Showing papers in "IEEE Transactions on Antennas and Propagation in 1961"

On the guided propagation of electromagnetic wave beams

Abstract: Any field in a half-space can be described by a continuous spectrum of cylindrical waves. If this spectrum comprises substantially only waves whose propagation constant is very close to the plane wave propagation constant, the field can be resolved into a set of elementary wave beams which are characterized by Laguerre polynomials. They satisfy orthogonality relations like the wave modes in a waveguide. The elementary beams or "beam modes" can be reiterated and guided by reconstituting the cross-sectional phase distribution at certain intervals. Reiterative beams are utilized in the beam waveguide. The finite size of the phase resetting devices effects a modification of the reiterative beam modes and causes diffraction losses. These losses decrease very rapidly with increasing diameter of the phasing devices.

Sidelobe reduction by nonuniform element spacing

Abstract: A perturbational procedure for reducing the sidelobe level of discrete linear arrays with uniform amplitude excitation by using nonuniform element spacing is presented. The calculation of the required element spacings is quite simple. The method can reduce the sidelobe level to about 2/N times the field intensity of the main lobe, where N is the total number of elements, without increasing the beamwidth of the main lobe. Several examples are given.

Some remarks on green's dyadic for infinite space

Abstract: The validity of the often used dyadic -(\jmath + \frac{1}{k^{2}} abla abla )\frac{e^{-jkR}{4\piR} to compute the electric field inside a current-carrying region is investigated. It is found that care must be exercised in the definition of the integrals, which should be taken as principal values around the field point.

Microwave antennas derived from the cassegrain telescope

Abstract: A microwave antenna can be designed in the form of two reflecting dishes and a feed, based on the principle of the Cassegrain optical telescope. There are a variety of shapes and sizes available, all described by the same set of equations. The essential performance of a Cassegrain double-reflector system may be easily analyzed by means of the equivalent-parabola single-reflector concept. Techniques are available for reducing the aperture blocking by the sub dish of the Cassegraln system: one method minimizes the blocking by optimizing the geometry of the feed and sub dish; other methods avoid the blocking by means of polarization-twisting schemes. The former method yields good performance in a simple Cassegrain antenna when the beamwidth is about 1\deg or less. The latter methods are available for any application not requiring polarization diversity, and an optimized set of polarization-operative surfaces has been developed for these twisting Cassegrain antennas. Experimental results, presented for practical antennas of both types, illustrate the feasibility of these principles. A number of unusual benefits have been obtained in the various Cassegrain antenna designs, and additional interesting features remain to be exploited.

Multiple beams from linear arrays

Abstract: The problem of devising a passive RF transmission line feed system to provide independent multiple outputs from a linear array is considered. It is shown that a lossless, matched feed network is possible only for uniform aperture distribution. The general feed system for connecting 2^{n} inputs to 2^{n} elements is shown to consist of conventional hybrid junctions with associated phase shifters. In order to increase the possible number of elements in the array, the problem of finding applicable junctions more complex than the hybrid is considered. Junctions with three inputs and three outputs and with four inputs and four outputs are derived for use in multiple feed networks, expanding the number of elements in the array to 2^{l}3^{m}4^{n} .

