# [TCML] New 3.5" Coil - A kick in the right direction please!

bartb bartb at classictesla.com
Fri Sep 19 20:48:48 MDT 2008

```Hi Grant,

Something that caught my eye is that you "want" the secondary wire
length at 1/4 wave. I hope this is only a "want". A coil of course does
not require this for best spark performance. If you "really" want the
secondary wire length to be 1/4 wavelength (such as antenna theory,
will capacitively load the secondary significantly and change the
distributed capacitance affecting the coils frequency (far from the wire
length). Spark propagating coils are tuned to their fundamental
frequency which may be close or far away from the 1/4 wire wavelength
situation. The wire length has no bearing on the resonant frequency. The
resonant frequency of a coil is a composite of all the capacitances
(objects) which affect the coil as well as the inductance. But it
doesn't stop at simply object capacitances causing the change. With
increasing frequency and with coil geometry (h/d ratio, spacing ratio,
etc..), the currents along the length of the coil are also affected.
This causes variations in inductance along the coil. If you could
measure the inductance of each turn say on a 1000 turn coil, not all
turns would measure the same inductance (even though the wire size,
geometry, and length are the same). The current affects the emf and thus
mutual inductance to adjacent turns, which in turn affects the
inductance of each turn itself. If the current varies, so does the
inductance. But if you calculate those currents along the form at the
fundamental and lump the inductances, you'll find the "real" inductance
at the fundamental frequency (which is different than say a DC
inductance or even 1kHz LCR inductance measurements). By the word
"real", I mean the inductance which is affecting the fundamental
frequency. This situation also affects the 1/4 wave wire length
precursor because of the inductance at high frequency (which is any
frequency above DC which adversely affects the inductance). So even our
lowish frequency 100kHz coils will have an effective inductance at the
fundamental which will be different than the wire inductance.

By all these "words", I'm simply saying that to actually make a 1/4 wave
wire length equal to the fundamental is no small task. Wheeler and
Medhurst equations won't help here. To do this, you really need a bare
secondary by itself. Even a primary coil is seen as an object to the
secondary that will change the situation. Coil height to ground, walls,
and miscellaneous objects do the same. But to do it in a dual-resonant
coil situation can only be done best with just those 2 coils and nothing
else. Even using a little small sphere as a top load (to reduce Ctop)
affects the coil a tad. It can be done that way, but if you want a spark
performing coil, then this isn't the way to go about it. It causes many
limitations to the coil design which affects performance.

If we ignore the 1/4 wave length wire theory, then we really only need
to worry about the fundamental frequency regardless of the objects on or
around the coil. This type of design makes for good sparking coils
because we now have freedom to play with primary LC values to match
whatever fundamental frequency the secondary coil ends up at.

So it's hard to give advice. If your set on "1/4 wave wire length" as a
precursor, then you have your work cut out for you. Is this really your
train of thought on this coil?

BTW, 9 to 12 turn primary? No. This is only a "common" value we tend to
end up at. Some coilers use over 20 turns and a few only use 2 or 1.
What is meant I think is that higher turns adds impedance which helps
attenuate the voltage on the primary for less problems and may even help
with quenching. But no one is saying this as though it is wrong to use
less than 9 turns. Simply that "some impedance" has been noted to help
with gap quenching.

ALSO, coupling you mentioned. No, coupling is fully adjustable. It's
just that you "can" make a coils coupling to whatever you want and still
have it even with the bottom secondary winding. For example. Suppose a
coil builder is use to building a set distance from primary to
secondary. And if the primary is even with the base, he usually ends up
about 0.13 k on the coupling for the same coil size. Now say he lowers
the primary to reduce the coupling and ends up at 0.11 which he finds
best. Well, the primary itself could be redesigned so that when the
primary is even with the sec base, he ends up at his found 0.11 ideal
value. Thus, he ends up not needing to adjust the coupling. That's all
there is to it. But adjusting the primary height is also perfectly
acceptable and usual (since coilers don't seem to know what works best
for a particular coil geometry).

