Re: Self capacitance and Medhurst

To: tesla-at-poodle.pupman-dot-com
Subject: Re: Self capacitance and Medhurst
From: Tesla List <tesla-at-stic-dot-net>
Date: Thu, 20 Mar 1997 01:24:56 -0600
Approved: tesla-at-stic-dot-net

Subject: 
            Re: Self capacitance and Medhurst
       Date: 
            Thu, 20 Mar 1997 15:36:08 +1200
       From: 
            "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization: 
            Wellington Polytechnic, NZ
         To: 
            tesla-at-pupman-dot-com


Steve wrote....

> I have seen two methods of finding the self capacitance of a coil.  One
> is closed-form:
> 
>         V = L/R  (in inches)
>         CD = 5.08*R*(.0563*V+.08+.38*SQR(1/V)) 'uufd.
> 
> And the other is table-lookup, attributed to Medhurst:
>         
>         C  = K x D      (D in centimeters)
>         
>         H/D       K
>         5.0     0.81
>         4.5     0.77
>         4.0     0.72
>         3.5     0.67
>         3.0     0.61
>         2.5     0.56
>         2.0     0.50
>         1.5     0.47
>         1.0     0.46
> 
> These two methods agree at the low end of aspect ratio, but begin to
> diverge at the high end, with the closed-form yielding the smaller
> values.
> 
> Does anyone on the list have a feel for which approach is more
> accurate?  Does anyone know where the closed-form originated?
> 
> Note: I have played around with gnuplot, and can easily come up with a
> closed-form which matches the table points much more closely.  But I'd
> like to know if the closed-form above is better than the table, or is
> merely an approximation of the table.
> 
>         Steve Falco
>         sfalco-at-worldnet.att-dot-net

I have found Medhurst to be reliable for all the coils I've tested it 
on (quite a few) to within a few percent. It is conditional on one end 
being grounded though. It cannot be used to predict Cself for an 
unearthed halfwave coil. We need to develop or find another one for 
that.

Malcolm