Coherent and incoherent scattering of microwaves from the ocean

Abstract: This report, the third in a series,1,2summarizes experimental studies of microwave over-ocean propagation on line-of-sight paths. Measurements at 5.3, 3.2, and 0.86 cm wavelength in 1955, in the Gulf of Mexico, between two oil drilling platforms one mile apart are compared with the results of the phenomenological model developed earlier. The total field was measured at maxima, and minima of the interference patterns using a sequence of receiver beam widths. The scattered field alone was measured by means of a narrow beam. The 1955, Gulf of Mexico data, are consistent with the previous 1953, Golden Gate data, in regions of overlap of certain parameters, and provide extended coverage for other values under different conditions. Specifically, some of the new findings are: a) The experimental values of the coherent reflected field are larger than exp [ -2(2\pi h\psi /1000\lambda)^{2} ] for values of "apparent ocean roughness" ( h\psi/\lambda ) greater than 110 milliradians, h =standard deviation of water surface, \psi =grazing angle in milliradians, and \lambda electromagnetic wavelength, b) After rising from zero, the incoherent scattered power exhibits a downtrend for h\psi/\lambda greater than 110. The incoherent power divided by the square of the smooth sea reflection coefficient shows no polarization dependence, c) The distribution along the surface as measured by the narrow-beam antenna shows that the coherent and incoherent power are markedly peaked in the specular direction, d) The ratios of coherent-to-incoherent power (m2) obtained from the shapes of the probability distributions agree with the power ratios obtained by measurements of the magnitudes of the signals, e) Total signal spectra broaden with increasing h\psi/\lambda as found at the Golden Gate; the relative shapes of spectra agree with those from the Golden Gate. Horizontally- and vertically-polarized total signal spectra are the same.

A reaction theorem and its application to antenna impedance calculations

Abstract: The reaction theorem is generalized to allow the fields of an antenna in one environment to be employed in calculations of mutual impedance in another environment. Several expressions for self-impedance and mutual impedance are presented. These are in the form of surface integrals or volume integrals of the field intensities or the current density. It is shown how the fields of an antenna in free space can be useful in calculating the impedance in the presence of scatterers.

A new technique for electronic scanning

Abstract: The concept of time-modulated antennas has recently been demonstrated as a means of overcoming many of the limitations currently restricting advances in the antenna art. Of special importance is the mathematical possibility of generating a pattern complex capable of providing simultaneous scan operation. This characteristic is realized by periodic time modulation of the aperture distribution. This paper discusses the theory of simultaneous "scanning" using time modulation techniques and shows that the required pattern complex is generated by a progressive-pulse aperture excitation. The fundamental equations and relationships concerning the form of pulse excitation and "scanning" coverage are derived. In addition, practical methods of physically generating the proper pulse-excited aperture are described, and the necessary detection requirements are delineated.

A triangular arrangement of planar-array elements that reduces the number needed

Abstract: In this paper, it is shown that by arranging the elements of a beam-scanning planar antenna array in a triangular pattern rather than a rectangular pattern, the number of elements needed in the array is reduced. (The number of elements needed in an array is determined from the requirement that no spurious beams form in the array pattern.) The reduction in the number of elements depends upon the solid angle over which the main beam is positioned. If the main beam is positioned within a constant angle about the array normal, then the number of elements can be reduced by 13.4 per cent by arranging the elements in a pattern of equilateral triangles rather than in a square pattern. If the main beam is positioned within a "pyramid," centered about the array normal, then the reduction is usually less than 13.4 per cent. Graphs are included showing for both element arrangements the solid angle over which the main beam can be scanned without the formation of spurious beams.

Arbitrary polarization from annular slot plannar antennas

Abstract: This paper describes the analysis and design of a class of antennas which can radiate and receive constant-shape pencil beams with either circular sense, any linear or elliptical polarization by a simple adjustment in the feed circuit. Such radiators are called arbitrarily polarized antennas. The apertures described are located on upper plates of radial waveguides and are composed of annular slots, with each annulus consisting of a discrete number of crossed slots. The annular slots are positioned so that each arm of the crossed slots can couple by a constant factor with the radial or circumferential currents flowing over the aperture plate to produce a common instantaneous direction for the electric field in each slot pair Both standing-wave and traveling-wave array types are employed. The standing-wave array requires only one radial waveguide mode for its operation. The traveling-wave array requires two modes and results in greater bandwidth and greater freedom in arraying many annuli. The methods used to excite the various radial waveguide modes are discussed; theoretical and experimental radiation patterns at X band are compared.