Best regards,
Bart

Grant Visser wrote:
> Hi there,
>
> Suffering from sleep deprivation or 18 hour research / calc / sim /
> research .... info overload at the moment, or BOTH! lol
>
> With my limited budget I am on a mission to get a solid base design
> into my head before I begin throwing things together as I did last
> time! *grin*
>
> The problem is that I have now done so much reading of content from so
> many different viewpoints (over the years) that I have confused myself
> COMPLETELY! Thus, I humbly beg your assistance ... or at least just a
> darn good kick in the right direction!
>
> I have in front of me 26.06" of 3.543" Plexiglass tubing with 3mm
> (0.1181") wall thickness (and don't want to waste a single mm of it!)
> lol
> Assuming 220V 50Hz mains and 2x 9KV/40mA NST's to power my new
> coil(/s) (could add another 2 very soon!!!)
> 3Kg of Double insulated magnet wire about to be ordered, ex works in
> either 0.335mm (0.013188"), 0.355mm (0.013976"), 0.375mm (0.014763"),
> or 0.4mm (0.015748") OD
> Have tried my best but am not going to be able to afford the (less
> than ideal) caps for a solid MMC so have decided that I will build a
> quality bank of vertical plate caps for this coil. They'll never go to
> waste!
> I have also decided (I think) to go for a balanced split cap tank design.
> Until I can find a decent blower I'll be using my current suction
> quenched adjustable 2 gap design.
> 1/4" copper tubing for the flat pancake primary coil
> Oh, and yes I do now have a SOLID and dedicated RF Ground.
> I do also have 3phase at my disposal .......
>
> Although it didn't even cross my mind initially, one of my biggest
> stumbling blocks at this point is at what frequency I am going to run
> this coil. This quandry started when I read a few posts from awhile
> back regarding the primary winding and that it should have no less
> than 12 to 15 turns and tuned at no less than 9 turns, and ensuring
> that the final diameter or the primary is the same or close to the
> winding height of the secondary! Apart from the fact that it actually
> made sense to me from a coupling point of view, it also meant that I'd
> be able to use a smaller main cap, so I was very happy right up until
> I started doing the math / sims and it meant that I was going to have
> to use a "SMALLER THAN RESONATE" capacitor which, seems to be an
> absolute no no if I have understood the numerous other posts and
> articles I have been reading!
>
> A further brainmangler that has crept in now is the other concept that
> I had forgotten about ..... and that is that I would want to design my
> secondary to have the length of wire I need to resonate with my
> primary at a 1/4 wave! ie. At 220KHz the 1/4 wave is at 1118.1818ft
> and I should thus use exactly that length of wire to coil my
> secondary. Again this all makes perfect sense ....... right up until I
> start doing the math and the sims ....... at which point, depending on
> which program I use to model my coil, either the program yells insults
> at me, or I simply cannot find a workable solution which allows me to
> use all of the data above AND have a resonate coil with a decent hd
> ratio! Going much below 220KHz means that I would have to increase the
> length of my coiling wire to seemingly crazy HD ratio's!
>
> Now I do absolutely want to achieve the longest possible sparks from
> this coil and I would like it to be an unstressed system allowing long
> run times. I would also however like to use this coil to begin
> demonstrating and researching the concepts of powering up light bulbs
> etc from the ground waves for which I am going to have to construct a
> top load with no breakout! With this in mind I was hoping to either
> split the 26" form in half allowing me to coil two seperate identical
> secondaries and use the second in a magnifier configuration. OR .... I
> was even considering (but had not yet begun much research) using the
> form for a BIPOLAR coil!
>
> I do realise that 3.5" is not exactly a MEGA COIL but it did seem to
> me that it could be a nice little testbed to get me started with some
> of the more interesting experiments! ???
>
> SO! Thats where I stand at this point! More confused than ever and
> really hoping that you will be able to help me back onto the straight
> and SPARKY!
>
> Grant Visser
> Randburg, South Africa
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```