On Jun 9, 1:06*am, Camelot wrote:
Hello, I have some doubts about standing waves on antennas that I hope
you could clarify!

Please forgive the unfinished posting above. It happened without any
action on my part.

First of all, one needs to get one's nomenclature correct. Antennas
like single-wire dipoles and full-wave loops are Standing Wave
Antennas. Standing Wave Antennas always have standing waves on the
antenna. They may or may not have standing waves on the transmission
line.

The 50-75 ohm feedpoint impedance of a 1/2WL dipole is a virtual
impedance, the result of forward and reflected waves superposing into
standing waves on the Standing Wave Antenna. The SWR on the 1/2WL
antenna wire is approximately 20:1. Only about 20% of the total energy
in the RF waves on a 1/2WL dipole is radiated. If 100 watts is being
delivered to the antenna and ~100 watts is being radiated, there is
about 500 watts of joules/second stored on the antenna wire during
steady-state.

Maximum power transfer to the antenna occurs when the antenna *system*
is matched but it is not an impedance match. It is instead a
*conjugate match*. A system with an SWR of 10:1 on the ladder-line may
(or may not) be a near conjugate match. If it is a near conjugate
match, maximum *available* power will be delivered to the antenna.

To summarize: Standing Wave Antennas, like 1/2WL single-wire dipoles,
*require* standing waves on the antenna wire.

A flat system, e.g. 50 ohm coax to a 50 ohm antenna, is an impedance
match but it is also a (trivial) conjugate match. In a low-loss
system, a Z0-match guarantees a near conjugate match at the antenna
feedpoint.

A low-loss system with tuned feeders is a (conjugately matched) system
if the impedance looking one direction in the feedline is very close
to the conjugate of the impedance looking in the other direction.

When a Z0-match to 50 ohms is achieved by a tuner (or other network) a
near conjugate match is achieved in a low-loss system. In general, all
low-loss systems that are transferring the maximum amount of available
energy are matched, by definition, i.e. they are a near conjugate
match.

The reason that I say "near" conjugate match is that the conjugate
matching theorem doesn't allow resistive losses in the analysis. Of
course, there is no such thing as a lossless system in reality.
--
73, Cecil, w5dxp.com