Theoretical calculations verified by experiments have shown that suitably designed ribbons made of alumina can serve as low-loss dielectric waveguides for electromagnetic radiation at frequencies from 30 to 300 GHz. Prior to this development, low-loss waveguides for this frequency range were unknown. The achievable attenuation factor for an alumina-ribbon waveguide is less than 10 dB/km; as such, it is less than a hundredth of that of a typical ceramic dielectric rod waveguide, less than 1/200 of that of a customary metallic waveguide, and less than 1/300 of that of a microstripline at a frequency of 100 GHz.
The exceptionally low loss factor is not achieved primarily, as it has been on some past occasions, through selection of ultra-low-loss dielectric material (since ultra-low-loss material is not obtainable in this frequency range). Instead, it is achieved primarily through selection of a cross-sectional geometry in conjunction with a reasonably low-loss dielectric material of suitable relative permittivity to support electromagnetic propagation in an inherently low-loss waveguide mode. In such a mode, the dielectric core (in this case, the alumina ribbon) acts as a surface waveguide: The interaction of the propagating electromagnetic wave with the dielectric core (and thus the attenuation) is minimal because the field configuration of the mode is such that only a small fraction of the electromagnetic energy propagates along the dielectric core while the remaining major part of the electromag netic energy propagates, parallel to the ribbon, through the surrounding free space.
The attenuation coefficient (a) of a dominant mode guided by a simple solid dielectric waveguide surrounded by lossless dry air depends on the loss factor and the permittivity of the dielectric material as well as the size and shape of the waveguide and the electromagnetic-field configuration in the particular mode. The equation for the attenuation coefficient is
α = (8.686p/l0)e1Rtan(d1),
where l0 is the free-space wavelength in meters, e1 is the relative permittivity of the dielectric material, R is a ratio between two integrals that depend on the electromagnetic-field configuration in the particular mode, and d1 is the loss tangent of the dielectric material. The product e1R is of particular significance and is denoted the geometric loss factor.
A systematic study involving computation of a for a variety of dielectric materials and cross-sectional geometries was performed. This study led to the following conclusions (among others):
- A ceramic ribbon waveguide can support two dominant modes with no cutoff frequency — a transverse electric (TE)-like mode with most of its electric field aligned parallel to the major axis of the cross section of the ribbon, and a transverse magnetic (TM)-like dominant mode with most of its electric field aligned parallel to the minor axis of the cross section.
- Unlike the TE-like mode, the TM-like mode is a low-loss waveguide mode as described above. In the TM-like mode, a suitably dimensioned ribbon waveguide made from alumina or other low-loss, high-permittivity ceramic can exhibit an attenuation coefficient of less than 0.005 dB/m.
- Whereas the geometrical loss factor of a circular rod is very sensitive to changes in diameter, that of a ribbon is insensitive to small changes in cross-sectional area. This signifies that the TM-like mode on the ribbon is very stable in the sense that it is not easily disturbed by geometrical imperfections.
The figure shows measured and calculated attenuation coefficients for the low-loss dominant mode in several different waveguide structures, including alumina ribbons with an aspect ratio (width÷thickness) of 10 and three different loss-tangent values. These and other data show that high-aspect-ratio alumina ribbons are suitable as low-loss waveguides, opening up possibilities for the development of communication systems operating in the 30-to-300-GHz frequency range.
This work was done by Cavour Yeh, Farzin Manshadi, Phillip Stanton, Vahraz Jamnejad, William Imbriale, and Fred Shimabukuro 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.
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
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Refer to NPO-21001, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Ceramic Ribbons as Waveguides at Millimeter wavelengths
(reference NPO-21001) is currently available for download from the TSP library.
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