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>

Dave wrote:
> My impression is that HAMs used to work Spacelab with
> 1W ish Walkie talkies?

No problem there, at VHF/UHF.  My comment referred to TC
operating frequencies.  My 400Mhz receiver happily picks up
the minute radiation from a tree, simply because the tree,
at 300K is hotter, therefore brighter at 400Mhz than the
cold galactic background of space at about 50K.  Down at TC
frequencies, a receiver sees the diffuse underside of the
ionosphere: a bright, chaotic light, like a disco held in
a hall of mirrors.  Flashes of light, 10 or 100 per second
coming in from all directions, reverberating around to
produce a steady cacophany of 'atmospheric' noise from a 
receiver at TC frequencies.  No chance of spotting that
tree at these low frequencies.  A powerful TC really has
a struggle making itself heard against this strong
background.  mW for mW, the VHF/UHF radiation is of more
concern partly because of the lower background and partly
because the TC contains more structures that are
efficient radiators of it.

10mW of VHF or UHF escaping from a TC could be picked
up tens of miles away, so long as most of that 10mW fell
within the passband of the receiver. If the VHF is being
created by the arc dumping energy into the natural resonances
of the primary wiring, then we'd expect it to be broadly 
peaked.  I wonder how wide these noise peaks are on the
spectrum, compared to typical comms bandwidths.  If they
were several Mhz wide, they would be of most trouble to things
like TV reception, which requires a similar bandwidth,
and relatively invisible to say, narrow band voice comms.

Commenting on a subject mentioned in a related thread,
I don't see much point to constructing an artificial
ground plane (counterpoise) when you're operating directly 
over the ground.  My tests show a slight increase in Q
but nothing that justifies major engineering.  Ok, that's
over a damp lawn.  Your mileage may differ if you're out
in the desert.  From the point of view of interference
it's important to return ground currents to the coil base
by the shortest possible route.  You may be tempted to
run a wire from the coil base out to a good stake that
happens to be available 30 feet away.  Ok, you get a 
good ground, but it forces the RF currents to travel
the 30 feet to your ground terminal in order to find
their way back to the secondary base.  Thus you have
grafted a (poor) loop antenna onto your coil, in series
with the base connection.  The other downside of a long
ground lead is the temptation to hook other things
onto it, on the basis that it is, so to speak, earthed.
But this merely ensures that the items attached are placed
at some RF potential with respect to the distant earth,
and probably with respect to the nearby earth too.

I'd rather connect the coil base to a tent peg beside
the coil than a good stake 30 feet away.  That helps to
reduce the area over which RF ground currents are present,
helping to reduce EMF induced in nearby equipment,
peoples feet, etc.  I tend to use a bunch of small stakes,
circled around the coil.  Not very deep - it's surface 
current we're collecting, so no point in hammering down
to the bedrock!  I water the lawn.  A large foil sheet
counterpoise only does marginally better than this (in
terms of Q factor).

Not much need for big hefty conductors, so long as
your're only thinking of carrying a few tens of amps
at a few hundred kHz.   A different matter if you plan
to strike arcs to ground terminals.  Consider how many
kV can be induced across a bend in a wire when you're
taking a few hundred amps discharge current at a 
few tens of Mhz.  Then, low self inductance of the ground
wiring becomes a criteria.  The very high arc currents are
discharging the field set up by the coil and topload, which
is localised (hopefully) in the close vicinity of the
coil.  The discharge currents flowing from the grounded
terminal receiving the arc will be trying to return to the
patch of ground (or counterpoise) beneath and around
the coil.  Not a good idea to make those currents travel
a circuitous route, for safety if nothing else.  The lead
from the secondary base to the (nearby) ground terminal 
doesn't need to be a thick one, but those coming from
discharge terminals will need to be wide, straight, and
take a short route back to the central ground terminal,
for safety, and to minimise the area of the loop around
which the discharge current flows, thus minimising radiation.
--
Paul Nicholson
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