W.V.T. Rusch

Forward scattering from square cylinders in the resonance region with application to aperture blockage

The relationship between the induced field ratio (IFR) of a cylinder and aperture blocking of a constant-phase aperture by cylindrical struts is discussed. An analytical technique is presented whereby the IFR of rectangular cylinders can be calculated using the method-of-moments with internal constraint points. An experimental technique using a forward-scattering range is used to measure the IFRs of square and circular cylinders in an anechoic chamber. These experimental results are compared with the theory, and their implications on aperture blocking losses and boresight cross polarization are discussed.

Derivation and application of the equivalent paraboloid for classical offset Cassegrain and Gregorian antennas

The equivalent paraboloid is derived for classical offset Cassegrain and Gregorian antennas. The important practical case of systems with circular exit apertures is discussed in detail, and a condition for a symmetric equivalent paraboloid is derived. For such systems, diffraction effects are investigated using tapered and scanned feeds to illuminate the equivalent paraboloid and the two-reflector system. >

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 increasing D/\lambda until 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.

The current state of the reflector antenna art

The current state of the reflector antenna art is presented in the form of a review article, intended primarily for engineers who are new in the field. The properties of paraboloidal antennas are related to aperture efficiency, surface tolerance, feed defocusing, and aperture blocking. Other reflector shapes considered are offset-fed paraboloids, classical and offset Cassegrain systems, shaped-dual reflectors, and scanning reflectors. Properties of contoured-beam systems are reviewed. Structures and materials are reviewed for ground stations and satellite-borne reflectors. Emphasis is placed on specific hardware examples for each principle or concept. Near-field probing, dichroic surfaces, and techniques of surface metrology are presented as examples of promising areas for new developments.

Radiation cones from feed-support struts of symmetric paraboloidal antennas

Wide-angle radiation associated with the plane-wave component of the field in the focal space of a paraboloidal reflector is computed by integrating the currents induced on the feed-support struts. This component of radiation is locally maximum on radiation cones which lie along the axis of each strut. Comparison with data measured for the Dwingeloo radio telescope indicates good agreement for the position, width, and intensity of these cones, even at levels 50 dB below the peak of the main beam.

Determination of the maximum scan-gain contours of a beam-scanning paraboloid and their relation to the Petzval surface

The scan-plane fields in the focal region of a beam-scanning paraboloid are determined from physical optics. Amplitude and phase contours are presented, and comparisons are made with the geometrical-optics results. Contours for maximum scan-gain are determined as a function of F/D and illumination taper and compared with the Petzval surface. Unless the F/D is very large or spillover is excessive, a higher scan gain is achieved when the axis of a directional feed is parallel to the axis of the reflector than when the feed is directed toward the vertex. The contour of maximum scan-gain is a function of both illumination taper and F/D . In general, larger F/D values tend to have a maximum-gain contour close to the focal plane, while the smaller F/D values tend to have a maximum-gain contour closer to the Petzval surface. Increasing the illumination taper moves the maximum-gain contour closer to the Petzval surface. Normalized maximum-gain contours are presented as a function of beamwidths of scan. The frequency dependence of these results is discussed.

Moment-method analysis of large, axially symmetric reflector antennas using entire-domain functions

The moment-method technique utilizing entire domain basis functions is applied to the analysis of large, axially symmetric reflector antennas. The electric surface current is modeled as a finite series of sinusoids whose domain consists of the entire generating curve. This expansion results in a matrix size of less than 5% of that produced with subdomain basis functions. Only a slight increase in the CPU requirements occurs from this analysis. The results from this technique show good agreement when compared to both physical optics and a subdomain-based moment-method formulation on small, axially fed paraboloidal and hyperboloidal reflector antennas. Extension to a large 100- lambda paraboloidal reflector with f/D=0.4 produces results comparable to that obtained using physical optics. Convergence is obtained with as few as two expansion terms per wavelength. Discretization of the generating curve with four points per wavelength leads to results which agree within 0.5 dB over data from a more densely defined curve. >

Boresight-gain loss and gore-related sidelobes of an umbrella reflector

It is shown that energy in the gore-related sidelobes of an umbrella reflector comes from energy lost from the main beam. Both boresight gore-loss and sidelobe level correlate directly with cyclic aperture phase error caused by the geometry. The gore-sidelobe peak is located near \theta_{p} , where \sin \theta_{p} = 1.2N_{G} (\pi/D\lambda) where N_{G} is the number of gores and D is the reflector diameter. An expression is also derived for the amplitude of the gore-related sidelobe.

Forward scattering from cylinders of triangular cross section

The induced-field ratios (IFR) of conducting cylinders of a triangular cross section have been calculated. When the width of the cross-section is in the range of 1-2 wavelengths, the E -wave IFRs are substantially less than the corresponding values for square or circular cross sections. From the standpoint of RF aperture blocking of a reflector antenna, the triangular cross section thus offers an advantageous cross section for feed-support struts.

The current state of the reflector antenna are-entering the 1990s

Modern reflector theory and practice, with emphasis on the recent past are reviewed. Included are a brief historical review and performance definitions for the nonspecialist. Special sections are devoted to shaped multiple reflectors, reflector surface metrology, and reflectors for the next decade and beyond. In spite of decreasing budgets for scientific, commercial, and military space programs, the 1990s promise to be a true golden age for the reflector antenna art. >