Resonant n>0 Modes Trapped by a Dielectric in a Coaxial Line

Lawrence R. Doolittle
Tunnel Dust, Inc.
Walnut Creek, California, USA
January 14, 2002

drawing of coaxial window

Scaling laws provide an incentive to make a high power window as large as possible, since losses scale inversely with radius, and both heat flux and temperature rise scale inversely with the radius squared. One phenomenon that limits how large a window can be made is the presence of higher order modes in the structure. Conventional wisdom is to limit the average circumference to one wavelength, so that these modes cannot propagate. An analysis of the resonant mode properties of low impedance coaxial windows shows how to design a window to keep these modes from interfering with power transmission.

You may download this Tech note in its entirety in PostScript (187K) or PDF (129K). The PostScript version can be printed on any PostScript printer, or previewed with the Ghostscript suite. The PDF version can be viewed and printed using any of xpdf (font problems), the Ghostscript suite, or (untested) Adobe Acrobat,

You may download a kit (8K) that allows a complete regeneration of the note, including all calculations. This kit is tested on a Linux box with Octave and LaTeX installed. Unpack it, and type "make" to get as far as the .dvi and .eps files. Other choices include "make print" and "make web".

You may look at the individual pieces:

LaTeX source file
Figure 1, Coaxial window geometry
Figure 2, Bessel function ratios
Figure 3, Normalized transverse separation constants
Figure 4, Normalized resonant frequencies for epsilon=10
Figure 5, Longitudinal H_z dependence of resonant modes discussed in the example

Larry Doolittle
January 14, 2002