General formulas for 2-D leaky-wave antennas

General formulas for a two-dimensional (2-D) leaky-wave antenna (LWA) are obtained, which are applicable to any general class of 2-D leaky-wave antenna that consists of a partially reflecting surface that is mounted on top of a grounded substrate, and excited by a simple source. Closed-form expressions are obtained for the radiated fields, the field peak values in the E- and H-planes, the beamwidths in the E- and H-planes, and the pattern bandwidth. The formulas are obtained from a simple transverse equivalent network model of the structure. An accurate formula for the substrate thickness necessary to give a beam at any desired angle is also obtained.     

Depolarization properties of offset reflector antennas

The cross polarized radiation for linearly polarized excitation and the beam displacement for circularly polarized excitation have been investigated for offset reflector antennas. Numerical calculations are given to illustrate the dependence upon the angletheta_{0}between the feed axis and the reflector axis as well as upon the half-angletheta_{c}subtended at the focus by the reflector. In the casetheta_{0} = theta_{c} = 45deg, measured results have been obtained for both linearly and circularly polarized excitations with a dual mode feed illuminating an offset paraboloid. The cross polarized radiation of horn reflector and open Cassegrainian antennas rises sharply to rather high values off the beam axis; however, in general, the maximum cross polarized radiation of offset reflector antennas can be made small by using a small angle between the feed and reflector axes. The cross polarization caused by offset is compared with that caused by an unbalanced feed pattern. The effect of the longitudinal current distribution and of departure of the surface from a paraboloid on cross polarization are also examined. The clarification of these cross polarization properties is found to be valuable in the design of reflector antennas.     

Saturation properties of large-aperture photoconducting antennas

The authors describe the saturation properties of ultrafast pulsed electromagnetic radiation generated by large-aperture photoconducting antennas as a function of optical excitation fluence. A theory that predicts this effect is presented. The amplitude saturation of the radiation has been observed form antennas incorporating GaAs, InP, and radiation damaged silicon-on-sapphire consistent with theoretical expectations. The radiated fields were measured directly with a time resolution of 0.6 ps with the use of a large-aperture antennas as a detector. From these experimental studies, information about the high-speed response (i.e., the transient carrier mobility in the first few picoseconds after optical excitation) of the photoconductors incorporated in the antenna can be obtained under conditions of high applied electric fields and optical fluences     

The properties of antennas in plasmas

The last five years have witnessed remarkable progress in the theory and measurement of both the radiation and impedance properties of antennas in plasmas. Increased motivation for research in this area has been provided by the Space Shuttle program and by the prospect of nuclear fusion. The focus of attention has been on resonance cones in linear, anisotropic plasmas, including radiation patterns, wave interference, pulse propagation, reflections from boundaries and inhomogeneous media effects. Under nonlinear conditions, the focussed field of a resonance cone can significantly depress the plasma density. Under both linear and nonlinear conditions, the input impedance of dipole and loop antennas has been studied extensively, for both anisotropic and isotropic plasmas. A continuing challenge has been the as-yet-not-fully-explained experimental observation of linear, non-collisional enhanced resistivity of the sheath region around an antenna. Numerical impedance calculations employing simplified velocity distributions have shown particular promise. Ion and electron wave radiation patterns for various antenna shapes have been calculated and checked experimentally. The response of both single antennas and pairs of antennas to plasma fluctuations has been studied and found to have applications to plasma diagnostics.     

Spatial coherence properties of planar antennas

Lateral spatial coherence of an antenna is modeled by random phase front tilts. An effective source coherence length is introduced, and a generalized Fresnel coefficient which takes into account the coherence of the source is abstracted. Generalized reciprocity relationships for beamwidth and source coherence as well as for observed coherence and source power distribution valid in the near and far field are developed     

Fundamental transmitting properties of carbon nanotube antennas

Fundamental properties of dipole transmitting antennas formed by carbon nanotubes are investigated. Since carbon nanotubes can be grown to centimeter lengths, and since they can be metallic, the properties of carbon nanotubes as antenna elements are of fundamental interest. In this paper, dipole carbon nanotube antennas are investigated via a classical Hallen's-type integral equation, based on a quantum mechanical conductivity. The input impedance, current profile, and efficiency are presented, and the radiation pattern is discussed, as are possible applications.     

Main-beam polarization properties of modified helical antennas

The polarization properties of modified helical antennas are investigated for a wide range of directions using a two-dimensional polarization diagram. The current distribution on the antenna and the radiated field are calculated for several variations of the antenna shape to show the influences of the helix design on the main-beam polarization characteristics. This extended knowledge about typical polarization diagrams yields conditions for the antenna design. Special emphasis is placed on the choice of the geometrical parameters for the open end and the feed region of the antenna to meet different system requirements. The calculated results are confirmed with measured polarization characteristics and radiation patterns for experimental helices     

Off-axis properties of silicon and quartz dielectric lens antennas

The theoretical far-field patterns and Gaussian-beam coupling efficiencies are investigated for a double-slot antenna placed off aids on extended hemispherical silicon and quartz lenses. Measured off-axis radiation patterns at 250 GHz agree well with the theory. Results are presented that show important parameters versus off-axis displacement: scan angle, directivity, Gaussicity, and reflection loss. Directivity contour plots are also presented and show that near-diffraction limited performance can be achieved at off-axis positions at nonelliptical extension lengths. Some design rules are discussed for imaging arrays on dielectric lens antennas     

