Antenna In Free Space Research Articles

Precise Incident Power Density Pattern Measurement through Antenna Pattern Deconvolution

Future cellular and fixed wireless loops (FWL) systems employing highly directive base stations antennas (5 to 8 degrees beamwidth) and moderately directive subscribers' antennas (15 to 25 degrees beamwidth) offer very large capacity due to reduced interference. An important property of the environment of such systems is the interference caused by scattering of the signal from a subscriber transmitter into directions other than the direct line of sight between the subscriber and the base station. In particular, for multibeam base station applications scattering could arrive at the base station in the direction being used by a different beam, resulting in interference that is difficult to reduce by normal nulling techniques. Thus interference can be highly dependent on the incident power density pattern (IPDP) caused by scattering of the signal radiated from the subscriber. We discuss herein the use of a uniformly illuminated array accompanied by electric field deconvolution to measure the crucial IPDP with the same performance as a low sidelobe array of the same size. The mathematical correction technique uses deconvolution of the measured complex electric field pattern with that of the antenna in free space by means of the Fourier Series and limiting the range of Fourier coefficients to those that are not negligible in the free space pattern. Application of the technique to an experimental uniform array with a 2.5 degree azimuthal beamwidth shows the practicality of the deconvolution with real antennas in real environments. The improved resolution and accuracy provided by Taylor weighting versus unweighted deconvolution when trying to measure weak scattered components in the presence of a nearby strong specular component is demonstrated. The IPDP was measured from many sites surrounding a suburban base station. A plot of the cumulative distribution of the ratio of the widely scattered power to that within a prescribed beamwidth summarizes the result of using the deconvolution technique on this experimental data.

Near-Field Distribution of Radial Line Slot Antenna for Surface-Wave-Coupled Plasma Generation

In plasma processing systems for large-size wafers, quick evacuation of reaction gas by making the process space narrow is urgently required to realize higher productivity and better quality for microelectronic devices. Wafers must be placed close to the antenna aperture in this case. A radial line slot antenna (RLSA) for surface-wave-coupled plasma (SWP) is a promising candidate to this end. A novel slot arrangement in the aperture, consisting of densely arrayed transverse slots, was fabricated and tested. A uniform, high-density (3×1011 cm-3) and large-area plasma at low electron temperature has already been realized in previous experiments. It dispenses with the DC magnetic fields required in ECR systems. For further development of the slot design, as well as the mechanism determination of plasma generation, in this paper we evaluate the electromagnetic near fields of SWP-RLSAs. As the first step, antennas in free space without the plasma as well as a glass vacuum window, are analyzed using a periodic array model. The predicted near-field distribution is compared with those of previous experiments and a reasonable agreement is reported.

RECEIVING ANTENNA IN A MAGNETOPLASMA IN THE RESONANCE FREQUENCY BAND

We find the rms voltage on a receiving dipole antenna in the electromagnetic field of a quasipotential-wave packet in a magnetoplasma in the resonance frequency band. It is shown that this voltage can be determined as a product of the electromagnetic-field amplitude of the incident wave by the effective length Leff of the receiving antenna. For a short dipole whose length is much smaller than the electromagnetic-mode wavelength, the antenna effective length is proportional to the product of the dipole effective length in free space by the excitation coefficient of quasielectrostatic waves (non-normalized electric-field pattern of a dipole). In the case where the receiving antenna is located in proximity to the resonance cone on the lighted side, this excitation coefficient is much greater than unity. Therefore, the determined voltage differs significantly from the conventional estimate which is based on formulas valid for antennas in free space. We perform our derivations using the reciprocity theorem and the fluctuation-dissipation theorem applied to a regular electromagnetic field and a receiving antenna, which constitute the special case of a system which is not in equilibrium but allows an equilibrium (stationary state) to be reached between the incident radiation field and the electromagnetic field reemitted by the antenna. The emphasis is placed on the universal nature of the applied calculation procedure which is valid for arbitrary antennas and matters.

