Determination of reactance loading for circularly polarized circular loop antennas with a uniform traveling-wave current distribution

A simple theory is presented to predict the lumped reactance loading for circularly polarized circular loop antennas with a uniform traveling-wave current distribution. The reactive load is located on a circular wire loop of one-wavelength circumference at a position of 45/spl deg/ away from the feed point. To achieve a uniform traveling-wave current distribution, the loading reactance and the input impedances of the loaded and unloaded loop antennas need to satisfy certain conditions. First, the input resistance and the input reactance of the unloaded loop antenna should have the same absolute value. Second, the input impedance of the loaded loop must be purely resistive and its value needs to be two times of the input resistance of the unloaded loop. Third, the loading reactance should be chosen to be two times in value and opposite in sense of the input reactance of the unloaded loop. These conditions can be approximately met when the circular loop is placed above a ground plane. The loading reactance is determined from the input impedance of the unloaded loop and is optimized for an optimal performance of circular polarization. It is found that the reactive load must be capacitive and its value depends on the height of the loop above the ground plane and the thickness of the wire. The characteristics of the circular polarization and the input impedance of the capacitance-loaded circular loop antennas are investigated. An experimental example is presented to verify the theoretical prediction.     

Cylindrical transmitting antenna: Tapered resistivity and multiple impedance loadings

A general formulation is presented which treats with accuracy the perfectly conducting antenna, the uniformly resistive antenna, the tapered resistive antenna, and the multiple impedance loaded antenna. The current distribution is obtained in analytic form that is easily used to obtain the effective length and radiated field. The presented theory has application in traveling-wave antenna design. Traveling-wave antennas are of interest for broadband and directional communication. Extensive numerical results are obtained, and comparisons are made with experimental data. The results indicate the feasibility of cylindrical traveling-wave antennas.     

Short cylindrical antennas with enhanced radiation or high directivity

The feasibility of enhancing the radiated power or improving the directivity of a short cylindrical antenna by double impedance loading is investigated. An approximate solution for the current on a doubly loaded short antenna is developed, and typical current dlstributions, impedances, and radiation patterns of antennas appropriately loaded to implement enhanced radiation or high directivity are presented. Significant improvements in radiated power or directivity can be achieved with optimum impedance loadings. Theoretical predictions are verified by the results of an experimental study.     

Self- and mutual impedances of traveling-wave linear antennas

A traveling-wave distribution of current can be produced on a conventional linear antenna, such as a dipole, through resistive loading. The self- and mutual impedances of parallel traveling-wave linear antennas are measured and compared with those of the corresponding conventional linear antennas. The effects of coupling on the input impedance of an array element is discussed, and it is shown that these effects are much less pronounced for traveling-wave antennas, which may be advantageous for some array applications     

FDTD analysis of probe-fed cylindrical dielectric resonator antennaoperating in fundamental broadside mode

An analysis of a cylindrical dielectric resonator antenna operating at the fundamental broadside mode using the finite-difference time-domain (FDTD) method is presented. Good agreement between theoretical and experimental results on the input impedance of the DR antenna is obtained. The effects of the probe length, feed position and the dielectric constant on the input impedance are presented     

Experimental investigations on the impedance behavior of a cylindrical antenna in a collisional magnetoplasma

An experimental study of the input impedance of a short cylindrical antenna immersed in a laboratory magnetoplasma is reported. The antenna is fixed in length and is aligned with its axis parallel to the dc magnetic field. Of particular interest is its impedance behavior in the vicinity of the resonance and cut-off conditions of the magnetoionic medium. Independent measurements are made to determine the electron density from the ion-current characteristics of a spherical Langmuir probe and the strength of the applied magnetic field with a Hall-effect probe. Measured impedances are compared with Balmain's theory. Essential features observed experimentally are a peak in the antenna input resistance near the upper hybrid resonance frequency and a peak in the antenna conductance near the cyclotron frequency. An additional small increase in the input resistance is observed near the plasma frequency when the plasma frequency is not close to either the upper hybrid resonance frequency or the cyclotron frequency. These results suggest the possibility of using a short cylindrical antenna as a diagnostic probe for measuring properties of a magnetoplasma such as the electron density and collision frequency.     

A loaded antenna system applied to VHF portable communication equipment

The technique of impedance loading to the antenna element has been applied to an antenna system for portable VHF communication equipment and its practical usefulness has been shown with a brief theoretical analysis and some experimental results. By means of impedance loading to the antenna element, the driving point impedance may be varied so as to make matching feasible, even though the element length is very short, and the effective length may be increased, so that the gain of the loaded antenna system relative to a non-loaded antenna system may be increased. As a practical example, an inductance loaded antenna system has been applied to a 34-54 MHz range portable communication equipment and about 5 dB relative gain increase has been observed.     

