Re: Halloween Coiling and the FCC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Hi All,

The RF ground lead is the most likely source of excess radiation. 
For some reason folk believe that hammering in loads of huge
earth stakes will automatically ensure a good ground for the coil,
but if the ground lead has to travel from the coil base down three
flights of stairs, through the garage and out to the yard, that
doesn't really do anything for the coil, except to give it a
reasonably good antenna.

I would always advocate confining the E-field close to the coil,
by using an artificial ground plane beneath the coil, if it can't
be operated directly over the earth.

Imagine a line of the E-field coming off the topload and bending
down to meet the ground alongside the coil, from which point the
RF conduction current makes its way back to the coil base. The RF
current thus forms a loop.  The radiation is roughly proportional
to the area of this loop, so if you unwittingly extend it, eg by
taking the coil base connection out into the yard, (thus forcing
all the return current from the coil's E-field to travel out to
the far end of the ground lead before returning), then you
increase the radiation quite a bit. 

To pick up a TC at 20 miles is really some feat!  At TC
frequencies, the background RF noise against which the TC signal
is received is quite high, so a fair bit of signal must be being
radiated - rather more than we would expect from the coil and
topload alone.  Can't put figures to it without more data, etc,
but there's probably something not right about the grounding
arrangements.  Other possibilities are that a nearby overhead
line is catching and propagating some of the field.  Either way,
if you're detectable at 20 miles, you could invite problems.

Effect on neighboring TVs and radios is more likely to be due
to RF getting onto the mains wiring, as with ham equipment.
That ought to be fairly easy to sort out at source.  At all costs
try to avoid RF getting onto the mains, eg when the coil is out
of tune.  Apart from the interference there's the chance of
starting a fire in an unexpected place.

Radiation is possible from
a) the secondary resonances (up to a few Mhz); Ensure a short
return path for the E-field back to the secondary base.
b) from the primary resonances (up to 10 Mhz or so);  Keep primary
wiring compact, use parallel line if long runs are needed.
c) from the spark gap, which with its wiring forms a nice
center fed dipole (up to several 10s of Mhz);  Fit ferrite beads.
d) from arc discharges ie the resonance of the arc loop (up to
VHF); Tricky.  Use short ground return path for discharges.
e) from surface resonances of the topload while breaking
out (VHF/UHF);  I don't know how much of a problem these could be,
but some quite large current pulses seem to accompany leader
formation, and they are of a duration short enough to excite the
surface resonances of a toroid.  It would be nice to try to 
relate a measured spectrum to the dimensions of the topload.
(should peak at a wavelength a little longer than the circumference
of the toroid).
--
Paul Nicholson
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