Re: Flat coils & undamped waves (was Wire Length)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

Why is flat coil geometry significant in this or any other context?  Is
there any electrical or functional difference between using a flat
secondary and a helical secondary, beyond one being much more difficult
to fabricate?

Perhaps I'm not understanding something correctly, but my understanding
of an "undamped wave" is just a waveform that stays at a constant
amplitude, as is the voltage coming out of an AC socket, an RF
transmitter, or CW oscillator.  A "damped wave" is what you get in a
disruptive Tesla coil, where the amplitude begins at some peak value and
diminishes exponentially with time.  Aren't all of these references to
undamped waves just referring to the output of a very high frequency
mechanical CW alternator - a high frequency variant of what powers our
power grid?

Vacuum tube Tesla coils and some non-disruptive solid-state coils do
generate CW, or undamped waves, but Tesla lacked the technology to do
this using active devices.  Did he couple a mechanical high frequency
alternator into a matching resonant transformer, to boost the voltage
and produce a true CW Tesla coil?  Was resonant-rise involved?  While in
theory he had the technology to do this and could have built a CW coil,
the only coil topology he used that I'm familiar with is with disruptive
spark gap technology.  I can't figure out just what he was doing from
the text you cited.  Perhaps vagueness was his intent?

Gary Lau
MA, USA"

   Answer to your question is that "inductors are inductors" no 
matter what the shape and flat spirals have no special and desireable 
electrical properties.  They appear in may of the Tesla photographs, 
probably because they were more photogenic than cylinderical coils of 
wire.  Some of the patent drawings appear to show spiral coils but I 
suspect that was draftsman's license.  I don't think there are any 
flat spirals shown in actual use in CSN, although there is at least 
one picture of same..  Flat spirals were used in some transmitters in 
the "early days of wireless", primarily for mechanical 
reasons.  Examples - they were convenient to mount on large flat 
panels.  Some were mounted "swinging gate fashion" where it was 
desired to adjust the degree of coupling between two different 
coils.  By the time that vacuum tube transmitters came into general 
use (circa 1917-1918) solenoidal coils came to be used 
universally.  In general they have higher Q for the same amount of 
copper and that was more important for CW than for damped waves where 
the circuit Q's tended to be much lower.

Ed