Showing papers in "IEEE Transactions on Antennas and Propagation in 1975"
TL;DR: In this article, the problem of absorption of microwaves by molecular oxygen in the atmosphere is treated by means of a first-order approximation to the impact theory of overlapping spectral lines.
Abstract: The problem of absorption of microwaves by molecular oxygen in the atmosphere is treated by means of a first-order approximation to the impact theory of overlapping spectral lines. By including only the coupling between adjacent rotational states in molecular collisions, we have devised a simple approximate method for computing the interference between lines from measurements on the resolved lines. The need for an empirically determined function describing the linewidth/ pressure ratio is eliminated. Comparisons with measurements at atmospheric pressures show that the first-order interference accounts for the low absorption at the band wings near 1 atm pressure. It also predicts the correct amount of asymmetry between high and low frequency wings. Improvement over previous models for the pressure broadening is obtained at frequencies \gsim 55 GHz. This approach is not specific to oxygen and could be adapted to other similar molecules.
469 citations
TL;DR: The theoretical study of UHF radio communication in coal mines, with particular reference to the rate of loss of signal strength along a tunnel, and from one tunnel to another around a corner is the concern of as discussed by the authors.
Abstract: The theoretical study of UHF radio communication in coal mines, with particular reference to the rate of loss of signal strength along a tunnel, and from one tunnel to another around a corner is the concern of this paper. Of prime interest are the nature of the propagation mechanism and the prediction of the radio frequency that propagates with the smallest loss. The theoretical results are compared with published measurements. This work was part of an investigation of new ways to reach and extend two-way communications to the key individuals who are highly mobile within the sections and haulageways of coal mines.
393 citations
Journal Article•
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TL;DR: In this article, the authors reviewed the radiation power factor of a small antenna in terms of its effective volume, which is proportional to the effective volume of the antenna's antenna array.
Abstract: A small antenna is one whose size is a small fraction of the wavelength. It is a capacitor or inductor, and it is tuned to resonance by a reactor of opposite kind. Its bandwidth of impedance matching is subject to a fundamental limitation measured by its "radiation power factor" which is proportional to its "effective volume". These principles are reviewed in the light of a quarter-century of experience. They are related to various practical configurations, including flush radiators for mounting on aircraft. Among the examples, one extreme is a small one-turn loop of wide strip, tuned by an integral capacitor. The opposite extreme is the largest antenna in the world, which is a "small antenna" in terms of its operating wavelength. In each of these extremes, the radiation power factor is much less than one percent.
366 citations
TL;DR: In this article, the standard far-field approximation to the Kirchhoff formula for the field scattered by a flat metallic plate S of arbitrary shape is given by a certain surface (double) integral.
Abstract: The standard far-field approximation to the Kirchhoff formula for the field scattered by a flat metallic plate S of arbitrary shape is given by a certain surface (double) integral. This double integral can be reduced to a line integral evaluated around the boundary of S. Moreover, if S is a polygon, this line integral can be reduced to a closed form expression involving no integrations at all. The use of such line integral representations can easily reduce the costs of numerical calculation by orders of magnitude. If the integrands are to be sampled p times per wavelength to achieve an acceptable degree of precision, and if A is the area of S , then the numerical evaluation of the double integral requires p^{2}A/\lambda^{2} functional evaluations whereas the line integral only requires p\sqrt{A/\lambda^{2}} . If S is a polygon with N vertices, then only 2N functional evaluations are required to evaluate the closed form expression with no quadrature error at all.
329 citations
TL;DR: In this article, a novel approach for systematically deriving the complex poles and residues of a structure from a set of time-domain data is presented, based on Prony's algorithm which involves the inversion of two matrices and a solution of the zeros of an n th degree polynomial, n being the number of desired poles.
Abstract: A novel approach for systematically deriving the complex poles and residues of a structure from a set of time-domain data is presented. The method is based on Prony's algorithm which involves the inversion of two matrices and a solution of the zeros of an n th degree polynomial, n being the number of desired poles. Two numerical examples are presented and several numerical advantages of this method are discussed.
292 citations
TL;DR: In this paper, a technique for combining moment methods with the geometrical theory of diffraction (GTD) is presented, which permits the application of the method of moments to a larger class of problems.
