# RE: twin coil transmission lines (fwd)

```Original poster: List moderator <mod1@xxxxxxxxxx>

---------- Forwarded message ----------
Date: Wed, 30 May 2007 21:06:13 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: RE: twin coil transmission lines (fwd)

At 03:38 PM 5/30/2007, you wrote:
>Original poster: List moderator <mod1@xxxxxxxxxx>
>
>
>
>---------- Forwarded message ----------
>Date: Wed, 30 May 2007 17:44:13 -0400
>From: "Lau, Gary" <Gary.Lau@xxxxxx>
>To: Tesla list <tesla@xxxxxxxxxx>
>Subject: RE: twin coil transmission lines (fwd)
>
>Hi Jim:
>
>Is it really correct to compare the skin-effect resistive losses of the
>transmission line, to the primary reactance, to reach an incremental
>loss percentage?  The primary reactance isn't a loss at all.

No, but I was attempting to find if the loss in the transmission line
is a significant fraction of the overall impedance.. For example in a
classic LC tuned circuit, if the Q is 100, you lose 1% of the energy
in every cycle, and the X/R ratio is 100.

I think other losses in the system will dominate.. the 50-100 ft of
tubing in the primary, for instance, or, more likely the gap.

>In contrast, the AC resistance of various primary coils that I tested
>(http://www.laushaus.com/tesla/primary_resistance.htm) were 0.18 to 0.39
>Ohms for a 67uH primary at 100KHz.  So if the transmission line really
>did add 2 Ohms, I believe that would be significant.

My calculation of AC resistance must be way off, then.. Your
measurement of the 1/4" tubing coil (4" id, 10.5" od 13 turns should
be about 7.75*2*pi*13   52.7 ft...  16 meters..

I would assume that if you had 5 meters of transmission line 2.5 out
and 2.5 back, and it was 1/4" tubing, the Rac would be a third what
you measured (0.21/3 or 0.07 ohms).  I must have massively
miscalculated... it's probably something like 0.02 ohms instead of 2
ohms (since my tube is 3 times the size of yours)  Probably a
centimeter/meter thing.

Makes the transmission line and primary R losses even less of an issue....

I think gap losses are your big loss (and, of course, coupling to the
secondary and power dissipated in the sparks)...  A couple hundred
volts drop across the gap wouldn't surprise me, and the rms gap
current is probably in the tens of amps during the bang.

>Regards, Gary Lau
>MA, USA
>
>
> > Date: Wed, 30 May 2007 14:16:00 -0700
> > From: Jim Lux <jimlux@xxxxxxxxxxxxx>
> > To: Tesla list <tesla@xxxxxxxxxx>
> > Subject: Re: twin coil transmission lines (fwd)
> >
> > ADate: Wed, 30 May 2007 14:54:42 +0000
> > >From: David Rieben <drieben@xxxxxxxxxxx>
> > >To: tesla@xxxxxxxxxx
> > >Cc: drieben@xxxxxxxxxxx
> > >Subject: twin coil transmission lines
> > >
> > >Hi all,
> > >
> > >I have a question regarding the transmission lines between large
> > >twin coil systems that's been bugging me for a while now. I have
> > >never seriously considered building a large twin coil or magnifier
> > >system, mainly due to the large space that one of these systems
> > >eats up when assembled. However, I know that there's also
> > >considerable line losses in the transmission lines that carry the RF
> > >currents from the SG and capacitor to the primary coil(s) and that's
> > >the reason that we usually try to keep the tank circuit wiring
> > >between the SG/capacitor to the primary coil as short as possible
> > >in a basic 2-coil SG system.
> >
> > Maybe the losses aren't all that huge, in comparison to other
> > losses... Let's gedanken this a bit...
> >
> > Say you're running at 100kHz, and you're using 2cm diameter copper
> > tubing as your conductor.  Skin depth is 200 microns or there abouts,
> > and the tube is "large" compared to the skin depth, so we can treat
> > it as a sheet 6.28 cm wide and 200 microns thick, 0.126 square cm
> > cross sectional area.
> >
> > Let's further assume you've got 5 meters of line (2.5 out and 2.5
> > back).  So, the resistance is
> > 1.673 micro ohm cm * 500 cm/0.126 cm^2 = about 2 ohms...
> >
> > Is 2 ohms important, loss wise?
> >
> > Using CV^2 = LI^2 and some plausible numbers, I get a few hundred
> > amps as the peak current.
> >
> > Perhaps a better way to look at it is to compare the 2 ohms to the
> > reactance of the primary at the resonant frequency = L*2*pi*f = 104
>ohms...
> >
> > This implies a loss of a few percent.
> >
> >
> >
> >
> >

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