A theoretical limitation on the formation of lossless multiple beams in linear arrays

Abstract: It is well known that through the use of lenses, several independent beams can be formed from a single antenna, with each beam having essentially the gain corresponding to the aperture of the lens Recently, feed systems have been developed for linear arrays which achieve similar performance through the use of directional couplers In this paper it is shown that the shape of the beams which can be formed from an equispaced array by such a feed matrix is not arbitrary, unless one is willing to accept losses in addition to normal plumbing losses It is shown that the array space factors associated with the individual beams must be such that they are mutually orthogonal over a period of the space-factor pattern

The traveling-wave linear antenna

Abstract: It is shown experimentally that an essentially traveling-wave distribution of current can be produced on a linear antenna by inserting a resistance of suitable magnitude one-quarter wavelength from the end of the antenna. A theory for the resistively-loaded dipole antenna is formulated on the basis that the inserted resistors (one in each arm) can be replaced by equivalent generators and that the resulting triply-driven antenna can be solved by the superposition of singly- and doubly-driven dipoles. Approximately 50 per cent of the power is dissipated in these resistors. With a traveling-wave distribution of current on an antenna available, the properties of this antenna are then investigated and compared with those of the conventional linear antenna. It is found that the input impedance of the traveling-wave antenna remains essentially constant as a function of antenna length, whereas that of the conventional linear antenna varies considerably. It is also shown that the input impedance of the traveling-wave antenna varies only slightly over a 2 to 1 frequency band. The directional properties of the traveling-wave and conventional dipole are compared, and it is shown that a minor lobe does not appear in the radiation pattern of the traveling-wave dipole until it is much longer than the conventional dipole. Also, it is shown that where the directional properties of the conventional dipole are quite sensitive to a change in frequency, those of the traveling-wave dipole are not.

A solution to the frequency-independent antenna problem

Abstract: A solution of Maxwell's equations is obtained for an antenna consisting of an infinite number of equally spaced wires in the form of coplanar equiangular spirals. Radiation amplitude patterns obtained from this solution agree closely with measurements on two-element spiral antennas. The phase pattern shows the approximate validity of a phase center at a distance behind the antenna which decreases with the tightness of the spiral. The current distribution clearly shows increased attenuation with increase in the tightness of the spiral, thus showing how the frequency-independent mode depends on the curvature. A remarkable feature of the solution is that the current consists of an inward traveling wave at infinity when the antenna is excited in that sense which produces an outward wave at the center.

Angular accuracy of a phased array radar

Abstract: One type of phased-array radar of current interest employs an array of separate receiving elements, each followed by an individual amplifier. These individual signals are combined coherently to form one or more receiving beams for searching, tracking, or performing both functions simultaneously. This paper presents an approach to the theory of angle measurement with a phased array of this type. In the one-dimensional problem considered here, the receiving antenna consists of a linear array of individual antenna-amplifier elements. The receiver-noise-limited case is considered, in which accuracy is limited by the additive normally distributed noise present in each channel. An expression is derived for the limiting accuracy of angular measurement when a single set of samples is available. This set of samples is obtained simultaneously, one sample from each channel. Next, two methods of implementing the angular measurements are discussed. These are amplitude comparison monopulse and a coherent or phase comparison technique. For large signal-to-noise ratios and for either a square law or a linear envelope detector, the accuracy of amplitude comparison monopulse approaches the theoretical limit. The same accuracy can be achieved with the coherent technique by proper weighting of the individual signals.

Experimental studies on a beam waveguide for millimeter waves

Abstract: The beam waveguide utilizes reiterative wave beams which are guided by resetting the cross-sectional phase distributions at periodic intervals. The major purpose of the measurements was to obtain data concerning the inherent losses of the guide, including the diffraction loss, caused by energy by-passing the phase transformers, and the dielectric and reflection losses of the transformers. Resonance measurements with an "open cavity," consisting of a section of a beam waveguide terminated by plane reflector walls, yielded the over-all losses of the phase transformers. In order to isolate the diffraction loss, pulse measurements were made between two reflectors, slightly curved to reset the phase distribution in the reflected beam. These measurements also gave information on the build-up of the reiterative wave beam. The experimental data support the theory which predicts that the diffraction losses of reiterative beams can be made extremely small.

