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Old January 27th 08, 03:26 PM posted to rec.radio.amateur.antenna
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Default Derivation of Reflection Coefficient vs SWR

Keith Dysart wrote:
When the generator is matched to the line so that
the reflected wave does not encounter an impedance
discontinuity when it arrives back at the generator
(and therefore is not reflected), ...


On the contrary, it is redistributed back toward
the load in the process of destructive interference
and becomes constructive interference associated
with the forward wave. Whether you call that a
reflection or not, the fact that the forward
power equals the source power plus the reflected
power tells us that reflected power being dissipated
in the source would violate the conservation of
energy principle.

where does the reflected power go?


Power doesn't flow so
reflected power doesn't "go" anywhere. It is the
reflected energy that is flowing, i.e. going
somewhere. If the power were flowing, its
dimensions would be joules/sec/sec. Maybe
you would like to try to explain the physical
meaning of those dimensions?

Does it enter the generator?


forward power = source power + reflected power

A component of the forward energy is equal in
magnitude to the reflected energy so no, it
doesn't enter the source.

Is it dissipated somewhere?


Not during steady-state. During steady-state it
is being used for impedance transformation. After
steady-state, it is dissipated either in the
source or in the load as a traveling wave.

Answers to these questions will quickly lead to
doubts about the *reality* of "reflected power".


Reflected power is the Poynting vector associated
with the reflected wave. It exists at a point. It's
average magnitude (indirectly measured by a Bird) is
Re(ExH*)/2. It's direction is the direction of the
flow of reflected energy.

If you don't believe in reflected energy, let your
fingers become the circulator load resistor for
an open-circuit stub being driven by a KW source.
--
73, Cecil http://www.w5dxp.com
 
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