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Re: Resonant Frequency



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

[finding a coil whose Fres matches the original wire 1/4 wave]
Steve Young wrote:
> A few coilers have built cone shaped secondaries...

> are there any technical advantages or efficiencies to be gained?

Don't know.  We'll find out.  But there's a dearth of data to be
found on these.   The folks who've built them, did they take any
measurements?  Dimensions, turns, wire size, DC inductance and
resistance, resonance frequencies, Q factor, input impedance?
Often, all we get reported on the Net is just a picture and maybe
a comment about spark length,  which is a waste of a coil, IMO.

Without this kind of raw material, we cannot validate the computer
models,  and without these models,  there's no other way to figure
out what's going on with the voltages and currents on the winding,
except by some very delicate and time consuming measurements.

General appeal to all - when documenting those coils, measure
everything *accurately*, ie don't say your coil's 12" diameter, you
may have intended 12", but in reality it will have come out a little
under or over.  We need lengths measured to 1 part in 100 at the
very least.  Outer dimensions are always more critical than inners,
so try to measure these to 0.1%.   Measure winding resistances and
record the room temperature when you do that.   Don't just measure
Fres, but look for the next two higher resonances - they give info
about the coil's capacitance, and provide a strong cross-check on
any computer model. 

David Thomson wrote:
> Can you come up with some ideas on how I can measure this dual
> cone coil to test the quarterwave hypothesis.

The way to do it is to measure Fres on a number of cones of 
different shapes, so that we can find coils which bracket the
quarter-wave wire length resonance with their Fres. 

Unfortunately, the double cone bipolar, balanced with the center
grounded (by virtue of the capacitance to the primary) is a
different animal to the quarter wave cones we were thinking of. 
Does your double cone secondary come apart in the middle? It
would be nice to measure each half separately.  We need a measure of
Fres, as you did with your flat spiral, along with measurements of
L, R, and the dimensions.  Also turns if possible, but as you've
seen, we can reliably deduce turns from the other data if necessary.

If your double-cone doesn't come apart, then remove the primary and
put the signal generator into one end, leaving the other o/c, so
that you measure a quarterwave resonance [*].

Ed Phillips wrote:
> ...I suspect that at some ratio of height to diameter the
> condition would be met ... However, NOTHING magic happens!!!

Thanks for pointing that out!  This is purely an academic exercise,
to help bury a myth, and to improve our understanding of a wider
range of coils than we normaly look at.  Once it's been wound into
a coil, the original straight-line resonances of the wire are of
historical interest only.

>From related threads...

[Response to noise source as measurement of Q]
> The time resolution of the trace is far too poor for that, as is
> the accuracy of triggering.
OK.  Just that I always seem to find Q measurements hard and fiddly
to do, never get the same answer twice. I'm forever on the lookout
for better methods.  Anyone got any ideas?

[maximising inductance]
Spent an hour or so yesterday modeling spherical coils.  Found the
best inductance for a sphere (for a given length of wire) occurs
when the windings extend from the equator each way for 24 degrees of
latitude - that's enough to drive the cranks wild with excitement :)

[*] To avoid (or maybe cause) confusion, I'll just point out that
the quarter-wave and half-wave resonances are one and the same
'mode' of the coil. It's down to the arrangement of the coil's
distributed capacitance (and our choice of reference potential)
whether we interpret it as a 1/4 or 1/2 wave.  One can be
continuously deformed into the other by a gradual change in the
external capacitance.  What about 'earth' connections you may
ask?  Well from the coil's point of view, when you ground one
end, you're simply altering the capacitance distribution by
connecting a big electrode to one end.
--
Paul Nicholson
--