New circular-polarized frequency-independent antennas with conical beam or omnidirectional patterns

Abstract: A conical beam may be obtained from balanced equiangular spiral antennas by constructing an antenna with more than two spiral arms and symmetrically connecting these arms to provide a suppression of the radiated fields on the axis of the antenna. The angle of this conical beam can be controlled and, with proper choice of parameters, confined to the immediate vicinity of the azimuthal ( \theta = 90\deg ) plane. An antenna with four symmetrically spaced arms can provide a radiation pattern that is within 3 db of omnidirectional circularly polarized coverage in the azimuthal plane. The standing-wave ratio of this antenna referred to a 50-ohm coaxial cable is less than 2-to-1 over the pattern bandwidth. This four-arm version retains the wide frequency bandwidths of the basic conical log-spiral antenna, and it provides a coverage which heretofore has been difficult to obtain even with narrow-band antennas.

The design and capabilities of an ionospheric radar probe

Abstract: Staff members of the Cornell University Center for Radiophysics and Space Research have designed an ionospheric radar probe to be located near Arecibo, Puerto Rico. The radar will have the following general specifications: 1) Antenna reflector, 1000-foot-diameter spherical bowl, illuminated by a 430-Mc dual-polarized feed. 2) Transmitter of 2.5 Mw peak, 150 kw average power, or 100 kw CW power. 3) Dual-channel receiver, capable of measuring total power, polarization and received spectrum. The radar will initially be used to measure the variation of electron density with height, the fluctuations of electron density at fixed heights and electron temperatures and magnetic field strengths at various heights. Ionospheric drifts may also be measured. The radar will also he able to obtain echoes from planets, information of the moon's surface and possibly echoes from the sun. Hydromagnetic shocks may also be detected and a study of cislunar ionization can be made. The passive system with the large antenna may be used as an instrument in radio astronomy to observe radio emission from planets and from true stars, and to make a survey of radio sources. With additional facilities, many radio astronomy measurements can be made taking advantage of the large antenna aperture and resulting high resolving power.

ELF waves in the presence of exponential ionospheric conductivity profiles

Abstract: Based on the theory of Nicolet and Ailkin, the profile of ionospheric conductivity exhibits a nearly exponential variation with altitude through the D layer and the lower edge of the E layer. Propagation of ELF waves below this exponential layer is considered after calculating the surface impedance Z_{s} at an altitude h where the local refractive index n does not necessarily satisfy |n|\gg1 . The propagation constant is determined by an iteration process. This model of an isotropic nonhomogeneous ionosphere is a closer approximation of the propagation geometry than are earlier models, where Z_{s} was defined at h where |n|gg1 . The present model accounts simultaneously for ELF attenuation rates as measured by Jean, and earth-ionosphere cavity resonances as observed by Balser and Wagner.

Scattering by a periodically apertured conducting screen

Abstract: This paper presents a generalized variational method for calculation of equivalent-circuit parameters or scattering coefficients of any thin obstacle. The equivalent susceptance is obtained from an eigenvalue of a general-energy operator. The method is applied here to the scattering of a plane wave by a conducting screen containing a doubly-periodic array of square apertures. Previous variational formulations of analogous waveguide problems were largely restricted to cases in which only the dominant mode can propagate. In applying these methods to scattering problems, the ratio of aperture spacing to wavelength had to be restricted to less than unity at normal incidence so that no additional propagating modes were excited. The formulation presented here can be applied, however, for any number of propagating modes. Numerical results are presented for scattering by a periodically apertured screen, giving scattering coefficients as a function of wave number of the incident wave. The distribution of energy into higher-order diffraction lobes is evaluated.

Folded dipoles and loops

Abstract: In Section I the theory of linear arrays consisting of two or more closely spaced elements that are interconnected by lumped reactances is reviewed. Specific application is made to two-element end-loaded folded dipoles and monopoles constructed of conductors with different diameters, to series tuned three-wire folded dipoles and monopoles, and to a three-wire-line reactor and impedance transformer. In Section II the circular folded dipole or Halo antenna is treated.

