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.

This Dichroic Plate was designed to reflect at a frequency of 118 GHz and transmit at a frequency of 240 GHz at an angle of incidence of 40°. The slots, each 0.036 in. (0.91 mm) long and 0.0145 in. (0.37 mm) wide, were end-milled in a 0.030-in. (0.76-mm)-thick plate at length-axis intervals of 0.038 in. (0.97 mm) and width-axis intervals of 0.185 in. (4.7 mm). The area containing the slots is an ellipse with axes of 2.78 and 1.18 in. (70.6 and 30.0 mm).

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.