Richard Clark wrote:
Lest there be any confusion: an antenna IS a transmission line.

In fact, there is a formula for calculating the Z0 of
a single horizontal transmission line wire above ground.
#14 wire at 30 feet is very close to 600 ohms. #14 wire
at 30 feet describes a lot of dipoles.
--
73, Cecil http://www.w5dxp.com

Richard Clark wrote:
On Mon, 20 Oct 2008 17:57:03 -0700, Jim Lux
wrote:

All of this kind of confusion is trying to make one sort of model (a
transmission line) fit something else (a radiator).

Hi Jim,

I've seen this kind of assertion made before, generally as a blanket
prohibition/warning/incantation/supplication/condemnation - but never
with any demonstrable problem that wasn't an example of designed-in
failure suited to the argument.

Lest there be any confusion: an antenna IS a transmission line.


You're right, but in many situations, it's not a uniform transmission
line, by any means. The Schelkunoff analysis is quite elegant.

In article , Richard Clark
wrote:

Lest there be any confusion: an antenna IS a transmission line.


Hello, and I think one would have to include two antennas and the
intervening medium(s) for the above statement to make sense. In any
event, the behavior of an antenna-medium-antenna as a passive 2-port
device can be considered as a transmission line at a given frequency. The
"loss" associated with this topology can be mitigated by keeping the two
antennas within a near, rather than far, field separation. Over a range
of frequencies the behavior of this 2-port can easily differ from that of
a transmission line, though.

In some electromagnetics textbooks an antenna is developed mathematically
via the gradual unfolding of a twin-lead transmission line. And many hams
know that a quick and dirty dipole can be created by simply folding the
braid back on a length of coax so that the braid and the exposed center
conductor become the radiating elements.

A more correct statement might be that a transmission line can be an antenna.
This can include unintended radiotion (e.g. RF flowing on the outside of
caox due to imbalance and/or stray coupling) or intended such as Andrew's
"Radiax" brand of leaky transmission line for installation in tunnels and
elevator shafts as a convenient means to extend the reach of over-the-air
broadcasts. Sincerely, and 73s from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

On Oct 20, 5:57*pm, Jim Lux wrote:
....
Some might argue, though, that the reason the effective velocity is less
is because the sqrt(1/LC) term is smaller because C is bigger because of
the increased surface area. *And that might not be far from the truth
for a restricted subset of antennas.

On the other hand, the propagation velocity of coaxial cable of
constant outer conductor ID is independent of the inner conductor
diameter, even though the capacitance per unit length increases as the
inner conductor diameter is increased. Clearly one must be careful
about attributing the effect to a single cause like increased
capacitance.

I haven't noticed in this thread any reference to Ronold W. P. King's
work. His writings should give more insight into the subject, if you
can get deeply enough into them. It's discussed empirically in
"Transmission Lines, Antennas and Waveguides," (with lots and lots of
interesting graphs showing the effect from various viewpoints) but you
can probably go deeper into the theory than you need in his other
books and papers on linear antennas.

Cheers,
Tom

K7ITM wrote:
On Oct 20, 5:57 pm, Jim Lux wrote:
...
Some might argue, though, that the reason the effective velocity is less
is because the sqrt(1/LC) term is smaller because C is bigger because of
the increased surface area. And that might not be far from the truth
for a restricted subset of antennas.

On the other hand, the propagation velocity of coaxial cable of
constant outer conductor ID is independent of the inner conductor
diameter, even though the capacitance per unit length increases as the
inner conductor diameter is increased. Clearly one must be careful
about attributing the effect to a single cause like increased
capacitance.

Which was the original intent of my comment. Fat radiators are shorter
at resonance than thin ones, and the details of why are not simply
explained by something like "capacitance effects", although such an
explanation may sort of work over a limited range.