Abstract: A technique for combining moment methods with the geometrical theory of diffraction (GTD) is presented, which permits the application of the method of moments to a larger class of problems. The fundamental idea used to develop the hybrid technique is to modify the usual impedance matrix that characterizes, for example, a wire antenna such that a metallic body or discontinuity on that body is properly accounted for. It is shown in general that one can modify the impedance matrix for any basis and/or weighting functions if one can compute the correct modification to the impedance matrix element. The modification is readily accomplished using the geometrical theory of diffraction and/or geometrical optics. Several example problems are considered to illustrate the usefulness of the technique. First, the canonical problem of a monopole near a conducting wedge is investigated. Second, a monopole at the center of a four-sided and an eight-sided flat plate is considered. Impedance results for the latter case are in good agreement with measurements. Third, a monopole at the center of a circular disc is examined and compared with experimental measurements in the literature, and fourth, the problem of a monopole near a conducting step is solved and the dependence of the input impedance upon the step height shown.
220 citations
TL;DR: The Wheeler method and the Q method are used to find the efficiency of electrically small multiturn loop antennas and are applicable at VHF and frequencies below where the design of an antenna range or anechoic chamber becomes increasingly difficult and expensive.
Abstract: Two methods for measuring antenna efficiency are described. The two methods, referred to as the Wheeler method and the Q method, are used to find the efficiency of electrically small multiturn loop antennas. The principal advantage of both methods is that they can be quickly and easily applied. Further, both methods relate the antenna efficiency to the input impedance rather than a far-field pattern integration. Thus, the methods are applicable at VHF and frequencies below where the design of an antenna range or anechoic chamber becomes increasingly difficult and expensive.
191 citations
TL;DR: In this article, an approximate equivalence between an impedance loaded surface and a thin dielectric shell is given, which is used to compute the backscattering from a thin circular dielectron and the results are compared to the exact solution.
Abstract: An approximate equivalence between an impedance loaded surface and a thin dielectric shell is given. This approximation is used to compute the backscattering from a thin circular dielectric tube and the results are compared to the exact solution. Computations for backscattering from a thin dielectric cone-sphere and resonant wire loop inside of a thin dielectric cylinder are also given as further illustrations of the method.
186 citations
TL;DR: In this article, the problem of scattering of a plane wave by an infinite periodic array of thin conductors on a dielectric slab is formulated, and numerical results are given as well as a comparison of this theory with experimental data.
Abstract: The solution to the problem of scattering of a plane wave by an infinite periodic array of thin conductors on a dielectric slab is formulated in this paper. Numerical results are given as well as a comparison of this theory with experimental data.
165 citations
TL;DR: In this article, the distribution of average excess delay and delay spread for Gaussian wide-sense stationary uncorrelated scattering channels associated with 100 small scale areas at different locations within a 2 \times 2.5 -km region of New York City are presented.
Abstract: Distributions of average excess delay and delay spread for Gaussian wide-sense stationary uncorrelated scattering channels associated with 100 small scale areas at different locations within a 2 \times 2.5 -km region of New York City are presented. For delay spread the mean is 1.3 \mu s, the standard deviation is 0.6 \mu s, the maximum value observed is 3\frac{1}{2}\mu s, and 10 percent of the areas exceeded 2\frac{1}{2} \mu s; for average excess delay the mean is 1.1 \mu s, the standard deviation is 0.9 \mu s, the maximum is 4 \mu s, and 10 percent of the areas exceeded 2 \mu s. The region is representative of the heavily built-up areas of many large cities in the United States.
163 citations
TL;DR: In this paper, the mutual impedance of nonplanar-skew sinusoidal dipoles is presented rigorously as a summation of several exponential integrals with complex arguments.
Abstract: The mutual impedance of nonplanar-skew sinusoidal dipoles is presented rigorously as a summation of several exponential integrals with complex arguments.
TL;DR: In this article, an analytical method was developed for the maximization of the directivity of a Yagi-Uda array by adjusting the lengths of the dipole elements, where the effects of a finite dipole radius and the mutual coupling between the elements were taken into consideration.
Abstract: An analytical method is developed for the maximization of the directivity of a Yagi-Uda array by adjusting the lengths of the dipole elements. The effects of a finite dipole radius and the mutual coupling between the elements are taken into consideration. Currents in the array elements are approximated by three-term expansions with complex coefficients that convert the governing integral equations into matrix equations. Array directivity is maximized by a perturbation procedure that adjusts the lengths of all array elements simultaneously and that converges very rapidly. This method can be combined with the previously developed spacing-perturbation method to form a double-perturbation procedure and obtain a Yagi-Uda array of nonuniformly spaced elements of unequal lengths. which yields a maximum directivity.
TL;DR: In this paper, it was shown that shaped-conical radiation patterns can be realized by extending the resonant fractional-turn quadrifilar helix to an integral number of turns, i.e., the radiated energy can be concentrated into a cone with the gain decreasing from a maximum at the edge of the cone to a local minimum at the center.
