K. Woo

Bio: K. Woo is an academic researcher from California Institute of Technology. The author has contributed to research in topics: Radiation pattern & Periscope antenna. The author has an hindex of 5, co-authored 8 publications receiving 144 citations.

Diffraction by an arbitrary subreflector: GTD solution

Abstract: The high-frequency asymptotic solution of diffraction by a conducting subreflector is studied. By using Keller's geometrical theory of diffraction and the newly developed uniform asymptotic theory of diffraction, the scattered field is determined up to an including terms of order k^{-1/2} relative to the incident field. The key feature of the present work is that the surface of the subreflector is completely arbitrary. In fact, it is only necessary to specify the surface at a set of discrete points over a random net. Our computer program will fit those points by cubic spline functions and calculate the necessary geometrical parameters of the subreflector. In a companion paper by Y. Rahmat-Samii, R. Mittra, and V. Galindo-Israel, the scattered field from the submflector is used to calculate the secondary pattern of an arbitrarily shaped reflector by a series expansion method. Thus, in these two papers, it is hoped that we have developed a "universal" computer program that can analyze most dual-reflector antennas currently conceivable. It should also be added that our method of calculation is extremely numerically efficient. In many cases, it is one order of magnitude faster than the conventional integration method based on physical optics.

Realizable feed-element patterns for multibeam reflector antenna analysis

Abstract: The radiation pattern of a feed element is approximately described by a simple function (\cos \theta)^{q} . For a given element spacing of the feed array, we give simple formulas for estimating the practical value of q when the element is an open-ended rectangular waveguide, an open-ended circular waveguide, a pyramidal horn, or a cigar antenna.

Multi-ring element FSS for multi-band applications

Abstract: NASA's CRAF/Cassini (C/C) project requires the use of multiple RF frequencies at S, X, Ku, and Ka bands for science investigations and data communications links. A single high-gain antenna (HGA) will be used. It incorporates a frequency-selective subreflector (FSS) to allow a Cassegrain configuration at X (7.2 and 8.4 GHz) and Ka (32 and 34.5 GHz) bands, and a prime focus configuration at S (2.3 GHz) and Ku (13.8 GHz) bands. A four-based FSS design for the C/C application is reported. The selection of the concentric ring elements is based on the fact that the ring's geometry is particularly conformable to the present circular polarization requirement and ease of manufacture. >

A GTD study of pyramidal horns for offset reflector antenna applications

Abstract: An efficient computational method is demonstrated for determining the far-field patterns of reflectors illuminated by pyramidal horns. A uniform geometrical theory of diffraction (GTD) formulation is used to determine the near- and far-fields of the horn. The far-field patterns of the offset reflector are constructed using the physical optics (PO) formulation in conjunction with the Jacobi-Bessel expansion method. Many representative results are shown for the far-field patterns of the reflector. In particular, the concept of the optimum phase center for the best location of a horn illuminating a reflector is carefully studied and some useful observations are made.

Shooting and bouncing rays: calculating the RCS of an arbitrarily shaped cavity

Abstract: A ray-shooting approach is presented for calculating the interior radar cross section (RCS) from a partially open cavity. In the problem considered, a dense grid of rays is launched into the cavity through the opening. The rays bounce from the cavity walls based on the laws of geometrical optics and eventually exit the cavity via the aperture. The ray-bouncing method is based on tracking a large number of rays launched into the cavity through the opening and determining the geometrical optics field associated with each ray by taking into consideration: (1) the geometrical divergence factor, (2) polarization, and (3) material loading of the cavity walls. A physical optics scheme is then applied to compute the backscattered field from the exit rays. This method is so simple in concept that there is virtually no restriction on the shape or material loading of the cavity. Numerical results obtained by this method are compared with those for the modal analysis for a circular cylinder terminated by a PEC plate. RCS results for an S-bend circular cylinder generated on the Cray X-MP supercomputer show significant RCS reduction. Some of the limitations and possible extensions of this technique are discussed. >

Techniques for High-Frequency Problems

Abstract: Techniques based on the method of modal expansions, the Rayleigh-Stevenson expansion in inverse powers of the wavelength, and also the method of moments solution of integral equations are essentially restricted to the analysis of electromagnetic radiating structures which are small in terms of the wavelength. It therefore becomes necessary to employ approximations based on “high-frequency techniques” for performing an efficient analysis of electromagnetic radiating systems that are large in terms of the wavelength.

Four-band frequency selective surface with double-square-loop patch elements

Abstract: Design and experimental verifications are presented for a four-band frequency selective surface (FSS) with perfectly conducting double-square-loop (DSL) patch elements. A single screen DSL element FSS is demonstrated for (1) a four-band FSS which reflects the X- and Ka-band signals while passing the Sand Ku-band signals, and (2) a low-pass (or Ka-add-on) FSS that reflects the Ka-band signal while passing the S-, X-, and Ku-band signals. Cascading the above low-pass FSS with a previously published single screen tri-band FSS, a double screen FSS is also successfully demonstrated for the Cassini four-band application. The good agreement obtained between the measured and the computed results verified both the single and double screen four-frequency FSS approaches for the Cassini Project. >

Shaped reflector antenna analysis using the Jacobi-Bessel series. [design for space and satellite communication]

Abstract: The physical optics approximation is employed to derive the radiation integral for a doubly curved offset reflector antenna illuminated by an arbitrary source. A novel procedure is presented for expressing the radiation integral in terms of a summation of Fourier transforms of an "effective" aperture distribution which includes the effect of the curvature of the surface. The Jacobi-Bessel series is used to evaluate the Fourier transforms. The vector nature of the far-field pattern is studied by evaluating its three Cartesian components in a unified fashion. The rapid numerical evaluations of the expressions obtained are demonstrated via extensive test cases. In particular, the scattering characteristics of symmetric and offset parabolic, spherical, and shaped reflectors are studied in detail, and comparisons are made with other available data.