Ribbon waveguides made of alumina or of semiconductors (Si, InP, or GaAs) have been proposed as low-loss transmission lines for coupling electronic components and circuits that operate at frequencies from 30 to 1,000 GHz. In addition to low losses (and a concomitant ability to withstand power levels higher than would otherwise be possible), the proposed ribbon waveguides would offer the advantage of compatibility with the materials and structures now commonly incorporated into integrated circuits.

Heretofore, low-loss transmission lines for this frequency range have been unknown, making it necessary to resort to designs that, variously, place circuits and components to be coupled in proximity of each other and/or provide for coupling via free space through bulky and often lossy optical elements. Even chip-to-chip interconnections have been problematic in this frequency range. Metal wave-guiding structures (e.g., microstriplines and traditional waveguides) are not suitable for this frequency range because the skin depths of electromagnetic waves in this frequency range are so small as to give rise to high losses. Conventional rod-type dielectric waveguide structures are also not suitable for this frequency range because dielectric materials, including ones that exhibit ultra low losses at lower frequencies, exhibit significant losses in this frequency range.

Normalized Power Densities were computed for a dominant eHE11 mode on a ribbon waveguide made of alumina (having assumed relative permittivity of 10), both uncoated and coated with various thicknesses polytetrafluoroethylene (having assumed relative permittivity of 2.06). The unit of thickness used in the computations was the free-space wavelength, λ0.

Unlike microstripline structures or metallic waveguides, the proposed ribbon waveguides would be free of metal and would therefore not be subject to skin-depth losses. Moreover, although they would be made of materials that are moderately lossy in the frequency range of interest, the proposed ribbon waveguides would cause the propagating electromagnetic waves to configure themselves in a manner that minimizes losses.

The basic principle for minimizing losses was described in “Ceramic Ribbons as waveguides at Millimeter Wavelengths” (NPO-21001), NASA Tech Briefs, Vol. 25, No. 4 (April 2001), page 49. To recapitulate: The cross-sectional geometry of a waveguide ribbon would be chosen in consideration of the permittivity of the ribbon material to support an electromagnetic mode in which most of the energy would propagate, parallel to the ribbon, through the adjacent free space and only a small fraction would propagate within the ribbon. As a result, the interaction of the propagating wave with the dielectric core (and thus the attenuation) would be minimal.

For straight runs, the ribbon waveguides would be uncoated. However, since most of the guided power would be carried in the nearly lossless air just outside the ribbon, a significant portion of the guided power could be expected to be radiated (and thus lost) where the guiding ribbon was sharply curved (for example, to bend it around a corner). In such a case, the short length of the ribbon containing the curve could be coated with a layer of a polymer having a suitable permittivity intermediate between that of air and that of the ribbon, so that most of the power would not be radiated but would remain confined within the polymer layer (see figure) while propagating around the corner.

This work was done by Cavour Yeh, Daniel Rascoe, Fred Shimabukuro, Michael Tope, and Peter Siegel of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Semiconductors & ICs category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
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Refer to NPO-30339, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Alumina or Semiconductor Ribbon Waveguides at 30 to 1,000 GHz

(reference NPO-30339) is currently available for download from the TSP library.

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This article first appeared in the May, 2005 issue of NASA Tech Briefs Magazine.

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