A line feed for a spherical reflector

A line feed for a spherical reflector is considered on the basis of a plane-wave spectrum of radiation angles. It is shown that a feed excited by circumferential slots results in a gain deterioration of at least 3 dB. The correct excitation of the feed is indicated. Expressions for field components in the focal region are obtained.     

Synthesis of the fields of a transverse feed for a spherical reflector

The problem of designing a transverse feed for a spherical reflector is considered and a method is presented for synthesizing the fields on a surface of a sphere enclosing a feed that will produce a specified reflected field at the surface of a spherical reflector. The method identifies the reflector and a spherical surface enclosing the feed as a boundary value problem and uses a finite set of spherical waves to approximate the boundary conditions. A feed designed to excite this field will in turn produce the desired reflected field at the surface of the reflector, under the condition that that portion of the reflected field which is scattered by the feed may be neglected. It is shown that the feed need produce only a small part of the synthesized field to obtain an antenna efficiency of more than 70 percent. Some typical field distributions will be shown so as to indicate a method for designing a feed and to point out the correlation between the polarization of the synthesized field and the polarization of the reflected field at the surface of the reflector.     

A high-performance line source feed for the AIO spherical reflector

An aberration-correcting line source feed has been designed, modeled, constructed, and tested in the Arecibo reflector. The feed is a linearly polarized flat wavegnide 40-foot-long array illuminating 700 feet of the 1000-foot-diameter spherical reflector at 318 MHz. The antenna, illuminated by the new feed, yields an aperture efficiency of 70 percent with a peak gain near 56 dB, half-power beamwidth of 16.2 minutes of arc, and sidelobe levels below 4 percent of the on-axis gain. Vignetting losses are approximately 30 percent at the highest zenith angle.     

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.     

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.     

Scale model development of a high efficiency dual polarized line feed for the Arecibo spherical reflector

The experimental development of a 1:6.535 scale model line feed for the Arecibo spherical reflector is described. The 14.7-ft long model at 2810 MHz simulates a 96.6-ft feed at 430 MHz capable of illuminating the full 1000-ft aperture of the reflector. The feed design requirements are discussed and an experimental program is outlined in which the necessary line source parameters were established using a number of leaky cylindrical test sections. Experimental measurements of both the near and far fields of the model feed are described and typical results are quoted and discussed. Finally, some results obtained after installation of a full size feed in the reflector are given and compared with predictions based on the model data.     

Focused aperture antenna using a spherical main reflector with (Cassegrain) convex feed

A focused aperture antenna has been realised using a (Cassegrain) convex subreflector with a spherical main reflector. The design was based on geometrical optics, and good agreement was obtained between the measured and calculated field in the focal region.     

Aperture blockage in reflector antennas

The aperture blockage effects on both the copolarized and the cross polarized components of the front-hemisphere pattern of a typical quadrupod supported primary-feed paraboloidal reflector antenna are evaluated. A detailed numerical model of the scattering process which involves the feed, the feed supporting struts, and the reflector itself is employed to compute the blockage effect.     

Optimum cylindrical near fields for illuminating a spherical reflector

A spherical reflector antenna can be illuminated by a radiating line source along its symmetry axis. The optimal fields on a cylinder around this axis are found using a modal expansion technique. The optimal fields are expressed as combinations of cylindrical harmonics with unknown mode coefficients and set equal to the required illumination on the sphere. The resultant integral equations are solved using steepest descent methods, giving the mode coefficients. These coefficients are then checked by numerical integration. The optimal cylindrical fields are found from these mode coefficients both by integrating numerically and by steepest descent methods. The results of both techniques are checked by numerical integration. The results give theoretical aperture efficiencies of greater than 90 percent at sufficiently short wavelengths.     

Dual-offset antennas with a spherical main reflector for smallearth stations

The design of dual-reflector antennas with a spherical main reflector for small earth stations is considered. An analysis of the field polarization throughout the system shows that it is possible to adjust the reflectors to obtain very low cross-polarization. The use of an elliptical main reflector projection is explored in order to enhance efficiency and lower side-lobe levels     

Double spherical cassegrain reflector antennas

There is a need for high gain antennas that are capable of rapid scanning and multibeam operations. This paper presents a study of the double-spherical Cassegrain reflector to ascertain whether this antenna is practical for these purposes. Aperture blocking, spread of the field, and phase and amplitude distributions are examined using geometric optical techniques. Some radiation patterns are calculated to show the effects of illumination taper, aperture blocking, and phase errors.     

