An offset paraboloidal reflector illuminated by a balanced feed horn constitutes an efficient launcher for coupling microwaves into quasioptical beams Measurements on a launcher with low blockage show low cross polarization The amplitude, phase, and polarization characteristics are predicted by two gaussian beam modes, and the resulting formulas are found to agree well with measurements at 19 and 28 GHz For example, with increasing offset angles, the ratio of the maximum cross-polarized signal in the radiation pattern to the on-axis co-polarized signal is observed to vary from −44 to −37 dB, within 1 dB of the predicted variation

Some far-field studies of an offset launcher

A microwave single-reflector scanning antenna derived from an ellipse (rather than the usual parabola) which gives a much greater field of view is presented. This reflector combines reasonable scanning in one plane with good focusing in the other, and its scanning ability is superior to the torus and other single reflectors because it has much greater aperture efficiency and is thus smaller while having the same performance. The reflector surface is derived in two steps: a fourth-order even polynomial profile curve in the scan plane is found using least squares to minimize the scanned ray errors; then even polynomial terms in x and y that minimize astigmatism for both the unscanned and maximally scanned beams are added to form the three-dimensional surface. Numerical simulations of radiation patterns for a variety of antenna diameter and field-of-view cases give excellent results. The 60 degrees scan case with 30- lambda -diameter aperture has only 0.2-dB peak gain deviation from ideal and first sidelobe levels below 14 dB down from peak gain. The 17 degrees , 500- lambda case has only 0.8-dB gain variation and -14 to -11 dB sidelobe levels for approximately +or-68 beamwidths of scan, with focal length to aperture diameter ratio equal to about one. >

High aperture efficiency symmetric reflector antennas with up to 60 degrees field of view

A scanning dual-offset reflector antenna using the simple motion of a flat subreflector to illuminate different parts of a shaped main reflector is presented. A particular case having a 20-degree linear field-of-view (for a DBS application) with a 30/spl lambda/ aperture diameter is explored. The main reflector surface is shaped to minimize the phase aberrations across the field-of-view. Once the main reflector surface is designed, the subreflector parameters are determined to ensure optimum focusing across the field-of-view. >

A shaped dual-reflector antenna with a tilting flat subreflector for scanning applications

Waveguide arrays with slots in the broad face use either longitudinal shunt slots displaced from the centerline or inclined series slots on the centerline. Despite the established popularity of the shunt-displaced slot, a case can be made for the superiority of the series-inclined slot. The resonant length of the series-inclined slot is substantially independent of the inclination, and the cross-polarized side lobes are easily reduced by a simple filter. For the shunt slot, resonant-length variations and unwanted side-lobes caused by the offsets are problems less easily solved. Some general features of four types of slotted waveguide arrays are discussed: (1) untransposed series or shunt slots, (2) transposed series or shunt slots, (3) interlaced series and shunt slots firing forward, and (4) interlaced series and shunt slots firing backward. Typical grating lobe spectra are shown and the relationship of the slotted linear array to the optical diffraction grating is discussed. The four types of slotted waveguide arrays presented are special cases of Rayleigh's beam-forming phased array in which the diffraction grating is "blazed" so that all the light falls in a particular direction. Slotted array design techniques are reconsidered briefly and "leakance" characteristics are shown for the one-third constant plus two-thirds cosine squared distribution (for different powers in the termination). The Fourier transform character of the input SWR versus frequency pattern is noted. Auxiliary fittings for slotted arrays (parallel plates, chokes, prisms, and horns) are also reviewed. Series-slotted waveguide arrays have been built by AIL for K_{u} -, X-, and S-bands, and for 332 Mc, where the array is 54 feet long with twenty-six 18-inch slots in a 26 by 13 inch waveguide. Two such arrays (side by side) were developed for an aeronautical glide-slope application. There is special interest in all of these arrays because of their use at or near the broadside beam-firing angle. Measurements of patterns made at or near the zero squint angle showed satisfactory beam formation with side lobes below -20 db and cross-polarized lobes below -27 db. Input SWR's were less than 2 to 1. Isolation between adjacent antennas was better than -45 db.

Series-slotted waveguide array antennas

The experimental results of investigation and utilization of the focal-region-field distribution of a Cassegrain antenna are presented. Aberrations caused by beam scan or reflector distortion can be significantly reduced through control of the phase and amplitude of feed sources distributed in the focal plane and transverse to the system axis. Inherent in this technique is the capability to correct for aberrations and also insure maximum antenna efficiency. A tubular feed array is used both as a probing instrument to determine the required focal-field distributions and as an active feed in which the phase and amplitude of the individual elements are independently controlled. The results are applicable to other reflector and lens antennas.

http://ieeexplore.ieee.org/iel6/8242/25847/01147335.pdf

Efficient aberration correction with a transverse focal plane array technique

By applying optical laws and ray tracing techniques to paraboloidal reflectors, it has been possible to derive the focusing and phasing criteria for obtaining wide angle performance of a paraboloidal microwave antenna system. Although this optics approach represents an approximation to the exact solutions, the experimental results indicate it is satisfactory for practical uses. The analysis indicates the sections of the paraboloidal reflecting surface best suited to line source correction. The reflector peripheral contour which tends to minimize sidelobes has been indicated. Two types of midpoint correctors are discussed. These will produce beams fanned in the elevation plane and well focused in the azimuth plane. Experimental results are presented from a midpoint corrector covering 60 degrees in elevation while maintaining well focused azimuth patterns over the entire elevation interval. Several pencil beams may be produced using one reflector by placing ridge line correctors at the appropriate angles on the focusing surfaces.

W. Mavroides

Papers

Corrective line sources for paraboloids