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Re: Formula for self C of a Coil (not Medhurst)



Tesla List wrote:
> 
> Original Poster: "Barton B. Anderson" <tesla123-at-pacbell-dot-net>
> 
> >
> > Original Poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
> 
> Hi Bob,
> I cannot confirm if the equation is for an isolated coil. I ran a post to the
> list quite a while back and received three responses from Antonio, Mike, and
> Reinhard. I compared each to the Medhurst table and ended up using Reinhard's
> 4th order equation since it appeared the most accurate "to the table". My
> assumption is that they were derived from Medhurst and will therefore include
> the fiddle factor. I did not look at ratios beyond Medhurst - these equations
> probably did not either (since that would require measurements of coils
beyond
> the tables range).
> 
> Bart

	The definition of self or distributed capacitance which was being
taught in "Radio Engineering" course when I went to college back in the
early '40s was that capacitance which, when connected across a coil of
stated inductance, would produce the observed self-resonant frequency. 
That's precisely what Medhurst was trying to determine when he did his
work, and his predictions agree very well with any practical
measurements which I have made or heard of being made.  To quote
Terman's "Radio Engineering", Second edition, 1937:

     "Distributed Capacity of Coils-Every electric circuit and every
piece of electrical equipment has capacity associated with it because
there are always dielectrics separating conductors between which voltage
exists.  These capacities are very often quite small, but at very high
frequencies even a small condenser has a low reactance and so becomes
important.  The stray capacity of an inductance coil is an important
example of this.

	In an air-core inductance coil there are small capacities between
adjacent turns, capacities between turns that are not adjacent, and
capacities between terminal leads.  In addition there can be capacities
to ground from each turn.  Some of the different capacities that may
exist in a typical coil are shown in Fig. 14.  [Picture showing same
thing.]  It is to be noted that every turn  has a capacity to every
other turn and also a capacity to ground.  Each of these capacities
stores a quantity of electrostatic energy that is determined by the
capacity and the fraction of the total coil voltage that appears across
the turns involved.  The total effect which the numerous small coil
capacities have can be represented to a high degree of accuracy by a
single condenser of appropriate size shunted across the coil terminals. 
This equivalent capacity is called either the distributed capacity of
the self-capacity of the coil."

	Notice the use of capacity in place of the "more modern" capacitance
(same thing, of course) "condenser" in place of capacitor, and the use
of both stray and distributed capacitance.  Although some modern purists
(read that snot-nose kids) might want to revise the nomenclature, it is
perfectly understandable and correct.  At the time this text was written
Terman was an accepted leader in his field, so assert that his
definitions should be the standard ones.  Medhurst looked at all of
these effects in his measurements.  Notice also that he uses the
expression "equivalent capacity".  Then, as now, distributed capacitance
was determined by measuring the self-resonant frequency of the circuit
and calculating the equivalent capacitance which, in  parallel with the
inductance of the coil, yielded that same SRF. 

Ed