Beam diffraction by planar and parabolic reflectors

In the complex source point technique, an omnidirectional source diffraction solution becomes that for a directive beam when the coordinates of the source position are given appropriate complex values. This is applied to include feed directivity in reflector edge diffraction. Solutions and numerical examples for planar strip and parabolic cylinder reflectors are given, including an offset parabolic reflector. The main beams of parabolic reflectors are calculated by aperture integration and the edge diffracted fields by uniform diffraction theory. In both cases, a complex source point feed in the near or far field of the reflector may be used in the pattern calculation, with improvements in accuracy in the lateral and spillover pattern lobes     

The design of linearly polarized slotted waveguide feeds for spherical reflectors

A procedure for the design of linearly polarized slotted waveguide line feeds for spherical reflector antennas is described. The necessary equations for determining the complex propagation constantalpha + jbetain a line source are given in terms of reflector parameters. Guide and slot dimensions that will yield the desiredalphaandbetaare then found by making use of experimental data, presented herein. These data consist of measured values ofalphaandbetaas functions of slot length for a number of slotted waveguide test sections of various widths. The technique is applicable over a wide range of frequencies and reflector sizes. An example is given in which a feed is designed at 1420 MHz to illuminate 700 ft of aperture in the Arecibo spherical reflector.     

Focalisation et pouvoir séparateur des antennes toriques multifaisceaux

Multibeam antenna installation is in project atCent Lannion. It consists of a symetrical spherical torus of a l,2m × 4 m projected aperture. Its total geosynchronous arc field of view is 60 °. Reflector and feeds have been sized to provide 3 ° orbital satellite spacing. After reminding of torus reflector operation, stringent geometrical constraints due to angular satellite spacing are reported. Solutions are provided to solve the torus focalisation problem by searching for its diffraction image.     

Factorization of the feed efficiency of paraboloids and Cassegrain antennas

The first approximation to the aperture efficiency of a paraboloidal reflector antenna is called the feed efficiency. The factorization of the feed efficiency into subefficiencies which account for losses due to spillover, cross polarization, nonuniform aperture illumination, and phase errors is considered. The relations between the radiation patterns of circularly and linearly polarized feeds are also derived.     

Coaxial feeds for high aperture efficiency and low spillover of paraboloidal reflector antennas

Coaxial feeds produce an approximate sector-shaped pattern, an almost optimum pattern of a feed for high aperture efficiency and low spillover of paraboloid antennas. Such a coaxial feed consists of a central circular waveguide which is surrounded by one or more conductors with circular cross sections. Theoretical and experimental investigations on coaxial feeds excited by H11modes have shown that the first ring yields the highest increase in the aperture efficiency of paraboloid antennas illuminated by them. Measurements performed on paraboloid antennas illuminated by a coaxial feed with only one ring yielded aperture efficiencies of 68 to 75 percent for angular apertures of the paraboloidal reflector of100degto160deg. Circularly symmetric patterns in conjunction with almost linearly polarized aperture fields can be achieved by multimode coaxial feeds. The values for the aperture efficiency, which are calculated for paraboloid antennas illuminated by multimode coaxial feeds, nearly reach the theoretical optimum. The measured values are 68 to 80 percent. In addition, the multimode feeds produce very little cross polarization.     

A comparison among 1-, 3-, and 7-horn feeds for a 37-beam MBA

A very common multiple beam antenna (MBA) configuration consists of a collimating device illuminated by an array of feeds. The collimating device is usually a reflector or a lens. The feeds are usually horn antennas with a circular aperture. The reflector is usually offset-fed to eliminate aperture blockage; the lens is center fed. The antenna's feeds are excited to produce a finite number of beams, so as to provide contiguous coverage of the field of view. The designer is forced to minimize the angular spacing between adjacent beams in order to maximize the minimum gain over the antenna's field of view. On the other hand, the feed horn's aperture gain is maximized when the feed horn spacing and its aperture diameter are equal. This results in antenna efficiency of the order of 30% when a single feed horn is excited to produce a beam. When a cluster of 3 or 7 adjacent feed horns are excited to produce a single beam, antenna efficiency can be increased to 50%. When it is tolerable, several identical antenna apertures can be used to replace a single aperture configuration. In this case, each of M apertures produces approximately N/M beams of an MBA that produces N beams. Horns producing adjacent beams do not illuminate the same aperture. This permits the use of a much larger horn aperture for a given beam spacing. This results in reduced spillover, higher gain of each beam, and increased antenna efficiency of each aperture. This paper investigates the maximization of gain for several lens antennas. It shows that antenna gain is increased as its focal length is decreased. That is, a focal length-to-diameter ratio (F/D) less than 1 is preferred     

