On Jun 6, 9:22*am, "JC" wrote:
Basic question (at least for me) for a very poor antenna matching :
-100 w reach the antenna and 50 w are radiated.
- 50 w are "reflected", what is their fate ?
Are they definitely lost for radiation and just heat the line, the final.....

What happens to the 50 joules/second of reflected energy depends upon
the phasing between the source wave and the reflected wave at the
source impedance. What most amateurs don't understand is there are
two mechanisms that can redistribute reflected energy back toward the
antenna. Those mechanisms are a re-reflection based on the physical
reflection coefficient (what RF engineers understand) and wave
interaction resulting in constructive/destructive interference (what
most RF engineers don't seem to understand) because, unlike optical
physicists, have not been forced to follow the energy flow.

If the reflected wave arrives 180 degrees out of phase with the source
wave, the two waves undergo destructive interference and all of the
reflected power is redistributed as constructive interference energy
back toward the antenna. This is what happens at the Z0-match
established at the input of an antenna tuner.

Thus your conditions of 100w forward power and 50w reflected power
could be accomplished with a 50w matched source.

If the reflected wave arrives 90 degrees out of phase with the source
wave, there is zero interference and the reflected power is dissipated
in the source resistor (in a source with a source resistor).

If the reflected wave arrives in phase with the source wave, all of
the reflected power and more than 1/2 of the source power can be
dissipated in the source resistor.

Such knowledge is old hat for optical physicists who don't have the
luxury of measuring voltages in light waves. They rely on a power
density (irradiance) equation.

Ptot = P1 + P2 + 2*SQRT(P1*P2)*cos(A)

where A is the phase angle between the electric fields of two waves.
The last term is called the "interference term" and that value is what
most amateurs are missing in their energy analysis. If the sign of the
interference term is negative, the interference is destructive. If the
sign of the interference term is positive, the interference is
constructive.
--
73, Cecil, w5dxp.com