance at the edge. Both the feed pin size and the boundary admittance are shown to be important in deriving the analytical expression for the input impedance. The boundary admittance is obtained by considering the radiated power and the electric and magnetic stored energies in the fringe capacitance. The analytical expression for the input impedance includes the feed location, the feed pin size, the disk size and thickness, and the dielectric constant of the material, and is useful for optimizing various parameters. The calculations are compared with experimental data showing a good agreement.     

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     

Input impedance of annular-ring microstrip antennas with adielectric cover

A theoretical and numerical analysis of the annular-ring microstrip antenna with a dielectric cover is performed. The problem is formulated in the Hankel transform domain, taking a coaxial feed into consideration. The integral equations for the surface current density on the patch are derived. Galerkin's method is used to solve for the surface current density, and a stationary formula for the input impedance is obtained. Numerical results showing the effects of a dielectric cover on the TM₁₂ mode of the annular ring are given. It is found that the dielectric cover reduces the resonant frequency, decreases the resonant resistance, and increases the impedance bandwidth     

The Annular Slot Antenna in a Lossy Biological Medium (Short Papers)

An integral equation is formulated for a coaxially fed annular aperture antenna. The integral equation in terms of the unknown tangential aperture electric field is solved numerically by the Method of Moments. The coaxial feed line is air filled while the exterior region consists of i) air, ii) fat or bone, and iii) muscle. Results are given for the aperture electric field, apparent input admittance, and contours of constant power absorption when the excitation frequency is 2.45 GHz.     

Mutual coupling between slots printed at the back of ellipticaldielectric lenses

This paper presents calculations of the mutual admittance between two slots on a ground plane that are the primary feed of an elliptical dielectric lens antenna. This admittance may also be regarded as the basic constituent of a Galerkin method of moments formulation for the analysis of more complex feeding arrangements of slot antennas and coplanar waveguide circuits, it is found that the contribution of the reflection mechanisms inside the lens significantly affects the mutual admittance especially for H-plane coupling     

Theory and experiment of a coaxial probe fed hemisphericaldielectric resonator antenna

A hemispherical dielectric resonator antenna fed by a coaxial probe is studied both theoretically and experimentally. The Green's function for the evaluation of the input impedance is derived rigorously and expressed in a form convenient for numerical computations. The method of moments is used to obtain the probe current from which the input impedance of the DR antenna is calculated. Both delta gap and magnetic frill source models are considered. Moreover, the results using a reduced kernel as well as the exact kernel are presented. Both entire basis (EB) and piecewise sinusoidal (PWS) expansion modes are used and the results are compared. The effects of the probe length, feed position, and dielectric constant on the input impedance are discussed. Finally, the theoretical radiation patterns for the first three resonant modes (TE₁₁₁, TM₁₀₁, and TE₂₂₁) of the DR antenna are presented