On Wed, 03 May 2006 15:55:23 -0700, Roy Lewallen
wrote:

This is the paper in which Schelkunoff develops his often-quoted
approximate equations for antenna feedpoint impedance (the ones
including sine and cosine integral -- Si and Ci -- terms). He says,
basically, that an antenna acts like a transmission line -- a conical
antenna like a constant-Z line and a cylindrical (e.g., wire or tubing)
antenna like a variable-Z line -- *except at the ends*. At the ends,
modes other than TEM are excited, resulting in radiation, modification
of antenna impedance, and modification of current distribution.

Otherwise expressed as a finite Z instead of a zero current (infinite
Z) point at the end. Of course, finite and infinite are relative even
for Schelkunoff.

The
radiation, he says, can be modeled as either a terminating impedance or
as a distributed impedance (R and L) along the line. You can find an
abbreviated version of this explanation in Kraus' _Antennas_.

Hi All,

Pretty much what I've offered in the past and recently in this thread
(same source, Schelkunoff through Robert Collin). Anyway, I see no
formulas offered and as I don't have Kraus to see if they are missing
there too:
Zc = Z0 · ln (cot (theta/2)) / pi
for
Z0 = 377 Ohms
theta 5°
or
Zc = Z0 · (ln(2) - ln(theta))) / pi
for
theta 5°
where theta is the half angle of the cone section.

This, of course, says nothing of the variable Zc for a thick radiator
(which is not conical, but cylindrical). The "average" Zc:
Zc = 120 · (ln (l/a) - 1)
for
l: length
a: diameter

The Zc as a function of position:
Zc(z) = 120 ln (2 · z / a)

73's
Richard Clark, KB7QHC