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Old May 31st 10, 12:16 PM posted to rec.radio.amateur.antenna
K1TTT K1TTT is offline
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First recorded activity by RadioBanter: Apr 2010
Posts: 484
Default Question about "Another look at reflections" article.


how about this, i'll really mess things up. all comments assume
linear media or circuit elements and sinusoidal stead state conditions
(all transients ignored).

Standing Waves are a figment of your instrumentation... it is a well
known method to choose either voltage or current to do calculations in
a circuit, knowing that the other can always be calculated given the
impedance. this works for distributed as well as lumped circuits. it
is a consequence of superposition and ohms law in the generalized
complex form. This is a trap when looking at traveling and 'standing'
waves when there are reflections. Since you are only seeing one of
the components of the wave you get the impression that the standing
wave is indeed concentrating the energy and all the resultant effects
at 180 degree intervals. you must however look at both components to
get a full picture.

Wherever there are standing voltage wave peaks there are also standing
current waves 90 degrees (in distance) along the line from them. And
while you can calculate the power of the voltage wave peaks as V^2/Z,
you can also calculate the energy of the current wave peaks as I^2*Z.
Of course you can really get yourself confused if you consider the
lossless transmission line with a perfect short or open end... in that
case at those peak voltage and current points the Z is infinite or
zero respectively which gives you zero power in either case. Now to
REALLY have fun, integrate the power at those points into the energy,
of course the integral of zero is always zero.

Want to get even more confused. Look at an animation of the standing
waves (EITHER current OR voltage) over one full cycle in time, there
is a peak where the standing waves along the line all reach a peak
value at once, then 90 degrees later the whole line is zero, then
another 90 degrees and there is an opposite peak. Think carefully
about the zero point, where did the energy go? Surely it doesn't mean
that all the energy stored in those waves has left the line? that
would require some external storage and faster than light energy
transfer, a mean trick if you can do it!

Ah, but wait, the current wave is 90 degrees out of phase with the
voltage, so the energy has a place to go, into the other component!
but remember, when that component peaks the impedance is wrong so
there is still no energy. But of course then everything does balance
out, no energy in one standing wave transfers perfectly to the other
standing wave 90 degrees later in time and space.

conclusion: Standing waves are a figment of your instrumentation, be
that eyes seeing the peaks and valleys of a physical manifestation of
them, a fluorescent lamp showing them as you run it along an open wire
line, or a voltmeter measuring them... they don't exist except when
they are observed, therefore they have no physical existence... much
like whats his name's cat.