Re: Spaced Coil measurements.

Hi Terry,

> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> Hi Malcolm,
>     I suspect you will find that the 172.6uH value for the inductance of your
> large coil is correct.  Very small inductances are hard to measure due to
> the small inductance and the sensitivity of the self capacitance to the
> surroundings.  In order to measure these small inductances with accuracy, I
> place a 100nF cap in parallel with the inductor and find the resonant
> frequency.  Since I know the value of the capacitor to high accuracy, I
> simply back calculate to find the inductance.  This is basically what you
> were doing by adding your shunt capacitors but I would go for a much larger
> value to insure that only the inductance was being seen.  Adding a large
> shunt capacitance has the following effects:
> 1.  It will completely swamp out the effects of the coil's self capacitance.
>  The small Cself will be far down in the noise compared to a 100nF shunt
> capacitor.

I'm happy that the inductance is around the 172uH mark and that 
Wheeler is a problem. I agree that much larger values of C would be 
definitive and I will try one but am punting that the result will be 

> 2.  It will completely swamp out and RFI from radio and other sources.

The noise was not a problem for these msmts. It just upset the LC 
analyser. I wonder though whether it would give a good inductance 
figure with a large amount of shunt C? Doesn't seem terribly likely 
but you never know. This weekend then.

> 3.  It will lower the resonant frequency far below where wire length or
> transmission line effects would cause error (although that may not be a
> problem anyway).

Doesn't look like it.
>     I also wonder if when you add small (100pF) shunts that just the added
> wires going to the coil are having a significant effect on the resonant
> frequency by disturbing the fields around the coil.

>From past experience I don't think so. However, a much larger C 
should settle the matter. 

>     I have a secondary of about the same dimensions and you 17.8pF value for
> Cself seems right.  That would tend to indicate that the voltage
> distribution along you space wound coil has a sine distribution along its
> length.  As long as that is true, Medhurst's equation should hold valid.
> In non linear wound coils, the voltage distribution is not a sine function
> and Medhurst's equation fails to give the correct values.
>     I suspect that wheeler's equation is giving an error of -14% as you
> suspected.  It may have a problem with highly spaced coils like yours.

I'll have a look at a couple of references and see whether stringent 
conditions are attached. It would be interesting if the (slight) 
differences I saw in the "proper" coils were to be attributed to L 
being modified by the small degree of spacing.

>     In your follow up post, it appeared that the wire length again did not
> seem to influence the resonant frequency.  However, do try to look for
> peaks or other indications around 2.9 MHz of the wire length effects.  Of
> course the velocity factor may not be 1.000 either so it gets messy.

Yes, I should document those. There were a few of them for the bare 
coil and from memory, only one "major" surious resonance for the 
shunted coils within the 6MHz range of the signal generator. 

>     Perhaps you could start with a straight wire along a long form.  Then
> it one turn, two turns, etc. using the same long wire to look at the
> effects.  The long wire should look like any 1/4 wave antenna.  Slowly
> winding the wire step by step, you may be able to see where the effects of
> inner turn inductance take over.  Or even more strange, perhaps the
> resonant frequency of a straight wire is governed more by Cself (and less
> by it's length) than we suspect.

That is a good approach. Time was the killer. I was hoping to see 
something with a single wind but since that hope appears doomed 
(apart from highlighting the L anomaly), This will be the road to go 
down. The approach will be to reduce the coil one turn at a time 
while maintaining the same winding height. It has to be done so it 
might as well be me. Thanks for the useful ideas.