Re: EMI and Tesla Coils, calculation of

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>



Tesla list wrote:
> 
> Original poster: "Dr. Duncan Cadd by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>
> 
> Hi Jim, All!
> 
> Yet again, apologies if this is a repeat.  I promise to feed the
> server less cucumber in the future . . .
> 
> >Original poster: "Jim Lux by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>
> >
> 
> <snip>
> 
> >We're probably not up to that current, but, if you figure 1000 amps,
> a
> >voltage drop of 10 kV/meter might be reasonable, or even 100
> kV/meter, for
> >10-100 ohms/meter.
> 
> Well, I've substituted 100 ohms per metre and Strange Things Happen.
> The power input goes _down_. Not a little, two orders of magnitude . .
> . still using 500kV excitation.  FWIW, the efficiency rockets, now
> around 0,8% of the energy is radiated, a whopping 30W.  Input power is
> now a mere 3,6E3 watts and electric fields 0,35V/m horizontal and 4V/m
> vertical at 10m.  The input impedance for the previous (722k per
> metre) model was Z = 1,45E5 ohms -j3,7E4 ohms; this model gives Z =
> 2,7E1 ohms -j3E4 ohms.

Much more like what I'd expect..   With the very high spark resistance,
you're basically just looking at the secondary as an antenna.. a terrible
one.  With the lower resistance, it's not a great antenna (-20 dB
integrated over the hemisphere..) but, it is starting to radiate.  And, 30W
of radiated power is starting to seem more realistic.  Even at really high
pulse powers, you're not going to radiate a lot of power with such a short
antenna.. the radiation resistance is just too low.

As an interesting exercise, you might put a small loop in the "near field"
( say, 4 50 cm wires in square about 10 meters away), and see what sort of
currents are induced.  In the near field, the energy is in the magnetic
field rather than the E field.  Get very far away and the E field
contributions from the various parts of the system all cancel (why small
radiators are inefficient).

> 
> Me no understand . . . I am all the more puzzled because this of all
> things should be least problematic for NEC2.  Perhaps it's saying that
> with a low resistance loading, most of the power simply goes to earth
> and cooks the soil/concrete.

Exactly... (that, and heating up the secondary and primary..) Intuitively,
you know that the spark isn't a great radiator (it's too small) If you do a
thermal balance on the spark (i.e. figure out how many joules it takes to
heat the air up to 10,000 degrees)(1m * 0.1 cm diameter = 0.7 cc  = 1 mg of
air = .24E-3 cal/deg = 10 J) you can see that a fair amount of energy is
going into heat.

> 
> I have a nasty feeling this is only going to be resolved by sticking a
> "spark" onto the model of Skip's coil.  Small computer, big headache
> ;-)  More reading of manuals.

Why don't you post your NEC model file...?
> 
> Dunckx