Owen Duffy wrote:
. . .
In "running the numbers", I note that the radiation resistance
indicated by NEC for a short dipole in free space is quite different
to that predicted by Kraus for a dipole with uniform current,
(Rr=80*pi()**2(L/Lambda)**2)!

The only way to achieve uniform current on a short dipole is with large
capacity hats at the ends of the dipole. Otherwise, the current tapers
nearly linearly from a maximum at the center to zero at the ends. If
you'll look closely at Kraus' figure of the short dipole he analyzes,
you'll see that it has capacity hats. Nearly all other authors analyze
just a straight wire which doesn't have those hats, and consequently
linear rather than uniform current distribution. And of course get quite
a different result.

I'll bet you didn't include large capacity hats in your model. I haven't
tried it, but you should get results much closer to Kraus' if you do.
NEC analysis gives radiation resistance very close to theoretical when
analyzing a plain straight wire dipole, but this isn't what Kraus does
in his book. It is interesting, though, to see how much effect the wire
diameter has on the impedance, and that the wire has to be very thin
indeed to approach the theoretical impedance for an infinitesimally thin
dipole.

Roy Lewallen, W7EL