Leaky wave antennas II: Circular waveguides

Abstract: The propagation characteristics of leaky waves on slotted circular cylinders are derived by the methods described in an earlier paper on leaky rectangular waveguides. The cross section of a typical leaky circular wavegaide is represented by a radial transmission line network incorporating a lumped terminating admittance which characterizes the slot discontinuity and the external region. The expression for the lumped admittance is obtained by combining an integral equation solution with a variational procedure. The resonances of this transverse network, which yield the complex propagation constants for the leaky waves, are solved by perturbation techniques to produce results in simple and practical form. Solutions are obtained for leaky waves corresponding to two different excitations of a slotted cylinder and these results are shown to compare favorably with measured values.

The University of Illinois radio telescope

Abstract: The University of Illinois radio telescope is a reflector in the shape of a parabolic cylinder whose aperture is 400 feet \times 600 feet. A 425-foot-long phase-adjustable array of receiving antennas lies along the focal line and produces a pencil beam one-third degree in width, steerable in the meridian plane up to 30 degrees in either direction from the zenith. The array was designed by means of a novel procedure using both variable spacing and variable excitation to produce a prescribed beamwidth. The reflector is built of earth, utilizing a natural ravine. The purpose of the instrument is to compile a catalog of faint extragalactic radio sources.

Optimum feeds for all three modes of a monopulse antenna II: Practice

Abstract: In an amplitude-monopulse antenna whose main aperture is illuminated by a feed, it is desired to control the feed excitation independently in the sum, azimuth difference, and elevation difference modes. It is possible to describe a feed system, comprising an infinite array of radiators and hybrid junctions, which demonstrates the principle of complete independent control of the three modes. There are several practical feeds which approach a hypothetical ideal one in varying degrees. One representative type is the "twelve-horn feed;" although it has some disadvantages, it is versatile and quite useful. However the most attractive type for many applications appears to be one having a combination of multiple-horn excitation in one plane and multimode-waveguide excitation in the other. With a simple form of the "multihorn-multimode" feed, the ideal is substantially realized, and a major improvement in monopulse performance can be obtained.

Tolerance theory of large antennas

Abstract: The design of an antenna calls for definite amplitudes and phases of the currents, but when the antenna has been constructed and adjusted, there will be departures from the design currents because of several factors. The customary procedure of taking radiation patterns and making the final adjustments semi-empirically has usually been satisfactory, but two difficulties have been setting in with the trend towards large antennas of high gain. First, it is impossible to measure the radiation pattern of the largest existing antennas; even the determination of single sections through the pattern or the gain in one direction presents difficulty. Second, the adjustments themselves are more laborious on larger antennas. It is therefore very desirable that the theory of antenna tolerances should be pursued so that the effect of departures can be taken into account, statistically or otherwise, during the design. This paper considers the effects of systematic and random errors on the radiation pattern of antennas representable by a field distribution over an aperture, such as paraboloidal reflectors and large arrays of small elements. In the case of paraboloids, the deterioration in directivity is found to depend on the mean square departure of the surface from the paraboloid of best weighted least-squares fit and on the two-dimensional autocorrelation function of the departure. The variation of directivity with wavelength of a particular paraboloid is deduced by leaving out of account those two-dimensional Fourier components of the departure with spatial periods less than a wavelength. Practical steps are considered for unifying testing, adjusting, and design so as to lead to the greatest relaxation of the mechanical tolerances imposed on construction.

Phase adjustment of large antennas

Abstract: A technique is described for adjustment of phase paths within large antenna arrays or paraboloidal surfaces which are now in use, or are planned, for radio astronomy. After large paraboloids have been constructed, they suffer distortions which are very difficult to investigate and for which photogrammetry, millimeterwave radar and optical survey have been suggested. A new suggestion, based on experiment at Stanford with phase measurement of long paths, is to place modulated gas discharge tubes, acting as scatterers, at various points on the paraboloidal surface and to monitor the phase path from a signal generator through the feed at the focus to each discharge tube in turn, and back. By means of a second probe, say a dipole situated at the vertex of the paraboloid, it is possible to triangulate on deflections. The feasibility of this scheme has been established in connection with the large Stanford cross antenna which has an aperture of 1339 wavelengths at 9.1 cm. The phase of the modulated reflected wave produced by the discharge tube is determined by adding it to a reference continuous wave of large amplitude and applying the resultant to a receiver sensitive to the modulating frequency. A null is obtained when the two waves are in quadrature. The coherent detection system allows measurement of the phase of the modulated reflection even when its amplitude is below -130 dbm. Using a 10-mw S -band signal generator, no difficulty was found in detecting the reflection from a small discharge tube placed 100 feet away from a 3 by 4 inch horn, which is sufficient range for applying the method to large paraboloids.