Abstract: It is shown that shaped-conical radiation patterns can be realized by extending the resonant fractional-turn quadrifilar helix to an integral number of turns, i.e., the radiated energy can be concentrated into a cone with the gain decreasing from a maximum at the edge of the cone to a local minimum at the center. This pattern shape is near optimum for many spacecraft communications and navigation applications, providing approximately 3 dB improvement over cardioid shaped patterns.
TL;DR: In this article, the theoretical scattering solution for a plane wave incident normally on a lossy dielectric multilayer circular cylinder of infinite length is outlined, and numerical values of the modal scattering coefficient for TE and TM modes are given for several single and multi-layer cylinders.
Abstract: The theoretical scattering solution for a plane wave incident normally on a lossy dielectric multilayer circular cylinder of infinite length is outlined. Numerical values of the modal scattering coefficient for TE and TM modes are given for several single and multilayer cylinders. Verifications of the results are described. The values may serve as reference data.
TL;DR: Several numerical schemes for solving Pocklington's and Hallen's equations for thin-wire scatterers are investigated in this article, and convergence rates of solutions obtained from seven methods are given and reasons for different rates are delineated.
Abstract: Several numerical schemes for solving Pocklington's and Hallen's equations for thin-wire scatterers are investigated. Convergence rates of solutions obtained from seven methods are given and reasons for different rates are delineated.
TL;DR: In this paper, a new method for detection and discrimination of radar targets is described, where the gross structure of a radar target can be identified from scattered fields of the target at harmonic radar frequencies located just in the low resonance region.
Abstract: A new method for detection and discrimination of radar targets is described. The basis for this method is that the gross structure of a radar target can be identified from scattered fields of the target at harmonic radar frequencies located just in the low resonance region. This is in sharp contrast to many target signature schemes that operate at much higher frequencies and observe many of the details of the target in lieu of its gross features. Multiple frequency radar scattering data and the complex natural resonant frequencies of radar targets are integrated into a predictor-correlator processor. The method is illustrated using as target models both classical shapes and thin-wire configurations of simple geometry. Integral equation programs are utilized to calculate multiple frequency backscatter data for the wire geometries and to deduce the complex natural resonant frequencies of the wire structures. Discrete multiple frequency radar scattering data spanning a particular spectral range are shown to be desirable for optimum capability but discrimination and detection can be achieved using just two near-conventional radars, even if the radars are located at different sites and hence view the target from different aspects.
TL;DR: In this article, active microwave measurements of vegetation backscatter were conducted to determine the utility of radar in mapping soil moisture through vegetation and mapping crop types using a truck-mounted boom, spectral response data were obtained for four crop types (corn, milo, soybeans, and alfalfa) over the 4-8 GHz frequency band, at incidence angles of 0\deg-70\deg in 10deg steps, and for all four linear polarization combinations.
Abstract: Active microwave measurements of vegetation backscatter were conducted to determine the utility of radar in 1) mapping soil moisture through vegetation and 2) mapping crop types Using a truck-mounted boom, spectral response data were obtained for four crop types (corn, milo, soybeans, and alfalfa) over the 4-8 GHz frequency band, at incidence angles of 0\deg-70\deg in 10\deg steps, and for all four linear polarization combinations Based on a total of 125 data sets covering a wide range of soil moisture content, system design criteria are proposed for each of the aforementioned objectives Quantitative soil moisture determination was best achieved at the lower frequency end of the 4-8 GHz band using HH polarized waves in the 5\deg-15\deg incidence angle range A combination of low and high frequency measurements are suggested for classifying crop types For crop discrimination, a dual-frequency dual-polarization (VV and cross) system operating at incidence angles above 40\deg is suggested
TL;DR: In this article, a piecewise-sinusoidal reaction technique is developed for scattering and radiation from perfectly conducting bodies of arbitrary shape, and the theory and numerical results for scattering patterns of rectangular plates and radiation patterns of corner-reflector antennas are presented.
Abstract: A piecewise-sinusoidal reaction technique is developed for scattering and radiation from perfectly conducting bodies of arbitrary shape. This paper presents the theory and numerical results for scattering patterns of rectangular plates and radiation patterns of corner-reflector antennas. In all cases, experimental measurements are included for comparison with the calculated data.
TL;DR: Numerical methods for antenna pattern synthesis are developed for point sources arbitrarily distributed in a plane, and to pattern synthesis in this plane, using the methods of finite dimensional vector spaces.
