Re: Primary question / Terman

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

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