Dichroic plates for cutoff wavelengths down to about a millimeter and high angles of incidence can be fabricated by numerically controlled milling of rectangular arrays of waveguide slots in half-wavelength-thick metal plates. A plate of this type is used to separate higher- and lower-frequency components of incident electromagnetic radiation linearly polarized along a specified axis; the plate is designed to reflect most of the incident electromagnetic radiation at frequencies below its cutoff frequency while allowing most of the radiation at higher frequencies to pass through.
Thick metal plates containing regular arrays of holes have been used before as dichroic reflectors. In the case of a plate containing circular holes in a rectangular or a triangular array, performance deteriorates substantially (sharpness of cutoff decreases and insertion loss increases) as the angle of incidence increases beyond 20°. Experience at submillimeter wavelengths has shown that the loss of performance at increasing angle of incidence can be mitigated by use of rectangular holes or slots instead of circular holes.
It is not practical to make arrays of precisely rectangular slots at the plate thicknesses needed for wavelengths in the millimeter range because (1) the preferred fabrication technique in this thickness range is numerically controlled milling and (2) the diameters of the end mills that must be used in this size range are such that the corner or end radii of the slots cannot be much less than the thicknesses of the plates. However, slots with rounded ends or corners can be used, as long as the effects of rounding are taken into account in design computations and acceptable frequency responses can still be obtained; this is the basis of the present development.
The frequency response of a dichroic plate containing a rectangular array of slots depends on the thickness of the plate and the shape and spacing of the slots. Typically, for a half-wavelength-thick dichroic plate, the half-power transmission frequency is close to the nominal cutoff frequency of the dominant waveguide mode. For a rectangular slot with sharp corners regarded as a waveguide, the cutoff frequency for the dominant mode is well known and is simply the frequency for which the width of the slot is a half wavelength. For a slot with semicircular ends and an aspect ratio (length ÷ width) of 2.5, the cutoff frequency is about 8.5 percent higher. The figure depicts a dichroic plate containing slots of this shape, along with its frequency response.
This work was done by Peter Siegel and Hamid Javadi of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
NPO-20826
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Millimeter-Wave Dichroic Plates for High Angles of Incidence
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Overview
The document titled "Millimeter-Wave Dichroic Plates for High Angles of Incidence" is a technical support package prepared under the sponsorship of NASA, specifically from the Jet Propulsion Laboratory (JPL) at the California Institute of Technology. It focuses on the development and application of millimeter-wave dichroic plates, which are essential components in various aerospace and communication systems.
Dichroic plates are designed to selectively transmit or reflect electromagnetic waves based on their frequency. This capability is particularly important in high-frequency applications, such as those found in millimeter-wave technology. The document outlines the innovative approach taken by the inventors, Hamid H. Javadi and Peter H. Siegel, in creating semi-circular-sided waveguide dichroic plates that maintain high performance even at steep angles of incidence.
The report discusses the background and novelty of the invention, emphasizing the challenges associated with traditional dichroic plate designs, which often struggle with efficiency and performance at high angles. The new design incorporates rectangular slots instead of circular holes, which enhances the separation of frequencies and improves overall functionality. This advancement is crucial for applications that require precise control over electromagnetic wave propagation, such as in radar systems, satellite communications, and other high-frequency technologies.
The document also includes a summary of the invention's potential applications, highlighting its relevance in modern aerospace engineering and telecommunications. The authors stress that the work was conducted under a NASA contract, and they clarify that references to specific commercial products or manufacturers do not imply endorsement by the U.S. Government or JPL.
In conclusion, this technical support package serves as a comprehensive overview of the advancements in millimeter-wave dichroic plate technology, showcasing the innovative solutions developed to address the limitations of existing designs. It provides valuable insights for researchers and engineers working in the fields of aeronautics, space exploration, and high-frequency communications, emphasizing the importance of continued innovation in these critical areas.