Large lateral feed displacements in a parabolic reflector

The radiation patterns of a parabolic reflector with large lateral-feed displacements are computed utilizing both the vector current method and scalar aperture theory, and compared to experimental results. The theory is general enough to include asymmetric primary pattern illumination. The scalar and vector solutions are derived from the same initial equation so that the approximations used in obtaining the scalar solution are clearly displayed. Results from the vector and scalar theories are compared and the range of validity of the approximate analysis is indicated.     

Circularly polarized feed for cylindrical parabolic reflector antennas

The feed design described is a linear array of crossed dipoles above a ground plane. The radiation patterns of the longitudinal and the transverse dipoles are made equal by parallel longitudinal rods, referred to as beam forming rods or beam matching rods. When used as a circularly polarized feed for an offset parabolic cylinder antenna, aperture efficiencies (spillover included) as high as 0.89 are potentially available if the field distribution along the array is uniform. The beam patterns of the feed were computed both by the method of moments and the geometrical theory of diffraction and are compared with patterns measured on a model at 1.5 GHz. A method of matching the impedance for both linear polarizations is proposed using parallel impedance matching rods.     

Calculated scan characteristics of a large spherical reflector antenna

A previously published numerical method to calculate the radiation properties of parabolic reflectors has been modified to also include very large spherical reflectors. The method has been verified by comparing the calculated and the measured results for a 120-wavelength spherical reflector.     

Fields in the focal region of a spherical reflector

An earlier analysis of the field structure and energy flow near the axis of any circularly symmetric focusing reflector is applied to the case of a spherical reflector. From the results an estimate is made of the attainable efficiency of corrugated-waveguide aperture-type feeds.     

Scattering from a hyperboloidal reflector in a cassegrainian feed system

The scattered field from a hyperboloidal reflector is calculated by integrating the induced current density over the front of the hyperboloid. The resulting integral expressions for the fields possess a stationary term which, when evaluated, yields the geometrical ray-optics approximation to the scattering problem. The complete field, including diffraction effects, may be obtained by numerical evaluation of the integrals. The formulas are applied to a hyperboloid illuminated by an idealized, sharply cut off uniform feed pattern. Characteristic diffraction phenomena are reduced with increasingD/lambdauntil the geometrical ray-optics result is obtained in the limit of vanishing wavelength. Theoretical field patterns are also obtained for a horn-fed hyperboloidal subreflector in a Cassegrainian feed system; they indicate that for moderately large hyperboloidal reflectors spillover may be reduced to an acceptable level, but there is a tendency toward increased forward spillover. The results of 9600-Mc model tests compare favorably with the theoretical patterns.     

Beam steering using feed displacement with a paraboloidal reflector

The field in an aperture plane for a symmetrical paraboloidal reflector with its feed displaced from the axis is obtained as a polynomial expansion in coordinates in that plane. From this expansion, the loci of feed positions for maximum gain and for narrowest beamwidths in the radial and tangential directions are found to be on spheres passing through the focus and with their centers near to the vertex, near midway between the focus and the vertex, and near infinity in the direction of the vertex, respectively. Expressions for the variation of the gain with offset are obtained, and are found to be consistent with results obtained by other workers by extensive computer modeling forF- numbers greater than one (the range that was covered in their models).     

Synthesis of the primary fields of a transmitting feed for a circular reflector

A two-dimensional electromagnetic boundary-value problem consisting in the synthesis of the primary fields of a transmitting feed antenna for a circular-cylindrical reflector is considered. A primary field that corrects for the phase aberration of the reflector is synthesized as a sum of circular waves whose common origin is displaced from the center of curvature of the reflector. Limitations on the gain of the feed-reflector combination as a function of the number of waves and the displacement are discussed. The synthesized fields are compared with those obtained from geometrical optics.