Some propagation characteristics of high UHF signals in the immediate vicinity of trees

Abstract: Results are reported of measurements made at very low angles of 2880-Mc vertically-polarized signals over wooded areas, with the elevation angle to the transmitter the principal variable. The effects of one tree and of many trees on the apparent location of a signal source, as determined with a narrow-beam antenna, are reported. A hypothetical direction-finding system is assumed and its pointing characteristics determined.

A new way of solving Maxwell's equations

Abstract: A general solution of Maxwell's equations for a single frequency can be expressed as the combination of two types of solution, each of which is characterized by an electric vector which is equal to the magnetic vector times the intrinsic impedance of free space but a quarter cycle out of phase. These two types are mathematically orthogonal, and each can be conveniently expressed by means of a single scalar. The method leads to circularly polarized surface waves propagating along anisotropic sheets which are perfectly conducting in one tangential direction and perfectly transparent in the orthogonal tangential direction.

The Stanford microwave spectroheliograph antenna, a microsteradian pencil beam interferometer

Abstract: A pencil beam interferometer has been constructed at Stanford, Calif., with multiple beams of 3.1 minutes of arc width to half power (0.8 microsteradian). It is composed of two equatorially-mounted, 16-element, Christiansen arrays of 3-m paraboloids, each 375 feet long (1255 wavelengths at a wavelength of 9.1 cm). The half power beamwidth of the fan beam of a single array is 2.3 minutes of arc. To form the pencil beam, the two arrays are switched together as in a Mills cross. Frequency range is from 2700 to 3350 Mc. Phase adjustment and monitoring are handled by a new technique of modulated, weakly reflecting gas-discharges maintained at the focus of the paraboloids. Television-type scanning yields maps of the sun (spectroheliograms) revealing fine details of the microwave source regions in the chromosphere and corona. All the transient bursts and a large fraction of the steady solar emission at 9.1 cm prove to originate in a small number of highly compact centers, whose brightness temperatures may exceed 5 /times 10^{5}/deg K. The sensitivity of the instrmnent also allows the thermal emission from the moon ( 250/deg K) and a number of galactic and extragalactic sources to be studied with high angular resolution. Illumination of the moon by terrestrial radar can be detected. The pencil beam interferometer furnishes the finest beams currently available from pencil beam antennas of any type. Examination of the fundamentals of extracting high resolution details of a source from its radiation field indicates the fitness of pencil beam interferometers, incorporating steerable multielement arrays, for future development to higher resolving power. Adequate technique of phase preservation over wide spacings is available.

Optimum feeds for all three modes of a monopulse antenna I: Theory

Abstract: In a monopulse antenna for use in a tracking radar, the requirements for the sum and difference modes are not the same. For the amplitude-comparison type of monopulse having an antenna whose main aperture is illuminated by a feed, these independent requirements can be met by a feed designed to fulfill two conditions. One is that excitation of the feed aperture in the difference modes be effectively about twice as wide as in the sum mode. The other is that the shapes of all the feed excitations be free of any avoidable irregularities. The amount of improvement available when these conditions are met depends on the optimization point desired for each mode, as well as the design of the reference antenna. In a typical case, the gains and slopes in the difference modes should increase by several db. Furthermore, the near sidelobes and the spillover radiation in the difference modes should decrease by an order of magnitude. A small increase of gain in the sum mode is also available, and finally, the criticalness of positioning the feed may be appreciably reduced.