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Re: Propagation velocity in long helical coils.
Hi Malcolm,
>Original Poster: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz>
>
>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?
Correct infact I can now say from ciruit analysis 3x Med for very short
coils at greater than 10x fr, 2x Med for long coils greater than 10x fr and
2.46x med (or (PI**2)/4 for long coils at fr 1/4 mode excluding end
effects etc.
>
>- 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).
Correct for isolated coils and may only be for greater than 10x fr.
Which is what Med used it for.
>PI seems to be missing from Bob's formula. Should it be?
Nothing missing, no PI
> 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.
The ends shorted are just for measuring the C.
The equation is for an isolated 1/4 wave mode
resonating coil.
>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?
See above
>OK - I'll check
>it on a coil this evening and derive C from the formula and measured
>Fr.
You need to measure L and C and fr. then compare Med f measured f and Jones
f.
I will be posting on a new thread on Med C and top loads.
>
>Regards,
>Malcolm
>
Regards Bob