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Re: Propagation velocity in long helical coils.
Hi Terry, Bob,
On 15 May 00, at 19:16, Tesla List wrote:
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
> Hi Bob,
>
> Sorry about not getting back to you on all this. With the holiday, list
> problems, work.... I just didn't get to it.
>
> I guess the equation you found for Fo from your May 10th post is:
>
> Fo = 1 / (4 x SQRT (L x C))
Thanks for posting that. I lost it along with other valuable material in
a server hiccup.
> Where C is the "true" capcitance of the coil with the top and bottom
> shorted (no resonance). Is this correct? I will set up a ground plane and
> try to do this test in a large room to get an accurate true C measurment.
> I am still doing some left over poly cap work tonight but I'll try this on
> my two well known coils ASAP!
Mulling over that makes me wonder:
- True Cself for the coil is higher than Medhurst's supposed value.
Correct?
- The recipe approach to determine 1/4 wave Fr is
1/(2PI.SQRT(LC)) where C is Medhurst's Cself. Correct? (well it
always works).
PI seems to be missing from Bob's formula. Should it be? The
formula in the form given would give Fr lower than when the coil is
1/4 wave connected since the denominator is smaller. (Well
*maybe* it isn't if C (true C) is considerably larger than Medhurst's
C). I would have expected a 1/2 wave type resonant mode with Fr
considerably higher than the 1/4 wave mode with the ends shorted.
In fact I would expect that it is greater than 2Fr (1/4 wave) because
no end capacitance effects are present.
Have I got this right or am I missing something? OK - I'll check
it on a coil this evening and derive C from the formula and measured
Fr.
Regards,
Malcolm