On Sun, 27 Jul 2008 19:46:40 +0100, Dave wrote:

I wish to know if the reactance of a dipole that is physically 0.5000
wavelengths in length depends on the diameter of the wire or not.

Hi Dave,

Yes, it does.

You are working with source material with conflicting agendas. One is
simply interested in what is called a dipole for the sake of field
studies and the characteristics of that dipole are a good first order
approximation. This means thin-wire by and large. The other source
is examining the antenna itself (or so it seems by both accounts).

The fatter the wire, the lower the inductance. Naturally the
reactance must follow. The fatter the wire, the more wavelength it
encompasses for a given length, hence the length can be shorter for
resonance. This shorthand hardly matters for conventional wire
antennas as "fat" is in the extreme, and wire is hardly the proper
nomenclature when we get into these gross dimensions.

Approximations of "fat" come with cage structures that attempt to
mimic a solid of revolution.

If you want to find the author who developed the first principles of
thin vs. fat, that is Dr. Sergei Alexander Schelkunoff (with Friis).

In what has been decried in this forum as the failed metaphor of an
antenna as transmission line, the antenna formulas from Schelkunoff
were derived from (beat) a transmission line, albeit a special one.

To attempt to draw parallels between transmission lines and antennas
is fraught with failures, true. Specifically, the traditional dipole
in its thin-wire implementation has no linear Impedance relationship
along its length. The wire separation is always growing with distance
from the feed point and thus the Z varies with distance. This failure
was anticipated by Schelkunoff, and folded into field theory through
using conic sections for the dipole arms. Hence the biconical dipole,
the conical monopole, and the discone. The transmission line analogy
survives through this legacy.

All formulas that you have probably recited are the degenerative forms
for his based on the conic sections.

Now as to that degeneration of the conic section into "thick" wire to
"thin" wire. The conic section is certainly thick at the distal end,
no doubt there. It is also thin at the feed point. The advantage is
lowered capacitance bridging the feedpoint compared to that if the
thickness were constant from the distal end - for a given
thickness/length/resonance. Also the conic sections most nearly
approach the shape of the emerging wave's initial spherical front.

Well, the long and short of it is to seek:
"Antennas: Theory and Practice,"
Sergei A. Schelkunoff and Harald T. Friis,
Bell Telephone Laboratories, New York :
John Wiley & Sons, 1952.

73's
Richard Clark, KB7QHC