Showing papers by "Per-Simon Kildal published in 2000"

Calculation of ring-shaped phase centers of feeds for ring-focus paraboloids

Abstract: Some primary feeds such as the hat feed radiate around the feed waveguide, which also acts as an axial support tube. Such feed antennas have phase centers in the form of rings rather than points. This paper presents a formula to calculate the location of the ring-shaped phase center. The optimum reflector is a ring-focus paraboloid with the ring focus coinciding with the ring-shaped phase center. The phase center formula is applied to two versions of the hat feed and it is shown that the aperture efficiencies can be improved by up to 0.5 dB when optimum ring-focus paraboloids are used instead of point-focus paraboloids.

Analysis of arrays of rectangular waveguides radiating through stepwise transitions with dielectrically loaded hard walls in one plane

Abstract: Planar antenna arrays of rectangular waveguides radiating through step transitions whose walls are loaded in one plane with a dielectric so that the hard-wall boundary condition is satisfied are considered. The analysis uses the multimode scattering matrix method. It is shown that finite waveguide sections with hard walls are capable of removing the blindness effects inherent in arrays of waveguides with a small cross section and homogeneous filling; the matching of the array remains sufficiently good. Characteristics of H-plane sectoral-horn and pyramidal-horn arrays are also calculated. The sectoral horns with hard walls are found to operate in a wider frequency band than similar sectoral box horns. The efficiency of hard-wall pyramidal-horn arrays is 2 dB higher than that of arrays of similar square box horns.

Scattering by screw heads in reflecting surfaces and their effect on the sidelobes of reflector antennas

Abstract: Screws are often used to fasten the main reflector of a reflector antenna to its support structure. The screw heads may then cause bumps in an otherwise smooth reflector surface. Thereby, they disturb the reflected fields and cause increased sidelobes. In this paper, a perturbation theory is introduced in order to calculate efficiently the increased sidelobes due to scattering from the screw heads. Screw heads are approximated as half spheres, whereby it becomes very efficient to calculate the scattering from them by using a spherical wave expansion algorithm, GIDMULT. This method has been verified by measurements in a low sidelobe antenna. We also describe some ways to reduce the scattering, such as dielectric coating of the screw heads and optimization of their locations. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 213–217, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11018