Scanning properties of large dual-shaped offset and symmetricreflector antennas

The scanning properties of shaped reflectors, both offset and circularly symmetric, are examined and compared to conic section scanning characteristics. Scanning of the pencil beam is obtained by lateral and axial translation of a single point source feed. The feed is kept pointed toward the center of the subreflector. The effects of power spillover and aperture phase error as a function beam scanning are examined for several different types of large reflector design including dual-offset, circularly symmetric large f/D, and smaller f/D dual reflector antenna system. It is shown that the Abbe-sine condition for improved scanning of an optical system cannot, inherently, be satisfied in a dual-shaped reflector system that is shaped for high gain and low feed spillover. The gain loss, with scanning, of a high-gain shaped reflector pair is demonstrated to be due to both aperture phase error loss and power spillover loss     

Transmitting, receiving, and scattering properties of antennas

In most modern textbooks on antennas, it is stated that receiving antennas scatter as much as they absorb under matched-load conditions. This is incorrect, as was shown 50 years ago. The reason for this discrepancy is an apparently incorrect use of the equivalent circuits. In this tutorial paper, a simple example of a two-element Yagi antenna and a test dipole in the far field is analyzed for transmission, reception, and scattering, through the use of the symmetric impedance matrix. It is shown that the correct equivalent circuit for the scattering case is more complicated, owing to the existence of more sources than in the transmitting case. Through a numerical example, it is also shown that the scattered power may be larger, equal to, or smaller than the absorbed power.     

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.     

Crosspolarisation properties of reflector antennas with random surface errors

The crosspolarisation properties of symmetric front-fed paraboloids with random surface errors are investigated. Average crosspolar far-field patterns and crosspolarisation discriminations are computed for different RMS surface errors. The influence of the correlation interval and phase-error model used is indicated.     

Equivalence of radiation properties of square and circularmicrostrip patch antennas

The radiation properties of a circular microstrip antenna are compared with those of an equivalent square microstrip antenna. On the basis of this comparison, a simple method of computing the far field of a circular microstrip antenna is suggested     

A summary of the transmitting and receiving properties of antennas

An expression for the electric field radiated by an arbitrary current distribution is given, which leads to a straightforward characterization of an antenna by a complex vector called the equivalent length. All classical transmitting parameters-such as the radiated power density, power intensity, radiation resistance, directional gain, and gain-are simple functions of the equivalent length. The same is true for the receiving parameters: induced voltage, available power, equivalent surface, and antenna factor. Eliminating the equivalent length between the gain and an equivalent surface provides a simple, direct, and general proof of the fundamental relation for antennas     

A method for the calculation of properties of wire antennas

A symmetrically-fed dipole antenna is considered in the paper. The rigorous integral equation is usually resolved by approximating the kernel, assuming a deltafunction feed, and iterating. To obtain consistent solutions, the sinusoidal current approximation needs to be modified («rounded») at the feed. A modified transmission line approach is equivalent to firstorder iteration by taking a certain value of characteristic impedance. The radiation impedance for a dipole contains the expression for the line reactance within it, and if this term is separated out, the remainder remains finite as the wire radius approaches zero. The iteration approach is equivalent to the usual induced EMF method result if a certain current amplitude is selected, but the result is unsuitable for the full wave dipole. An improved choice is indicated, and is suitable for any dipole length.     

Study on ellipticity properties of single-feed-type circularly polarised microstrip antennas

The letter presents the basic method for the design of single-feed-type circularly polarised microstrip antennas (SCPA). An SCPA designed to operate under condition of circular polarisation (OCCP) proved to produce an axial ratio below 0.5 dB at the boresight. Moreover, the measured and calculated axial ratios for SCPA agreed well within the effective range. These results indicate the use of the design method reported in the letter for construction of SCPA with excellent directivity.     

Directive properties of antennas for transmission into a material half-space

The directive properties of antennas for transmission into a material half-space are investigated. In a practical situation, the antennas might be located in air with the directive transmission into the earth. The field of a general antenna over the half-space is expressed as a spectrum of plane waves. The integrals representing the field are evaluated asymptotically to obtain the "geometrical optics" field of the antenna, and this field is used to define quantities that describe the directive properties of the antenna (pattern function, gain, and directivity). Numerical results are presented for infinitesimal electric and magnetic horizontal dipole antennas in a dielectric half-space, region 1, with directive transmission into the adjacent dielectric half-space, region 2, and the ratio of permittivitiesepsilon_{2}/epsilon_{1}greater than one. The theory for the infinitesimal dipoles completely explains the directive properties previously obtained for the resonant circular-loop antenna over a material half-space. Measured field patterns and gains for dipole and loop antennas near an interface between air and fresh water are in good agreement with the theory.     

The radiation properties of electrically small folded spherical helix antennas

The radiation properties of several electrically small, folded spherical helix antennas are presented. The primary variables considered in the design of these antennas are the number of helical turns and the number of helical arms. The principle design objectives are to achieve self resonance, a low quality factor (Q), and a practical radiation resistance for small values of ka. Designs are presented for ka less than 0.5, where the antennas are self resonant, exhibiting an efficiency in excess of 95%, a Q within 1.5 times the fundamental limit, and a radiation resistance near 50 /spl Omega/. The relationship between the number of helical turns, the number of helical arms, and achieving self resonance at low frequencies is discussed.     

Effect of substrate thickness on radiation properties of coplanar strip antennas

The effect of substrate thickness on the broadband (15-150 GHz) radiation characteristics of exponentially-tapered coplanar-strip antennas is explored using picosecond photoconductive pulse generation and sampling.<>