Re: Primary and Secondary winding direction

["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>

Date: 04 January 2001 14:57
Subject: Re: Primary and Secondary winding direction


Hi Tim, Alan, Pete et al!

>Original poster: "Tim by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <warpath-at-wtp-dot-net>
>
>Hi All, I, too, have heard of the "1/4 wavelength
secoundary winding".
>Could someone explain what this is and
>where it came from? The first person I talked to about
making a coil talked
>of this. Thanks, Tim
>


I think it was Nikola Tesla himself who came up with this
one, though it may have been a chap called Ferranti, as I
can recall reading in an old publication that "the voltage
increase in the secondary is due to the Ferranti rise" says
he paraphrasing wildly from memory.  I suspect that one of
the reasons for this hypothesis is that a hundred years ago,
finding the actual resonant frequency was *not* easy (no
digital meters, no oscilloscopes, no W W V transmissions, no
quartz crystal standards - thus no laboratory frequency
reference; also no copy of Grover's beloved inductance
tables, nor Nagaoka-San's constant so essential to accurate
coil winding) and that the quarter wavelength of wire was a
crude and somehow workable kludge to approximate the truth.

If the wire is not wound into a coil but is stretched out
"on the flat" as it were then it's actually a fair
approximation - at very high frequencies (hundreds of Mc/s)
quarter wave lines known as Lecher lines are still used as a
quick means of roughly locating a resonance or measuring a
frequency.  I suppose years ago the quarter wave assumption
for coils must have restricted the range over which you had
to hunt the resonance.  These days we have it all so easy in
comparison.

Further than this, a quarter wavelength of wire has the
peculiar property of impedance inversion.  If you earth one
end, and measure the impedance at the resonant frequency at
the other end, you might expect that since the wire is
earthed the impedance would be low, but not so.  It's high.
So is the impedance at the top end of a Tesla secondary, the
bottom of which is earthed and therefore of low impedance.
If you have that same piece of wire not connected to earth
at the far end (high impedance) and remeasure the impedance
of the near end at the resonant frequency, it will now be
low.  If the far end impedance is high, the near end
impedance is low, and vice versa - hence "impedance
inversion".  Likewise, the base feed impedance of a Tesla
coil (top end not connected to anything but the top load and
thus high impedance) is low, to an approximation.  I assume
that someone must've noticed the similarity between Tesla
secondaries and quarter wavelengths of wire and concluded
accordingly.

Certainly this impedance-inversion property of quarter wave
wires played an important part in wireless, as it formed the
basis of Professor Slaby's impedance matching technique used
in the German coherer-based receivers of a century ago.  It
also gave us the quarter wave aerial which of course is
still very much in use, and all manner of derivatives
thereof.  For old wireless stuff try
http://www.oldradios.de/

Dunckx