View Single Post
  #271   Report Post  
Old January 17th 08, 02:45 PM posted to rec.radio.amateur.antenna
Cecil Moore[_2_] Cecil Moore[_2_] is offline
external usenet poster
 
First recorded activity by RadioBanter: Mar 2007
Posts: 3,521
Default Standing morphing to travelling waves, and other stupid notions

Keith Dysart wrote:
Expanding on statement 2...
Cecil's claim is that the "energy in" the
forward and reflected waves travel from end
to end on the transmission line.


The forward wave and reflected wave each possess
energy and momentum which must be conserved.
Assuming the transmission line is a homogeneous
medium, i.e. a constant Z0, there is nothing
anywhere along the line that can possibly cause a
reflection. A reflection is necessary to reverse
the momentum of a wave and reflections are impossible
in a homogeneous medium. By the process of elimination,
there must be something wrong with statement #1.

This is easy to demonstrate on the bench using a TV
signal. Even though there are points of zero power
in the standing wave, ghosting will be clearly visible
proving that the forward wave has made a round trip to
the end of the stub and back.

The superposition principle allows us to analyze the
forward wave and reflected wave separately. When we
do that, it is obvious that energy is flowing both
ways across that zero power node location and at all
other points on the line. If the reflected wave is
equal in magnitude to the forward wave, the net power
is zero at every point on the line, not just at the
nodes. The Poynting vector for the forward wave is
equal in magnitude and opposite in direction to the
Poynting vector for the reflected wave.
--
73, Cecil http://www.w5dxp.com