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From: Bill KT4YE
Date: 23 Mar 2002
Time: 20:17:55
Remote Name: 12.93.229.78
Thanks David, for the information.
A question and an observation...
How and where was the Epsilon= 5 dielectric material applied? Ie, was it applied over the entire antenna or a selected portion? Was the antenna element "dipped" into the material or was it "painted on" in the form of a slurry? What was the nature of the dielectric material. Powder? Plastic? Were there significant voids in the material?
On lower frequencies, such as 80 M, I have built antennas using insulated wire. They have always been a percent or so shorter at resonance than the same antenna -- using the same gauge wire -- with bare wire. The thickness of the insulation/dielectric -- as a percentage of a wavelength -- is MUCH smaller than with the example cited. And the Epsilon is, I'm sure, lower than 5.
It would appear that a bit more empirical evaluation is in order.
Also, It seems that the application of high dielectric materials to antennas has some kinship to the problems associated with building a microwave "window" in a waveguide. These windows are used to let RF out of a klystron/TWT etc. without "letting all the vacuum escape." I used to build those kinds of things about 40 years ago, and my notes are all long gone -- as is my memory of them. But many of the situations seem similar: A (relatively) high-dielectric discontinuity in an otherwise homogeneous RF field.
Cheers, Bill