Focal shifts in parabolic reflectors

Given a parabolic reflector, the maximum directivity is not always achieved by placing the feed at the focal point. Depending on the nature of the feed, the maximum directivity can be obtained by axially displacing the feed either toward or away from the reflector. For low-tapered feeds, the shift should be toward the reflector. This result is similar to an optical phenomenon called the focal shift. We find that this positive shift depends mainly on the Fresnel number of the reflector. For highly tapered feeds, the shift should be away from the reflector. This negative shift becomes significant when the reflector aperture is small, in units of wavelength. A unified view is presented to explain both the positive shift and the negative shift in terms of spillover, aperture illumination efficiency and phase asynchronism. For a system with optimum aperture edge taper, no focal shift can exist.     

A dual chamber Gregorian subreflector system for compact rangeapplications

A dual-chamber compact range configuration is proposed wherein the main reflector and target zone are located in the main chamber and an oversized Gregorian subreflector and associated feed assemblies in the other. The chambers are isolated by an absorber fence except for a small coupling aperture which is used to transmit signals between them. The absorber fence prevents diffraction by the subreflector and spillover by the feed from illuminating the main reflector and target zone. System performance is analyzed with and without the absorber fence to show how the coupling aperture should be shaped to minimize diffractions     

Sectoral hoghorn: a new form of line feed for spherical reflector aerials

A sectoral hoghorn may be used as a line feed to correct spherical aberration in an offset spherical reflector. The bandwidth of this type of feed is considerably greater than that of other line feeds, and the construction is simple. The results of measurements on a prototype X band hoghorn are reported, and a technique for reducing the area of spherical reflector required for a given scan angle is described.     

Field correlation diffraction theory of the symmetrical cassegrainian antenna

The received field as focused by the parabolic main reflector of a Cassegrainian antenna at the surface of an arbitrary profile subreflector is calculated by a spherical wave expansion. This facilitates the application of the field correlation principle and leads to an expression for aperture efficiency taking into account diffraction effects. A comparison is made with numerical results previously published or obtained by other methods. The potential advantage of the technique is the speed of computation and the capability for synthesis as well as analysis of reflector shapes.     

Some examples of generalized cassegrainian and gregorian antennas

Some theoretical generalizations are given of two-reflector, rotationally symmetric microwave antennas fed by a plane wave across the feed aperture. So far as geometrical optics apply, the proposed designs offer 1) no reflection of energy back into the feed and 2) arbitrary illumination over the antenna aperture. The general solution is exhibited in terms of quadratures, and the reflector shapes for some simple cases are worked out in detail and plotted. Two families of antennas are found which image the feed aperture onto the secondary reflector. These antennas might be expected to have low spillover losses due to diffraction, but none of them is of practical proportions. It is shown in general that such imaging is incompatible with efficient illumination of the secondary aperture.     

Improved element pattern for the line feeds of the sphericalreflector antenna in Arecibo

The line feeds of the spherical reflector antenna in Arecibo are considered. A model for the element pattern is presented and used to give a better explanation of the central dip in the radiation pattern of the line feed than that given before. The model includes the effect of the fins between the elements by considering the line feed to be a circularly corrugated cylinder. Such a corrugated cylinder has, according to recent results, a soft outer surface at which the electric field vanishes     

Asymptotic transition region theory for edge diffraction. II.Calculation of diffraction losses in multireflector antennas

For pt.I see ibid., vol.38, no.9, p.1350-8 (1990). Asymptotic formulas that can be used to calculate diffraction losses in a multireflector antenna without having to integrate rapidly varying fields over the reflector surfaces and the aperture are presented. Two kinds of losses caused by edge diffraction are considered: the reduction in antenna efficiency and the increase in spillover. The asymptotic formulas are obtained from the standard transition region field introduced in part I and are expressed in terms at the Δρ defined there     

The SNFGTD method and its accuracy

The spherical near-field geometrical theory of diffraction (SNFGTD) method is an extended aperture method by which the near field from an antenna is computed on a spherical surface enclosing the antenna using the geometrical theory of diffraction. The far field is subsequently found by means of a spherical near-field to far-field transformation based on a spherical wave expansion of the near field. Due to the properties of the SNF-transformation, the total far field may be obtained as a sum of transformed contributions which facilitates analysis of collimated beams. It is demonstrated that the method possesses some advantages Over traditional methods of pattern prediction, but also that the accuracy of the method is determined by the quasioptical methods used to calculate the near field.     