Accurate model of arbitrary wire antennas in free space, above or inside ground

An accurate model of wire antennas in free space, above or inside lossy ground is presented in which the current is assumed to flow on the surface of the wire and the testing is also performed on the surface. To replace the traditional delta-gap source, a more accurate source model is developed by using the Huygens' principle. From this principle and reciprocity theorem, a variational formulation of the input admittance is derived. When the triangle function is chosen as both basis and weighting functions, all the elements of impedance matrix and source vector are formulated in closed forms, which can be rapidly computed. Several numerical results are given. Comparing with measured data, both the current distribution and input impedance by this model are more accurate than those of delta-gap model.

Effect of the imperfect flat Earth on the vertically polarized radiation of a cylindrical reflector antenna

The radiation of a circular cylindrical reflector antenna in the presence of imperfect flat Earth is treated in an accurate manner. The boundary value problem is formulated in terms of a full-wave integral equation converted to the dual-series equations and then regularized by using analytical inversion of the static part. The resulting Fredholm second-kind matrix equation is solved numerically with guaranteed accuracy. The feed directivity is included in the analysis by using the complex source-point method. Various antenna features, which include the overall directivity, efficiency, gain, and radiated and absorbed power fractions have been calculated and compared with the free-space antenna characteristics. They show some phenomena not predicted by approximate techniques.

An FDTD/MoM hybrid technique for modeling complex antennas in the presence of heterogeneous grounds

Calculating the current distribution and radiation patterns for ground-penetrating radar antennas is a challenging problem because of the complex interaction between the antenna, the ground, and any buried scatterer. Typically, numerical techniques that are well suited for modeling the antennas themselves are not well suited for modeling the heterogeneous grounds, and visa versa. For example the finite-difference time-domain (FDTD) technique is well suited for modeling fields in heterogeneous media, whereas the method of moments (MoM) is well suited for modeling complex antennas in free space. This paper describes a hybrid technique, based upon the equivalence principle, for calculating an antenna's current distribution radiation pattern when the antenna is located near an air-ground interface. The original problem is decomposed into two coupled equivalent problems: one for the antenna geometry and the other for the ground geometry, with field information passing between them via a rapidly converging iterative procedure. The fields in each region may be modeled using numerical techniques best suited to them. Results for several test cases are presented, using FDTD to model the ground problem and MoM for the antenna problem, that demonstrate the accuracy of this hybrid technique.

Microwave Dielectric Properties of Shelled, Yellow-Dent Field Corn

Dielectric properties of three hybrids of shelled, yellow-dent field corn were measured at different bulk densities and moisture contents in a Styrofoam container located between two horn antennas infree space. Relative complex permittivities were determined from measurements of attenuation and phase shift at frequencies from 11 to 18 GHz and at temperatures from 4 to 45 °C. The grain moisture content ranged from about 9% to 19%, wet basis.

Advances in measurement methods and reduction of measurement uncertainties associated with antenna calibration

A major source of uncertainty in electromagnetic-compatibility conformance testing is the antenna factor. Until recently, broadband antennas in the frequency range 30 MHz-1 GHz were calibrated typically to an uncertainty of ±2 dB. Antennas can now be calibrated to an uncertainty of less than ±0.5 dB. This has made it possible to quantify a second category of uncertainties which arise when the antenna is used for emission testing on an open-field site. Ideally, the most useful calibration is of free-space antenna factors, but when antennas are used above a ground plane mutual-coupling effects can alter the antenna factor. The directivity of the antenna affects the magnitude of the received signal; balun imbalance affects the received signal; and the phase centre of log-periodic antennas changes with frequency. On top of this there are site effects which include reflections from surrounding objects, reflections from antenna-support structures and input cables and edge diffraction from the ground plane. These effects can be quantified to uncertainties as low as ±0.1 dB using antennas with calculable antennas factors.

