T. Veruttipong

Time domain version of the uniform GTD

The uniform geometrical theory of diffraction (UTD) solutions can be inversely transformed analytically to obtain a time-domain version of the UTD. The time-domain solutions are valid in the early time period where an observation time t is close to the time after the arrival of the first diffracted wavefront. Comparisons with GTD (geometrical theory of diffraction) and also with available rigorous results (J.B. Keller and A. Blank, 1951) reveal that the UTD solutions are accurate for substantial early time periods while the GTD (Keller and Blank) results are valid for very early time periods. >

Design considerations for beamwaveguide in the NASA Deep Space Network

A generalized solution is found for retrofitting a large dual-shaped reflector antenna for a beamwaveguide. The design is termed as a bypass beamwaveguide. Both highpass design feed imaging and bandpass design feed imaging are considered. Each design was studied using geometrical optics, Gaussian wave analysis, and both low-frequency and high-frequency diffraction analysis. An important extension of the Mizusawa-Kitsuregawa criteria was discovered (M.M. Zusama and T. Kitsuregawa, ibid., vol.AP-21, pp.844-8, Nov. 1973). The principle revealed shows how a two-reflector cell, although in itself distorting, may be combined with a second cell which compensates for the first and delivers an output beam which is a good image of the input beam. >

Interpolation methods for shaped reflector analysis

The diffraction analysis of reflector surfaces which are described only at a discrete set of locations usually leads to the requirement of an interpolation to determine the surface characteristics over a continuum of locations. Two methods of interpolation, the global and the local methods, are presented. The global interpolation representation is a closed-form or series expression valid over the entire surface. The coefficients of a series expression are found by an integration of all of the raw data. Since the number of coefficients used to describe the surface is much smaller than the number of raw data points, the integration effectively provides a smoothing of the raw data. The local interpolation provides a closed-form expression for only a small area of the reflector surface. The subreflector is divided into sectors each of which has constant discretized data. Each area segment is then locally described by a two-dimensional quadratic surface. The second derivative data give the desired smoothed values. >

Low-loss offset feeds for electrically large symmetric dual-reflector antennas

Circularly symmetric, dual-reflector, high-gain antenna systems often require feeds placed off the systems axis because of the need for multiple feeds to use the reflector antenna. Also, the constraint requiring the hyperboloid or shaped subreflector to remain circularly symmetric is sometimes added. In a Cassegrainian system, the subreflector and feed may be rotated off-axis around the paraboloid focus and retain main reflector focusing. However, substantial spillover results in considerable noise in a high-gain/low-noise temperature system. In a shaped system, the tilt of the shaped subreflector and feed together results in substantial defocusing as well as spillover noise. If the subreflector is tilted approximately one-half the angle of the feed tilt in either the Cassegrainian or the dual-shaped reflector antenna, it is found that spillover and noise are substantially reduced with tolerable defocusing. An extensive numerical analysis of these effects was conducted to determine the characteristics of a 70-meter, dual-shaped reflector versus Cassegrainian antenna and to gain some understanding of the cause of the observed effects.

Interpolation methods for GTD analysis of shaped reflectors

The finding of smooth analytic representations for antenna reflector surfaces which are prescribed only by discretized data obtained by various synthesis methods is examined. Frequently the data are distributed in a nonuniform grid and contain noise. The smoothness required is to C sub 1 for physical optics diffraction analysis and to C sub 2 for geometrical theory of diffraction (GTD) analysis. The GTD analysis approach requires a surface description which returns data very rapidly. Two methods of interpolation, the global and the local methods, are discussed. They each have advantages and disadvantages; characteristics are discussed and examples are presented.

GTD, physical optics, and jacobi-bessel diffraction analysis of beam waveguide elipsoids

For certain applications of beam waveguides to large ground system antennas, the use of ellipsoid pairs in place of more conventional paraboloid pairs is appropriate. For example, in the application illustrated in Figure 1, the feed and maser system is rotated in azimuth with the antenna. In this case, the beam waveguide does not go directly to the axis of the main reflector but bypasses some in-place heavy machinery along this axis. In this configuration, ellipsoid pairs are more suitable because their convergent ray characteristic allows for a lower loss transmission and also because the three independent focii can be aligned in accordance with the rules discovered by Mizusawa and Kitsuregawa.l Thus aligned, the ellipsoid pair perfectly images the feed pattern according to geometrical optics (see Figure 2).

Asymptotic evaluations of near field physical optics integrals for shaped sub-reflectors

Diffraction analysis of the subreflector scattered field near the rim of the main reflector in a high-gain dual-shaped system was found to have singularity problems. A 34-m ground station dual-shaped reflector with a 20-dB subreflector edge taper was considered. The UTD (uniform geometrical theory of diffraction) and slope diffraction field singularities are due to a geometrical optics caustic on the reflection boundary. Physical optics evaluation has been carried out efficiently and accurately using asymptotic techniques for both symmetric and offset asymmetric subreflectors.<<ETX>>

Low-loss off-axis feeds for symmetric dual-reflector antennas

Circularly symmetric, dual reflector, high gain antenna systems often require feeds placed off the systems axis because of the need for multiple feeds to use the reflector antenna. Also, the constraint requiring the hyperboloid or shaped subreflector to remain circularly symmetric is sometimes added. In a Cassegrainian system, the subreflector and feed may be rotated off axis around the paraboloid focus and retain main reflector focusing. However, substantial spillover results in considerable noise with a high gain/low noise temperature system. In a shaped system, the tilt of the shaped subreflector and feed together results in substantial defocusing as well as spillover noise. If the subreflector is tilted approximately one-half the angle of the feed tilt in either the Cassegrainian or the dual shaped reflector antenna, it is found that spillover and noise are substantially reduced with tolerable defocusing. An extensive numerical analysis of these effects was conducted to determine the characteristics of a planned 70-meter, dual shaped reflector versus Cassegrainian antenna and to gain some understanding of the cause of the observed effects.