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ground problems
From: Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: bert.hickman-at-aquila-dot-com
Sent: Monday, September 22, 1997 9:11 AM
To: Tesla List
Subject: Re: ground problems
Tesla List wrote:
>
> From: randy-at-gte-dot-net[SMTP:randy-at-gte-dot-net]
> Sent: Monday, September 22, 1997 12:38 AM
> To: Tesla List
> Subject: Re: ground problems
<SNIP>
> > > Hello All:
> > >
> > > I've used soldered-together sections of "chicken wire" as a form of radial
> > > system (counterpoise) for my vertical ham antennas from time to time. It
> > > works fairly well in that application and is quite cheap. Any
> > > opinions/experience from other list members about how this might work with
> > > a TC that needs a grounding system? It's an RF ground, though, not a safety
> > > (green-wire, in the US) ground.
> > >
> > > Geoff (NQ7A)
> >
> > Geoff,
> >
> > Good point, Geoff. I considered suggesting a counterpoise, but it wasn't
> > clear if Mad Coiler's apartment was wood/frame construction or steel
> > beam/concrete. A counterpoise could work very well, especially if Mad
> > Coiler lived on the first floor. Since Mad Coiler lives on the second
> > floor, an elevated counterpoise would still work very well if he was in
> > a steel/concrete building, and less so if he lived in a wood/frame
> > environment.
> >
> >
>
> -- Bert --But, wouldn't he have to live on the top floor of the world's highest
> apartment building for this to be a problem, i.e. an odd-quarter
> wavelenth from the earth ground? At the frequencies in question...
> Randy
> N4MBL
Randy & All,
This can still be a problem, and it gets worse with increased power
levels. In the case of a Tesla Coil (and a vertical antenna), the
purpose of the counterpoise is to serve as a "localized" ground in the
absence of a true ground. A counterpoise is normally insulated from
ground, but physically close to ground. A counterpoise electrostatically
couples to the upper portion of the coil and the toroid, effectively
forming a significant portion of the other "plate" of the coil and
toroid's capacitance.
If the counterpoise is large enough (say 3-4X the toroid diameter and
2-3X the height of the coil), it can be fairly effective. Unfortunately,
in the process, the counterpoise will be raised to a substantial RF
voltage. If the building is concrete-steel construction, the
counterpoise will capacitively couple to the steel framework, reducing
its potential. If the distance to ground is much higher, there's a
degree of risk that the counterpoise itself may tend to arc to grounded
objects or even to other conductive objects (like you...!). Arc-overs
from an energized counterpoise (particularly to grounded objects) can
carry substantial RF current, and may be a real fire hazard. In a
building with wood-frame construction, I'd still suggest grounding the
counterpoise to an available ground for added safety particularly if you
are planning on running above the 500W level.
-- Bert --