Roy Lewallen wrote:
A transmission line is similar to an antenna in only some respects, and
assuming they act exactly the same leads to erroneous conclusions.

A capacitor and resistor is similar to a stinger in only some
respects, and assuming they act exactly the same leads to
erroneous conclusions.
--
73, Cecil http://www.qsl.net/w5dxp

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

"Roy Lewallen" wrote:
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_.

A transmission line is similar to an antenna in only some respects, and
assuming they act exactly the same leads to erroneous conclusions. Among
the many mistakes made in recent postings is the assumption that a
complete reflection takes place from the end of an antenna wire. As
Schelkunoff, Kraus, and others explain, this isn't correct.

What they seem to be saying is that a quarter-wave monopole
could be modeled like this:

======1/4WL 600 ohm line======12K load

The 12K load dissipates approximately the same amount
of power radiated by a 1/4WL monopole so the conditions
at the feedpoint will be similar to the 1/4WL monopole.

Just because it can be modeled in that fashion doesn't mean
that the radiation is from the same place as the 12k load.

This does seem to be a good way to understand the forward
and reflected waves occurring in a 1/4WL monopole. Guess
what the feedpoint impedance is?

Another way to model the antenna would be with resistance
wire instead of transmission line wire. Then we wouldn't need
the 12K load resistor. We could just specify 1 dB loss between
the forward power and reflected power.
--
73, Cecil, W5DXP