The shunt-driven circular loop antenna

The distribution of current, admittance, and far field of a shunt-driven circular loop antenna are derived and related to those of an equivalent series-driven loop in parallel with a section of transmission line. The effective length of a shunt-loaded receiving antenna is obtained in terms of the corresponding quantity for the series-loaded loop.     

Symmetric electromagnetic field of a circular loop antenna over the conductive cone of finite length

The diffraction properties of the field of circular loop antenna over the conic surface of finite length have been investigated. The relationships of the radiated power as a function of the antenna position on the symmetry axis of the cone were established. The directional patterns and field distribution in the near-field zone were obtained for different geometrical parameters of the diffraction system. The results obtained were compared with similar relationships for semi-infinite cones.     

Impedance of a large circular loop antenna in a magnetoplasma

The input impedance of a large circular loop antenna with arbitrary orientation in a cold magnetoplasma is calculated by using a transmission line theory. New impedance resonances for antennas of finite size in a magnetoplasma in the frequency region below and near the electron cyclotron frequency are indicated theoretically. The resonance peak of the impedance at the lower hybrid resonance frequency is also predicted to exist for arbitrarily oriented antennas of finite size. The experiments on the impedance of a large circular loop antenna are carried out for the cases of normal and parallel orientation of the magnetic field with respect to the plane of the loop immersed in a radio frequency-generated laboratory plasma. The newly predicted impedance resonances for the antenna of finite size are observed. It is also shown that the measured impedances agree fairly well with the calculated ones.     

The circular loop antenna immersed in a dissipative medium

Investigations were made of the radiating and circuit properties of circular loop antennas when immersed in conducting media with various loss tangents. The following quantities were experimentally examined and compared with the available theoretical results. 1) Driving point admittanceY_{in}of circular loops as a function of the circumference per wavelength,2pib/lambda = betab. 2) Current amplitude and phase distributions. 3) Field patterns. In each case the loss tangentsigma/omegaepsilon_{r}epsilon_{0}of the medium (conductivitysigma, permittivityepsilon_{r}epsilon_{0}) is taken as a parameter. The experimental results are in good agreement with the theory. A comparison between the behavior of the loop in a conducting medium and those of a linear dipole was made wherever possible. It was discovered that there is a cut off size beyond which the driving point admittance of a linear dipole is the same whether or not its ends are connected to form a loop. This particular loss tangent is 1.06, and the cut off size islambda/2. A circular loop antenna withbetab= 1.0is excited predominantly in a dipole mode and its field pattern resembles that of a two element dipole array. A study was made to determine how this pattern becomes more like that of a monopole as the loss tangent of the solution is increased.     

Wideband probe-fed circularly polarised circular loop antenna

A wideband probe-fed circularly polarised circular loop antenna is presented. The wideband performance is achieved by introducing a parasitic loop inside the original loop that is driven by a feed probe. It is found that the 2 dB axial ratio bandwidth can be increased from 6% for a single circular loop to 16% by the introduction of a parasitic loop.     

Transient field produced by a traveling-wave wire antenna

The electromagnetic fields of a transient traveling wave current on a straight wire segment are calculated directly in the time domain. Retarded potential theory is used to establish an impulse response valid in both far and near zones. A closed-form expression for the field due to a rectangular current pulse is obtained, and corresponding plots are given. Numerical results from this permit the investigation of the differences between near versus far zones and coated versus uncoated wires. Experimental findings for both coated and uncoated wires show good agreement with theory. A formula for far-zone radiation from an arbitrarily shaped wire is developed; results for a proposed directive antenna geometry are presented     

VHF TV full channel loaded circular loop antenna

A very high frequency (VHF) full channel room TV receiving antenna, with a voltage standing-wave ratio (VSWR)<2 in the full-band VHF is described. TheE- andH-plane patterns are nearly cardioid in shape, hence it has the advantage of eliminating the TV ghost interference caused by backward reflecting waves in a strong signal environment. The directional pattern and impedance of the loaded circular loop antenna are calculated. The theoretical and experimental data coincide with each other.     

Electromagnetic field due to a loop antenna in a borehole

The radiation of a loop antenna embedded in a borehole with complicated geometry is analyzed by perturbation method and an efficient fast multiple-integration technique. The perturbation method is not used directly because there is a drill-stem whose conductivity and magnetic permeability are very large in the borehole. The prolate ellipsoid equivalence of the metal drill-rod is made, then the drill-rod may be removed and the perturbation method is valid. Some numerical results indicate that the approach is efficient and stable at low frequencies     

Characteristics of a horizontal circular loop antenna over a multilayered, dissipative half-space

The problem of a horizontal loop antenna located above a stratified dissipative half-space is investigated. Current distribution is formally obtained by cosinusoidal expansion around the loop. The effect of the lower half-space on each modal current is discussed in the case of a small loop (k_{0}b = 0.3) and a resonant loop (k_{0}b = 1). Change in input admittance as a function of height and as a function of the electric parameters of a homogeneous dissipative haft-space are presented. Such change is found to be very pronounced when the loop is close to the interface and when the loop is near resonance. The effect of a second layer in the lower half-space is then discussed.     

