Schelkunoff's method is elegant, and one that lends itself to relatively
simple calculation -- in closed form -- with a computer. However, it
doesn't give results which are in as good agreement with measurements
than some other methods, so some assumptions made in his derivation
aren't completely correct. A good summary of various methods and their
validity appears in David Middleton and Ronold King, "The Thin
Cylindrical Antenna: A Comparison of Theories", _Journal of Applied
Physics_, Vol. 17, April, 1946.

The program for calculation of the "Field Day Special" antenna
(ftp://eznec.com/pub/fdsp~.exe) uses Schelkunoff's method, and it's
perfectly adequate for the purpose.

Of course, these days we can easily do very accurate calculations from
very fundamental equations with a computer using the method of moments
or other methods. There's a very good description of the method of
moments in the second edition of Kraus' _Antennas_. I assume it's also
in the third edition, which I don't yet have.

Roy Lewallen, W7EL

Richard Clark wrote:
On Thu, 17 Jul 2003 12:05:19 -0700, Roy Lewallen
wrote:


Most simple derivations for the input impedance of a dipole assume it's
infinitely thin. The general problem of a dipole made from wire of
finite diameter is a lot tougher, and is the topic of the papers by the
authors I listed in another recent posting. With EZNEC, you'll find that
the dipole impedance will continue to change as you make the wire
diameter smaller and smaller, until it gets too small for the program to
handle at all.

Roy Lewallen, W7EL


Hi All,

The derivation of dipole electrical characteristics comes by neither
thin nor thick (cylindrical) elements but through a simpler
(conceptually, not mathematically) work described by S.A. Schelkunoff
in "Advanced Antenna Theory," John Wiley and Sons, 1952.

Schelkunoff approaches the design as merely the extension of the
transmission line and he answers the issue of the antenna (the thin
wire form) being non-linear (the presumed incremental
inductance/capacitance is not constant along the length of the split
transmission line) by simply employing conical structures.

The Biconical Dipole "develops a transverse spherical (TEM) wave
analogous to that on a conventional transmission line" (reference
"Antennas and Radiowave Propagation," Robert E. Collin, McGraw Hill,
1985). "Thus the biconical antenna theory provides a theoretical
basis for assuming a sinusoidal current distribution on thin-wire
antennas."

Like any transmission line terminated in its own character impedance,
the Biconical Dipole (within limits imposed by size and apex angle)
also presents a wide frequency range exhibiting a constant radiation
resistance (about 160 Ohms across three octaves, by my margin notes).

The easiest validation of this is found in the Discone.
http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm

73's
Richard Clark, KB7QHC