Re: Magnetic Field in a Flat Spiral

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

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

[Day to day variation of Q factor]
Ralph Zekelman wrote:
> Can you explain the day-to-day variation a bit more?

There's not much I can say on this subject with any degree of
confidence, except the observations that:
(talking about isolated secondaries here, at small signal levels)

a) Q seems quite hard to measure.  Perhaps that's just my poor quality
gear and lack of patience. If I measure the Q of my big coil several
times over say an hour, I can get a spread of around 10% in the
answers, so I don't trust my own measurements.

b) Measured Q factors always seem to come out considerably lower than
anticipated on the basis of straight line AC wire resistance.  
That means there are other sources of loss.  Some of these will be
fixed, eg the proximity loss, and others perhaps will vary.

c) Some of the possible additional loss causes are: Shunt conductance
from the coil/topload (ie leakage currents to earth);  Ground circuit
resistance;  Tan delta of the coil's surroundings; Tan delta of the
coil former material; Eddy current losses in the ground and other
objects; 

d) I might put forward a guess that those coils which give the lowest
measured Q compared with the predicted Q will also show the most
day-to-day variation.  Really that's just wishful thinking, because
if the 'extra' losses were varying, then we have a hope of tracing
them (by systematically controlling the coil's environment) and we
can hope to bring these extra losses under control.

e) My own experiments have only gone as far as surrounding the coil
base with a large area of aluminium foil - 25 sq metres in order to
virtually eliminate ground circuit resistance. The best I can say is
that there was just a modest improvement in Q, but the results are not
reliable so I've not reported anything.

f) There's plenty of work to be done in this field, and if we were
able to do a proper job of figuring the energy loss budget of a 
resonator, there could well be considerable benefits for CW coilers,
for whom the output voltage is proportional to Q factor given a fixed
input power to a particular coil.  If I could get my coil to even
approach the Q estimated on the basis of skin loss and proximity loss,
then my output voltage would more than double!

> Also, I'm still trying to figure out "gobsmacked."  :-))

That feeling you get when you step outside a nightclub and admire 
someone else's girlfriend for a nanosecond longer than protocol allows.
--
Paul Nicholson
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