On Oct 24, 4:31 pm, K7ITM wrote:
On Oct 24, 12:11 pm, wrote:

And, "end capacitance effect" is a poor model for what's really going
on. It's been used as an "explanation" for the observation that an
antenna that is slightly shorter than half a wavelength is resonant(as
in has no reactive component at the feedpoint). The problem is that an
infinitely thin dipole is resonant at less than 1/2 wavelength, and in
that case, there's no real "end" to have an effect.

?? I have been under the impression that in the limit as the
conductor radius goes to zero, the resonance does go to a freespace
half wavelength. You have to make the antenna _really_ thin to get
anywhere near that, though. Even a million to one length to diameter
ratio won't do it.


Half wavelength of zero radius has a feedpoint impedance of about
73.1+j42.5 ohms
(see, e.g. page 639 of Orfanidis's online electromagnetics book
(chapter 16), which gives a one page derivation)
(http://www.ece.rutgers.edu/~orfanidi/ewa/)

Roy's right about the spiral not converging to 377+j0.. I misread the
graph.

Jim