Abstract: Some general numerical methods for antenna pattern synthesis, with and without constraints, are developed. Particular cases considered are 1) field pattern specified in amplitude and phase, 2) field pattern specified in amplitude only, and 3) these two cases with a constraint on the source norm. Both the source and the field are discretized at the beginning, and the methods of finite dimensional vector spaces are used for the computations. The theory is general, but is applied only to point sources arbitrarily distributed in a plane, and to pattern synthesis in this plane. Some numerical examples are given for ten sources approximately equispaced on one-half of an ellipse, with the desired field pattern chosen to be the csc \phi pattern in the first quadrant.
TL;DR: In this article, the propagation and radiation of electromagnetic waves from oscillating sources in a suddenly created plasma are studied in detail and field expressions are derived through the use of Laplace transformations.
Abstract: Propagation and radiation of electromagnetic waves from oscillating sources in a suddenly created plasma are studied in this investigation. Field expressions are derived through the use of Laplace transformations. The spatial distribution of sources is taken to be arbitrary but confined. Two cases are considered in detail: 1) plane wave propagation in a source-free region and 2) electric point dipole radiation. In the case of plane wave propagation, various aspects such as wave split, frequency shift, phase and group velocities, amplitude changes, power flows, and energy relations are discussed. In the case of electric dipole radiation, the electromagnetic fields and instantaneous radiated power are calculated and expressed in terms of Lommel functions of two variables. Asymptotic expressions and graphical results of numerical calculations of these quantities are presented. Many interesting properties of the spherical waves and power radiation are discussed.
TL;DR: In this article, two simple mathematical models are described for the prediction of the vector radiation fields from offset parabolic reflector antennas with offset feeds, and experimental support for the predictions obtained from the models has been obtained by comparisons with measured data from antenna systems operating at 30 GHz.
Abstract: Two simple mathematical models are described for the prediction of the vector radiation fields from offset parabolic reflector antennas with offset feeds. Experimental support for the predictions obtained from the models has been obtained by comparisons with measured data from antenna systems operating at 30 GHz. The principal radiation characteristics of the offset-reflector with offset-feed configuration are discussed.
TL;DR: In this article, the impedance of a simple open slot antenna was measured as a function of frequency and was then compared with the theoretical values obtained through Babinet's principle from the known values of the admittance of its complementary antenna, the strip dipole.
Abstract: The impedance of a simple open slot antenna was measured as a function of frequency and was then compared with the theoretical values obtained through Babinet's principle from the known values of the admittance of its complementary antenna, the strip dipole. A careful experimental study was then made of the impedance of the same slot with several different cavity backings. The impedance was found both as a function of frequency and of the various parameters of the cavity, namely, the cross-sectional size of the cavity, the medium filling the cavity, and the type of termination of the cavity.
TL;DR: In this paper, an equation for the deviations from homology if an antenna is adjusted at zenith angle \theta and observes at angle \phi is presented, and the best adjustment angle is discussed; this is basically different for both types of mount.
Abstract: An ideal telescope structure would deform, when tilted, in a homologous way, from one paraboloid of revolution to another one. Conventionally designed telescopes approach this condition already to some degree, special designs to a very high degree, and a measure for the degree of this approach is suggested. An equation is presented for the deviations from homology if an antenna is adjusted at zenith angle \theta and observes at angle \phi ; it contains only two structural parameters for alt-azimuth mounts (three for polar mounts). The choice of the best adjustment angle is discussed; this is basically different for both types of mount, and examples are given. Supported at two elevation bearings, conventional telescopes will generally show a strong gravitational astigmatism which may be corrected at the secondary mirror, thus improving the astronomical performance considerably. Two possible correction methods are suggested. Several of the equations presented are checked with the example of the 140-ft telescope at Green Bank, W. Va. Good agreement is obtained between detailed computer analysis and equations, and between analysis and astronomically obtained data. Suggested improvements for the 140-ft (applicable also to other telescopes) would diminish the deviations from homology by a factor of 2.5, and the remaining residuals then are brought below the internal inaccuracy of the surface panels. The astigmatic correction thus is advisable as well as sufficient.
TL;DR: In this article, a design method for the creation of line sources that will yield antenna patterns consisting of a narrow main beam and low sidelobes, with the sidelobe levels different on the two sides of the main beam.
Abstract: A design method is described for the creation of line sources that will yield antenna patterns consisting of a narrow main beam and low sidelobes, with the sidelobe levels different on the two sides of the main beam. The requisite aperture distribution is shown to be symmetrical in amplitude and asymmetrical in phase. Main beam offset and the characteristics of the associated difference pattern are discussed. The boresight error voltage curve is also displayed. Incremental gain resulting from asymmetric sidelobes is analyzed and data is given for typical cases. Experimental confirmation is, presented.