Radiation of a travelling-wave antenna in plasma in the whistler frequency band

The far field, the radiation pattern, and the radiation impedance of a traveling-wave antenna oriented in plasma along the external magnetic field in the resonance region of the whistler frequency band are studied. It is shown how the main characteristics of the antenna depend on its length and the current propagation constant along the antenna wire.     

Effect of dielectric jacket on a buried travelling-wave antenna

Transmission line theory has long been used to analyze the performance of dielectric-coated buried antennas. However, at UHF the ground cannot be assumed to be highly conducting and a new approach is necessary. A method is presented to predict the input impedance and pattern of a dielectric-coated traveling-wave antenna buried in lossless ground.     

Analysis and optimization of an electrically small receivingantenna

A theoretical and experimental study of an optimum receiving antenna configuration that fits within certain allocated space requirements is presented. This optimum monopole antenna configuration is applicable for Loran-C reception. The antenna analysis is based on a quasi-static numerical study of a conducting body of revolution above a perfectly conducting ground plane. A general numerical algorithm is developed to determine the input impedance and the effective height of the antenna. In addition, the amplifier noise and its role in the choice of the optimum antenna are examined. Results are presented for cylindrical and truncated conical structures both with and without a top load     

Analysis of a tubular monopole loaded with a shielded helical coil

An analysis of a shielded helical coil in a cylindrical tubular antenna is presented, based on a combination of aperture theory and a network model containing lumped RLC components, which is motivated by the desire to use this analysis in antenna-optimization routines. Measured input admittance data for an antenna loaded with a shielded helical coil are presented for comparison with computed data. Also, an experimental procedure by means of which one may measure the transverse electro-magnetic (TEM) input impedance of a coaxial guide loaded with helical coil is described.     

Minimization of backscattering of a loop by impedance loading--Theory and experiment

A study on the minimization of the backscattering of a conducting loop by the impedance loading method is presented. A conducting circular loop loaded symmetrically with two identical lumped impedances is assumed to be illuminated by a plane electromagnetic wave at normal incidence. The induced current on and the backscattered field by the loaded loop are determined. The optimum impedance loading for zero backscattering is obtained. The optimum reactive loading for minimum backscattering is also studied. An experiment was conducted to verify the theory.     

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.     

Theory and Experiment of an Aperture-Coupled Hemispherical Dielectric Resonator Antenna

A hemispherical dielectric resonator (DR) antenna using aperture coupling for excitation is studied both theoretically and experimentally. The reciprocity method is used to formulate the problem. The exact magnetic field Green's function due to the equivalent magnetic current in the slot is derived rigorously, and it is presented in a form which can be evaluated very efficiently. Moment method is used to solve the magnetic current from which the input impedance of the antenna configuration is obtained. The equivalent radius of the slot is used so that simple formulae developed for the cylindrical dipole can be applied directly. The effects of the slot's length, the slot's position, and the slot's width on the broadside TE₁₁₁ mode input impedance are studied, and reasonable agreement between theory and experiment is obtained     

Design and Prototyping of a Switching Beam Disc Antenna for Wideband Communications

The design procedure for a switching beam antenna for wireless communication systems is given. The antenna can rotate the beam over the whole azimuthal angle exploiting its geometrical and electrical cylindrical symmetry. The required directivity and the beam rotation are provided by a sequential insertion of metallic posts into the radial waveguide that forms the main body of the antenna. The first stage of the design involves setting up the dimensions of an omnidirectional antenna, to which the desired directivity is subsequently provided. After optimization to maximize the impedance bandwidth, the final antenna was prototyped. A comparison between simulated results and experimental data is presented     

On thin-wire multiturn loop antennas

A general theory of thin-wire multiturn loop antennas is presented. For convenience the windings of the loop are considered to form a circular helix. An integral equation is derived and solved numerically for the current distribution on the antenna. The antenna impedance, efficiency, and pattern are obtained.     

The cylindrical antenna with nonreflecting resistive loading

The distribution of current along a center-driven cylindrical antenna is obtained when the material forming the antenna is resistive. The particular case is considered when the impedance per unit length of the antenna is a function of the distance from the end. A solution is obtained specifically when the current is represented by an outward traveling wave with no reflected wave. The admittance of the antenna and the far-field pattern is determined. Field patterns are evaluated for a wide range of lengths. These are characterized by a single major lobe with a very small minor lobe structure.     

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.     

Computer analysis method for arbitrarily shaped microstrip antenna with multiterminals

An analysis method is presented for an arbitrarily shaped microstrip antenna with multiterminals. The method is based on the variational method and the modal-expansion technique. Eigenvalues and eigenfunctions are determined using the Rayleigh-Ritz method. Input impedance and other antenna parameters are derived at nonresonance. Furthermore, the network model, useful for the network analysis of a microstrip antenna with multiterminals, is presented by introducing an ideal transformer. Finally, numerical examples are compared with experimental results. The agreement is quite good, and the validity of the present method is confirmed.