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From: Bill KT4YE
Date: 16 Apr 2002
Time: 22:27:40
Remote Name: 12.93.229.252
Sometimes when I think about something long enough...
In the early 60s, when I worked in Eimac's Microwave Tube Lab, we noted that adding dielectric supports to coax hardline caused disturbances in the SWR.
To minimize these disturbances, we would routinely undercut the diameter of the center conductor where the dielectric was mounted. I believe this may still be common practise.
The effect of the dielectric was to make the conductor "act like" it had a larger diameter than it really had. There were some tech papers published then (proceedings of something or other) that related dielectric constant to "apparent diameter" but I have long since changed careers and no longer have access to this kinda stuff.
Anyway, perhaps thin (as a percentage of WL)coatings of dielectric do not act to SHORTEN the antenna; merely to FATTEN it!
Thoughts? 73 Bill