one for the theorist

[Tim Ressel]

Martin,

My book, "Noise Reduction Techniques In Electronic Systems" by Henry Ott, shows
the inductance of a wire above a ground plane to be:

L = (u / 2pi) * Ln(4h/d)    Henrys/meter

where:	u = permeability in free space = 4pi * 10e-7
	d = diameter of wire
	h = distance from ground plane
	
Examples:
	26 Ga wire 1" from ground plane = 0.028 uH per inch
	20 Ga wire 0.25" from ground plane = 0.017 uH per inch

Not a lot of inductance here. Just think of it as a 1/2 turn air-core inductor.

Tim Ressel -- Hardware DQ
Hewlett-Packard
Verifone Division
916-630-2541
tim_r1 at verifone.com



> ----------
> From: 	Martin Czech[SMTP:martin.czech at intermetall.de]
> Sent: 	Tuesday, July 13, 1999 8:58 AM
> To: 	synth-diy at mailhost.bpa.nl
> Subject: 	one for the theorist
> 
> I've recently read a book where the inductance of a single piece of wire
> was given. This is strange, I thought, because you can only speak of the
> inductance of a loop. And, after all what I know, the magnetic energy
> of a single wire without "back" current path goes towards infinity and
> so does L. 
> 
> Contrary a normal twisted pair arrangement makes sense.
> 
> 
> Example:
> 
> If you have a cylindric shaped wire then H=i/(2*PI*r).  The magnetic
> flux will be phi=u*l* int{H(r)dr}
> 
> = i*u*l/(2*pi)*ln(r/r0)
> 
> (u=my, r0 radius, r distance)
> 
> The total flux will need r->infinity and this is clearly also infinite,
> thus the inductance is also infinite.
> 
> This makes sense, you simply need the back current path to have finite flux.
> 
> So, is this a stupid book, or is it me who is the stupid?
> 
> m.c.
> 
> 
>