TL;DR: In this article, a parametric analysis of coherent electromagnetic losses due to scattering from volume weak inhomogeneities is presented, and it is shown that the loss could be appreciable in a resonance region and thus play a more dominant role than conductivity losses.
Abstract: A parametric analysis of the coherent electromagnetic losses due to scattering from volume weak inhomogeneities is reported. It is shown that even in the case of very weak inhomogeneities, the loss could be appreciable in a resonance region and thus play a more dominant role than conductivity losses.
TL;DR: In this article, the reflection of electromagnetic waves from a smooth conducting surface at high frequencies is studied and the incident and reflected fields are expanded in asymptotic series of the form E = exp [iks(r)]\Sigma\min{m=0}max{\infty}(ik)^{-m}e_{m}(r), where s is the phase function, and e is the amplitude vector.
Abstract: The reflection of electromagnetic waves from a smooth conducting surface at high frequencies is studied in this paper. Both the incident and reflected fields are expanded in asymptotic series of the form E = exp [iks(r)]\Sigma\min{m=0}\max{\infty}(ik)^{-m}e_{m}(r) , where s is the phase function, and { e_{m} } are amplitude vectors. Explicit formulas based on a ray technique are given for calculating the first two orders of the electric field, magnetic field, and surface current. When the conducting surface is a paraboloid (or a sphere) and the incident field is a plane wave in its axial direction, our solution recovers the exact solution (or the first two orders of the exact asymptotic solution), As a special case, our result is compared with the work of Schensted. It appears that the latter is only partially correct.
TL;DR: In this paper, the potential integral of thin-wire theory at the singularity of the exact kernel in cases where the current is representable as piecewise constant and/or piecewise linear over an interval containing a singularity is calculated.
Abstract: A technique is presented for calculating the potential integral of thin-wire theory at the singularity of the exact kernel in cases where the current is representable as piecewise constant and/or piecewise linear over an interval containing the singularity. The integral of the rapidly varying portion of the kernel is converted to a highly convergent power series. Examples of how one employs the results in computations are given.
TL;DR: In this article, the results of experimental studies on the backscattering properties of corn, milo, soybeans, and alfalfa were presented, and the measurements were made during the summer of 1973 over the 8-18 GHz frequency band.
Abstract: The results of experimental studies on the backscattering properties of corn, milo, soybeans, and alfalfa are presented. The measurements were made during the summer of 1973 over the 8-18 GHz frequency band. The data indicate that soil moisture estimation is best accomplished at incidence angles near nadir with lower frequencies while crop discrimination is best accomplished using two frequencies at incidence angles ranging from 30\deg to 65\deg . It is also shown that temporal plant morphology variations can cause extreme variations in the values of the scattering coefficients. These morphological changes can be caused by growth, heavy rain, and in the case of alfalfa, harvesting.
TL;DR: In this article, a new slope diffraction function for the half plane is presented along with applications, which is valid through the shadow region and is applied to find the far-fields for a source on the surface of a conducting wedge.
Abstract: The first order geometrical theory of diffraction (GTD) predicts vanishing fields along the surface of a conducting wedge for the incident electric field polarized parallel to the diffracting edge The slope diffraction coefficient is a valid correction term for incidence angles removed from the shadow boundary A new slope diffraction function for the half plane is presented along with applications This new form of slope diffraction coefficient for the half plane is valid through the shadow region Reciprocity is invoked to find the far-fields for a source on the surface of the conducting wedge In addition to applying the two-dimensional slope diffraction analysis to practical problems, the equivalent current concepts have been extended to include equivalent slope currents for the analysis of either finite or curved edges This new form of the slope diffraction function has been successfully used to provide an H -plane horn pattern analysis that is considerably less tedious than previously possible with GTD Both pure GTD solutions and hybrid solutions using conventional aperture integration for the main beam region and GTD for the far-out side and back lobes are compared with experimental results
TL;DR: In this paper, the effects of rain on transmission through large radomes of the type which may be used with ground station antennas in satellite links were investigated. But the results showed that the nonwetting nature of the radome surface significantly degraded with time, and this was accompanied by a large increase in the 20 GHz transmission loss during periods of rain.
Abstract: An experiment has been designed to study the effects of rain on transmission through large radomes of the type which may be used with ground station antennas in satellite links. Measured transmission data from the experiment are described and compared with predicted values. The nonwetting nature of the radome surface significantly degraded with time, and this was accompanied by a large increase in the 20-GHz transmission loss during periods of rain. The losses were in the range of 6-8 dB for rain rates of 3-40 mm/h. The cumulative distribution of transmission through the radome for a 12-month period is presented.