Transient radiation from a linear antenna with nonreflecting resistive loading

The transient waveforms radiated by step voltage or Gaussian pulse excited resistively loaded linear antennas are investigated by numerical means. Current distributions on the harmonically excited antenna are obtained as functions of frequency and for different values of the loading. The transfer functions of the antenna and the spectral densities of the radiated waveforms are obtained as functions of frequency and the loading and for different directions in space. The time dependent radiated waveforms produced by the antenna are obtained by using the fast Fourier inversion technique. The effects of the various antenna parameters on the radiated waveforms are also investigated. Some of the results are compared with available approximate analytical results.     

On the excitation of a circular loop antenna by travelling-and standing-wave current distributions

Presented here is an exact formulation of the electric and magnetic (EM) fields radiated by a circular loop antenna, assuming both travelling- and standing-wave current distributions. By using a differential current element positioned in an azimuthal direction as the starting point, the paper systematically develops the EM fields radiated by the circular loop via a vector potential theory. This approach leads to a general integral representation for the radiation characteristics of the loop antenna where we completely evaluate the resulting expressions, when the excitations of the loop assume travelling- and standing-wave distribution forms. In addition, this paper briefly examines a method to generate approximately such current distributions by coupling the loop to a two- or four-wire transmission line. Furthermore, the paper discusses a graphical representation of the current distribution plotted as a function of frequency or loop size. From the field expressions determined, we derive generalized closed-form results for some important design parameters for the loop antenna. These parameters include the radial component of the Poynting vector, the total power radiated by the loop, the directivity, the radiation resistance, and the effective area and height of the antenna. When we specialize to the important case of uniform current excitation, the exact values of the parameters deduced from the general expressions, are summarized and exhibited in a comprehensive table. This table facilitates the computations of these important physical parameters. Further analysis involving small argument and asymptotic approximations in the residts for the travelling-wave current excitation leads to closed-form expressions in terms of tabulated functions. Numerical results presented, include the fields radiated by the loop when the standing-wave current excitation admits a Fourier series representation. The present approach via potential theory reveals that the fields can be calculated in any arbitrary direction : this is consistent with a previous observation of Knudsen (1951, 1953) who employed a different approach.     

The circular loop antenna multiloaded with positive and negative resistors

Studies are made of both the driving-point impedance and the amplitude and phase distributions of the current in a circular loop which is driven by an arbitrary number of delta-function generators and loaded with an arbitrary number of lumped impedances. The only restriction imposed in the present treatment is that the elements (either generator or load) are spaced at equal intervals of2pib/malong the circumference wheremis the total number of elements andbis the radius of the loop. The numerical calculations include loads with negative resistances in order to take account of loops loaded with an element like an Esaki tunnel diode, with proper precautions against both self-oscillation and switching effects. A study of such precautions is to be considered separately. The experimental results agree quite well with theory except for the larger values ofbeta b. The radiation patterns of the loop antennas withbeta b=1loaded with various resistors were also measured.     

Theory of double parasitic loop counterpoise antenna radiation patterns

Concepts of ray optics and the results of Sommerfeld's half-plane diffraction theory are applied to obtain theoretical expressions for the elevation plane radiation field produced by double parasitic loop counterpoise antennas. Theoretical results are compared with results obtained from model measurements. Within the range of approximation the agreement between theory and experiment has been found to be satisfactory. A method of designing the antenna for special application to air traffic control systems is also discussed briefly.     

Table of the field patterns of a loaded resonant circular loop

A table ofE- andH-plane patterns of the circular loop antenna withbeta b = 1.0loaded withZ_{L} = R + jXatphi = 180degis presented. The table gives useful information for determining the values of the load impedance for a given field pattern. The results were experimentally examined.     

Near fields of the constant current thin circular loop antenna ofarbitrary radius

Assuming a known (constant) current distribution on the thin circular loop antenna of arbitrary radius in free space, an exact integration of the vector potential is performed without recourse to approximations. The only restrictions on the solution variables are that the observation point distance must be greater than the loop radius and that the polar angle must run between 0 and π. The resulting vector potential infinite series solution possesses a real part composed of linear combinations of complete elliptic integrals of the first and second kind and an imaginary part composed of elementary functions. Thus, it is possible to obtain an exact solution which is valid everywhere that r>a and 0⩽&thetas;⩽π. The electromagnetic field components of the constant current circular loop antenna are then determined by direct series differentiation. These solutions are valid in the near and induction fields, converging rapidly there, and are also valid in the far field, although many terms of the series are needed for convergence     

Frequency tunable monopole coupled loop antenna with broadside radiation pattern

A frequency tunable antenna made with an open-circuit monopole coupled loop antenna associated to a varactor diode is presented. The proposed antenna shows a 35.8% relative bandwidth, covering the 470?675 MHz frequency range. Theoretical and experimental results of S₁₁, radiation patterns and gain have been performed and show good agreement.     

圆口径正馈抛物面天线的近场分析

分析了圆口径正馈抛物面天线的近场.采用口径场绕射积分求取了天线的近场及远场方向图,绘制了波束宽度、方向性和最大副瓣电平三个关键参数随场点距离的关系曲线.通过与物理光学法的计算结果对比,验证了该方法的正确性.仿真结果是:随场点距离的减小,波束宽度展宽,方向性减小,最大副瓣电平在一定范围内波动.场点在D2/λ外,远场近似的波束宽度误差小于1%;在0.5D2/λ外,远场近似的波束宽度误差小于10%;在0.5D2/λ内,不能采用远场近似.