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transient electromagnetic fields due to dipole antennas located near an infinite perfectly conducting cylinder

AbstractFor full‐fledged applications of radar imaging of buried objects by using electromagnetic waves, it is important to analyze theoretically and experimentally the fundamental problems such as the scattering of the electromagnetic wave by various objects and the electromagnetic wave propagation from the antenna above the ground into the ground.As the first step toward such a goal, this paper studies the transient electromagnetic fields scattered by an infinite perfectly conducting circular cylinder placed near a dipole antenna in free space. First, the transient electromagnetic fields are calculated rigorously for the case where one dipole antenna is placed in parallel to a conducting cylinder. Furthermore, geometrical optics approximation also is derived.From the computed results, it is shown that the transient scattered field can be interpreted as the total sum of the scattered fields corresponding to optical paths. It is shown also that the geometrical optics approximation is extremely accurate in the backscattering region important for the analysis of inverse scattering problems such as the detection of a buried object. Next, the transient transmission and reception responses are analyzed theoretically in the case where two dipole antennas are placed in parallel to the circular cylinder and its validity is confirmed by experiments.

Finite length helical sheath antenna in a general homogeneous medium

The sheath helix is an anisotropically conducting cylindrical tube that represents a good approximation to an actual helix consisting of many turns per wavelength. In this paper an integral equation for the current distribution along a finite length, axisymmetrically driven helical sheath antenna in a general homogeneous medium is solved by the method of moments to obtain the current distribution and radiation pattern of the antenna. The characteristics of various helical sheath antennas in both dissipative and nondissipative media are presented. Comparisons are made of the current distributions along multifilar thin‐wire helical antennas and helical sheath antennas in free space.

On the relationship between the transmitting and receiving properties of an antenna

The response of a linear, passive antenna in free space to an incident plane wave is related to its far-field radiation in the reverse direction by \bar{h}^{r} = \bar{h}^{t} . The vector effective height \bar{h}^{r} characterizes the voltage across the open-circuit antenna terminals induced by an incident plane-wave field, and the corresponding \bar{h}^{t} characterizes the far-field radiation of the antenna when driven by a current injected into the same terminals. This relationship, which is shown to follow from the well-known reciprocity principle, both includes phase and makes mathematically explicit the polarization relation involved in the identity of the transmit and receive patterns of an antenna.

COMPUTER MODELING OF ANTENNAS NEAR THE GROUND

ABSTRACT An accurate and efficient numerical method based on the rigorous Sommer-feld theory is described for modeling antennas near an interface such as the ground. The Sommerfeld integrals are evaluated by numerical integration along contours in the complex plane and two-dimensional interpolation is used subsequently to obtain the many Sommerfeld integral values needed for the moment-method solution of an integral equation. These methods permit modeling an antenna within 10-6 wavelengths of the ground for about two to four times the computation time for the same antenna in free space. Results showing currents and radiation patterns are included.

Plane wave spectrum scattering analysis of near-field obstacle effects on directive antenna patterns

A plane wave spectrum scattering analysis of the effects of a near-zone obstacle on the pattern performance of a directive antenna is discussed. The free-space azimuth monopulse antenna is characterized by its sum and difference mode plane wave spectra, and a computed plane wave scattering dyad is used to characterize the scattering by a metallic right circular cylinder when it is excited by the incident antenna spectra. An efficient computer algorithm has been developed to compute the far-zone scattered and total fields for the antenna/cylinder system. Extensive experimental data have been obtained, and the patterns calculated using the present analysis are in agreement with the measured data.

Analysis of Wire Antennas in the Presence of a Conducting Half-Space. Part II. The Horizontal Antenna in Free Space

An integral equation approach for the analysis of a horizontal wire antenna(s) located in free space near a lossy half-space is described. Two methods, a rigorous one based upon the Sommerfeld formulation and an approximation technique involving the Fresnel plane-wave reflection coefficients, are used to account for the influence of the interface-reflected field upon the antenna current distribution. A moment method (subsectional collocation) solution procedure is used to obtain numerical results for antenna impedance and radiation patterns. The reflection coefficient approach is found to agree well (generally within 10%) with those due to the rigorous method at a fraction (approximately 1%) of the computer cost.

