Jim Lux wrote:
Ian White GM3SEK wrote:

So.. Terman's equation probably holds for coax where the inner
conductor is 20 skin depths,
Sorry, Jim, you lost me: why such a large number as 20?
At 2.5 skin depths, the current density is 10% of the surface
value; at 5 skin depths, 1%. If at least 5 skin depths are
available, we can be confident in the accuracy of the standard,
uncorrected equation for most purposes.


But it's round... (unless Terman rolled that into his constants)

I don't believe so.

Consider if you peeled that 2.5 skin depth layer and made it flat. It
would look like a pyramid, not a rectangular bar.

I see your point, just wouldn't have imagined that the circular geometry
would make such a very large difference compared with a flat surface.


Of course, if you assume that the cross sectional area is an annulus
(the pi*( r^2-(r-skindepth)^2) style calculation) this partially gets
taken into account.

The other factor is that in a wire that is comparable to skin depth in
radius, the current on the far side of the wire also contributes to
squeezing the current towards the near side surface. (and that's why
the actual math gets hairy.. you can't use a simple exponential
approximation for the current density)

Agreed. I do have the more detailed equations involving Bessel
functions, but no time to compute them just now.

At 20 skin depths, the difference is negligble.

No doubt; but it's the journey towards "negligible" that concerns us
more than the destination :-)


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek