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Re: Coupling Questions & coil measurements
Ross, Terry, All
I have always wondered what the k of my system is. However I have only very
basic equipment to test it with - an analog meter, a home-made RF probe and
a few bits with which I could make a GDO. Any hints on an easy way I could
measure k with these parts?
Ross, as for your question about 'double humping', this occurs when
coupling is more than 'critical', ie 1. As coupling is increased the two
peaks of resonance will separate further apart, and I think (correct me if
I'm wrong) the overall system Q is reduced. This phenomenon is often used
in RF bandpass filters and IF circuitry to ensure that there is sufficient
bandwidth to either cover a whole amateur band or to make sure that the IF
is wide enough to recover the original audio signal on demodulation. There
comes a point in Tesla coil use where too much coupling will shift a high
voltage point somewhere down the secondary where it causes a band of
inter-turn arcing. Naturally also you are inviting racing arcs and strikes
to the primary which can destroy your secondary in short order, and losing
quenching, particularly on single-gap systems. A heavily blown series gap
like I use seems to be happy with a fair bit of coupling though.
Alex Crow
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From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com
Subject: Coupling Questions & coil measurements
Date: 09 August 1999 01:41
Original Poster: Ross <ross-o-at-mindspring-dot-com>
Hey,
I have a few questions about coupling. I have measured the resonant
freq of the primary and secondary according to the technique described
in Malcolm's Electronics World (*) paper. I checked the coupling using
the technique where you run 5-10A of 120VAC through the primary and
measure the voltage across the secondary (**).
In my reading, I have found several references that discuss "Magic K"
values. Brent's book explanation says that Magic Ks relate to even
integers (multiples) of "RF cycles". That makes no sense to me because
"RF cycles" are different for each coil and it's unique Fsec.
The paper "Tesla Transformers" by Von Werner Heise also describes
"values of K to strive for". One of the magic K values is 1.33. I have
helped measure K on 3 well performing coils in the last week and all had
a K value of almost exactly 1.33!?! The coils varied in size from 3 ft
long secondaries to secondaries as tall as me. I may be
misunderstanding Mr Heise, but it seems that his argument is based on
first notch quenching.
Can someone explain why some of these K values are supposedly better.
Has anyone seen evidence of superior performance after moving from a sub
optimal K to a "magic" K? In other words, is it an artifact of the math
or does it really matter?
On a similar topic, I'm not sure that I understand the "double hump"
response. Can someone please describe how the resonant freq of the
primary/secondary system is different from the resonant freq of either
circuit measured independently and how this relates to coupling?
Once I get more of this figured out, I hope to devote a web page to
proper Tesla Coil measurement techniques and interpretation. I intend
to discuss it in layman's terms and then spend a little time describing
the differential equations and showing how a few things are derived (for
us engineer/math types). If anyone already has a page like this, please
let me know. Also, any references to this type of information is
appreciated.
(*) Electronics World + Wireless World, March 1995
(**) I will drop this on my page at
http://www.geocities-dot-com/CapeCanaveral/Cockpit/3377/tesla/measure.htm
Making arcs in SoCal,
--
Ross Overstreet
Huntington Beach, CA
ICQ 20762411