Analysis of Wire Antennas in the Presence of a Conducting Half-Space. Part I. The Vertical Antenna in Free Space

An integral equation method (based upon the electric field) for the analysis of a vertical antenna located in free space near the interface with a lossy half-space is presented. Two methods are considered to account for the influence of the interface-reflected fields upon the antenna current distribution: (1) the rigorous Sommerfeld integral approach and (2) a plane-wave reflection coefficient approximation. Numerical results for antenna impedance from the two methods are compared. It is found that for the parameter variations studied, the reflection coefficient approximation provides results within 10% of those obtained from the rigorous approach at approximately 1% of the computer time. The reflection coefficient technique has the further advantage of being readily extended to arbitrary antenna geometries and orientations.

Investigation of thin linear antennas with displaced feed points in a plasma†

This paper is an extension of the recent work of Gupta (1968) in which the properties of a thin resonant antenna in a plasma are analysed using linearized hydrodynamic theory and assuming a general propagation constant for the current distribution, for an antenna having feed points displaced by λ0/2. In this paper the analysis is extended to include a non-resonant antenna and also the dependence of the properties of both types of antennas on the size of the displacement and value of the propagation constant. The field patterns in the plasma are calculated for two different values of the general propagation constant and for various lengths of the antennas. The results are compared with the free space radiation field strengths which are evaluated by setting the plasma frequency to zero. It is shown that the maximum field strength of both the resonant and the non-resonant types of antenna in free space increases considerably when the feed points are displaced to a, certain optimum value. The variation of the...

Admittance of an infinite cylindrical antenna immersed in a lossy, compressible plasma

An expression is derived for the driving point admittance of an infinite, perfectly conducting cylindrical antenna excited by a finite uniform gap and immersed in a lossy, compressible, isotropic plasma. This expression is based on the twin assumptions that the gap width is much smaller than the wavelength of the plasma (electroacoustic) wave and that the radius of the antenna is much smaller than the wavelength of the electromagnetic wave; it is similar in form to the corresponding expression for an infinite antenna in free space, and it is obtained in much the same manner. Conductance and susceptance curves computed from the admittance function are in good agreement with those obtained numerically by other authors for f ≥ 0.7 fP. The behavior of the admittance function at frequencies in the neighborhood of the plasma frequency depends mainly on the electron – neutral particle collision frequency, not the plasma temperature. In general, the effect of the temperature is quite small, although a temperature-related effect can produce large admittance values in very low-loss plasmas at frequencies well below the plasma frequency (f ~ 0.03 fP for T = 1500 °K). This investigation supports the view that the propagation constant of the current along a cylindrical antenna in a compressible plasma is nearly equal to the plane-wave propagation constant in an incompressible plasma.

Coupled antennas in a dissipative medium

The Fourier-transform method used to solve for the current distribution and the driving-point admittance of a single antenna in free space and in a dissipative medium can be extended to cover the case of two coupled antennas.

Surface fields on linear antennas

Surface fields are calculated for a source-excited flat-strip antenna in free space by modifying a variationalimpedance formulation. The exciting fields are related by an integral to the antenna current, and approximate current distributions correspond to nonvanishing tangential-electric-field components on the metallic antenna surface. Current distributions consisting of sine and shifted cosine waves violate the edge conditions and excite strong tangential electric fields near the ends of the conducting antenna surface. These field singularities are avoided by making the antenna current decay in proportion to the square root of the distance from the antenna edge. The more accurate current distributions produce smaller surface fields on the antenna, and it is possible to distinguish the more accurate current distribution even in cases where the variationally determined antenna impedances differ insignificantly. The calculated current distributions and impedances are compared with experimental data and with results of iterative calculations.