Nonconfocal multimode resonators for masers

The case of a resonator composed of two concave spherical reflectors separated by an arbitrary distance is examined. The general problem of the electromagnetic field distribution over the nonconfocal aperture is first formulated by means of the Huygens principle. The solution of the resulting integral equation is obtained analytically in the highly nonconfocal limit. It was found that when the reflector spacing d is much larger than the radius of curvature b of the reflectors, the aperture field distribution is in the form of traveling waves. For arbitrary d/b, the eigenvalues and eigenfunctions of the lowest order mode is obtained by numerical solution using the IBM 7090 computer. The diffraction loss was found to increase rapidly when d→2b and a geometrical interpretation of this behavior is given. Furthermore, it was found that as the spacing departs from the confocal value, the apertures are no longer surfaces of constant phase. The optimum spacing for maximum Q of the resonator is also obtained.     

Leaky Wave Enhanced Feed Arrays for the Improvement of the Edge of Coverage Gain in Multibeam Reflector Antennas

The performance of multibeam focal plane arrays feeding a single aperture is usually reduced due to conflicting requirements on the feed elements. Dense packing is usually required to minimize the beam separation, while typically large feed apertures are needed to provide the high feed directivity to reduce spillover losses from the reflector. In this paper the use of dielectric super-layers to shape the radiation pattern of each feed is demonstrated. The shaping is obtained by exciting, according to design, a pair of TE/TM leaky waves. The spillover from the reflector is reduced without physically increasing the dimensions of each single element aperture. A prototype of a feed array composed of 19 waveguides arranged in a hexagonal lattice was designed, manufactured and tested. The measured embedded patterns provided an increase of the edge of coverage gain, with respect to the free space case, of at least 0.6 dB in an operating bandwidth (BW) of ap12%. Moreover when reactive loading of adjacent feeds is adopted the increase in the edge of coverage with respect to the free space case was demonstrated to be larger than 1.6 dB over a 3% BW.     

Improvement of prompt response from impulse radiating antennas by aperture trimming

Reflector impulse radiating antennas (IRA) traditionally have been constructed by terminating a self-reciprocal, transverse electromagnetic (TEM) transmission-line feed structure into a paraboloidal reflector. The section of the paraboloid used is usually circular in cross-section, with the outer boundary coinciding with the circle of symmetry of the TEM feed. The reflector converts the spherical TEM mode on the feed line into an approximate plane wave in the near field by geometric optics. The prompt radiated electric field in the direction of focus is given in the physical optics approximation in terms of the integral of the electric field of the TEM mode over the aperture plane inside the reflector boundary. Balanced feed structures have TEM modes that provide both positive and negative contributions to this integral in the aperture plane. Determination of the contour where the principal component of the electric field in the TEM mode is zero identifies portions of the aperture that contribute destructively to the integral. These portions are removed, thereby increasing the prompt radiated field without altering the feed structure or the applied voltage waveform. Furthermore, decreasing the size of the TEM feed relative to the aperture size, followed by appropriate aperture trimming, allows an even greater increase in radiated field. Results are presented that predict an increase in prompt radiated fields for all electrode configurations. Improvements are largest for electrode angles that are large (with respect to the vertical). The trends predicted by the numerical results are verified by an experiment conducted on a time-domain antenna range.     

Aperture efficiency and line feed phase center of parabolic cylindrical reflector antenna

Formulas for calculation of the aperture efficiency of a parabolic cylindrical reflector antenna fed by a line feed along the focal line are presented. The efficiency is factorized into a number of subefficiencies which include contributions from line feed end diffraction, and from blockage and reflections from line feed supports and from diffraction by gaps in the reflector surface. One of the subefficiencies is used to define a phase center for the line feed as well as to obtain a formula for calculating it.