Re: Primary question / Terman

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

Original poster: "Mark Fergerson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <mfergerson1-at-home-dot-com>


  Duncan, I think you and Terman are right. Here's why, and
practically without math:

  "Skin effect" and strap edge preferential conduction (Terman
effect?), if it exists, are both consequences of the fact
(semiclassical view) that electrons repel one another. With a
cylindrical conductor, they'll accumulate at the surface. With a
non-circular-cross-section conductor, they'll all show up as far from
the center of the conductor as possible.

  Mark L. Fergerson

P.S. There's also the field discontinuity (high voltage sense) at the
edge of the strip, which might increase the possibility of arcing from
the high edge of the primary to the secondary. Increasing the P/S
radial distance as much as is consistent with good coupling should fix
that.

> Original poster: "Dr. Duncan Cadd by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>
> 
> Subject: Primary question
> 
> >Original poster: "Chris Brick by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <cbrick-at-rebelbase-dot-com>
> >
> 
> Hi Chris!
> 
> >I am building a new coil and would like to use a copper strap primary as
> >opposed to copper tubing.  Are there any major concerns I should be
aware of?
> >
> >Corona issues from sharp strap edges?
> 
> My first coil (now semi-retired but still usable) had a primary from half
inch
> brass "boiler band" from a model engineer supply company, it was maybe 1/32
> thick.  It worked very well and I didn't have any noticeable problems
with it.
> I suppose it will depend to some extent on how sharp is sharp.  But also,
there
> will be some degree of electrostatic shielding due to the potential of the
> neighbouring turns so it may not be as sharp electrically as it is
> mechanically.
> I separated the turns with corrugated paper of maybe an inch wide and an
eighth
> thick, so the dielectric properties of the paper will also tend to reduce the
> local field (a bit lossy though, although most of it is air).  But the
> corrugated paper did stop the turns sparking to one another - you can
often get
> enough volts per turn to flash over with the close spacing of a compact strap
> primary.
> 
> >Decreased field due to more compact design?
> 
> If more compact, I expect the field is greater.
> 
> >Changes in coupling due to tighter field?
> >
> 
> More coupling if coils are closer as a consequence of compactness.  But
you can
> always move one of the coils up or down to compensate.  Not a problem, unless
> the inside turn is too close, mine was spaced around an inch and a half.
> 
> >Also, what type of tap connections you used?
> 
> I soldered a thin wire onto each turn, staggered around the turns to increase
> the spacing (i.e. tap every 0.9 or 0.8 of a turn) and made do with that -
> connect with a croc clip.  If I wanted finer tuning, I used a roller-coaster
> inductor in series.  You can then - with a long plastic handle - tune whilst
> running like Finn's coil.  The only disadvantage there is the roller-coaster
> isn't coupled to the secondary, so the coupling changes with the tuning, but
> then it will do that with a tapped coil anyway.  With a tapped coil, more
coil
> equals more coupling.  With a roller-coaster, more coil equals less coupling.
> Split the difference and keep the coupling the same ;-)
> 
> Seriously, it won't make much odds.
> 
> One thing puzzles me about strap primaries.  Dear old Frederick Emmons
Terman,
> ScD, bless him, says that flat strip conductors are very poor at radio
> frequencies because the current travels along the two edges ("Radio
> Engineering"
> p20) where it is constrained by the magnetic flux, and he uses this to
> illustrate the point that it isn't the surface area of the conductor which
> matters.  Everyone else I have read says (sometimes with heaps of partial
> derivatives and Bessel functions) that the current travels on the surface and
> hence flat strip is very good.
> 
> [Pained hush prior to near-heresy . . .]  Could Terman be wrong?  Or have the
> mathematical whizzos missed some fundamental physical reality hidden in
amongst
> the mathematical mud pies?  I'd like to believe Terman and I can see his
> logic -
> if you take a cross-section of a circular wire, the current flows along a
thin
> ring at the circumference (skin effect) so if you take a slice across the
> diameter, the current will be along the two opposite edges.
> 
> At what density of current flow, frequency, thickness, width and
resistivity of
> strap does the magnetic flux no longer hold the current at bay at the two
> edges?
> It will be connected with the penetration of the flux into the strip, and the
> linkage of that flux with the current I know.  A.G. Warren (chapter 17,
"Bessel
> Functions" in "Mathematics Applied to Electrical Engineering", Chapman &
Hall,
> fifth impression 1946, volume 9 in the "Monographs on Electrical Engineering"
> series, editor H.P. Young) effectively says Terman is in error, but he
assumes
> the current to flow on the faces of the strip and then proves it, if you see
> what I mean, and it seems to me that his model could apply just as well
if the
> current was flowing at the edges.  Answers on a hyperbolic function please. .
> 
> Chris, just go ahead and do it.  Never mind the academic pedantry, my brass
> strip primary worked just fine.
> 
> Dunckx