Interesting Gary, that you should mention this phenomenon.
It is the reason that shafts connected to devices such as variable
capacitors and roller inductors should be made of a nonconducting material
in a match box.
They why of it is as follows:
The shafts, traversing the grounded bushings in the front panel will, over
time, act as the center conductor of a transmission line as the shaft and
bushing gradually develop a less than perfect connection due to repeated
movement, metal fretting. The symptom of this will be that the apparent
tuning of the box will change as you touch the knobs.
We'd see this with large high power matchboxes used on semiconductor
manufacturing equipment using rf energy to perform some of the processes.
The energy (many kW @ 13.56MHz.) would leak out into either the surrounding
space with manually controlled boxes or into the motor / control area with
eventual loss of control as the rf got into the control circuitry.
With the environmental / personnel concerns of high levels of rf in the work
space you can see why it was important... like a leaky microwave oven door.
----- Original Message -----
From: "Gary Schafer" <email@example.com>
To: "'Dan Schaaf'" <firstname.lastname@example.org>; "'David Gilbert'"
Sent: Wednesday, December 28, 2011 4:43 PM
Subject: Re: [TowerTalk] Trees and Verticals
> Again, see "waveguide beyond cutoff".
> No signal will ever get inside of the tube unless there is a conductor
> running out the opening to couple it in. then the signal will end inside
> shortly after reaching the end of that conductor.
> Gary K4FMX
TowerTalk mailing list