NEC-LIST: Wave speed on wires (is it c?)

As Ian Flintoft wrote:
Sender: davem

> If we are talking about the propagation of a surface wave along a
> single wire then I believe the mode (a TM mode) only exists for
> finite conductivity and is loosely bound to the surface. The
> velocity of propagation is less than c. E.g for a copper wire at
> 3GHz and wire radius of 1mm there is something like a 0.004%
> reduction in velocity. I think this reduction goes to zero as the
> wire radius goes to zero but I'd have to check.

Now for a perfectly conducting wire in free space, the speed of this
mode becomes exactly c, the fields become: H=I/(2*Pi*r) with I the
wire-current and E=Z0*H (radial). It is just the field of a coaxial
cable with the outer conductor's radius moved to infinity. Therefore
it is a TM solution, which for all frequencies travels at c.

These fields both fall of as 1/r and the energy as 1/r^2, so for
instance 3/4 of the energy is within 3*r of the wire surface, and 99
percent within 99*r. So we can call it a loosely bound wave. An
interesting question is what happens if the end of the wire is
reached: how much remains bound (is reflected) and what is the
radiation pattern of the part that flies off?

Greetings,
Jos

----
 Jozef R. Bergervoet                            Electromagnetism and EMC
 Philips Research Laboratories,               Eindhoven, The Netherlands
 E-mail: bergervo_at_natlab.research.philips.com      Phone: +31-40-2742403