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The Extreme Failure of Poor Concepts in Discussing Thin Layer Reflections
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August 1st 05, 03:38 PM
Walter Maxwell
Posts: n/a
On Sun, 31 Jul 2005 15:52:19 -0500,
(Richard
Harrison) wrote:
Jim Kelley wrote:
"That`s because power doesn`t propagate, I hasten to add - neither do
Poynting vectors.:
Some world-class experts disagree with Jim. Here is a sample quotation
using the words "power flow".
From E.M. Purcell writing about "Antenna Gain and Receiving Cross
Section" on page 19 of "Radar System Engineering" edited by Louis M.
Ridenour:
"If the transmitting antenna were to radiate energy isotropically-that
is, uniformly in all directions-the power flow through unit area at a
distance R, from the antenna could be found by dividing P, the total
radiated power, by 4piRsquared."
Best regards, Richard Harrison, KB5WZI
Richard, in addition to Ridenour, I quote from Reflections 1 and 2,
Chapter 8, "A semantic problem with the term "power flow" also fuels
the erroneous belief that reflected power is fictitious. This brings
us to the question, "Does power flow?" To help us understand the
answer, let's examine an analogy that involves current.
When we talk about "current flow," we take the meaning for
granted. However, does current really flow? The basic electricity
sections of engineering textbooks (also The ARRL Handbook) say that
current does not flow--charge flows. Current is defined as the
quantity of charge flowing past a point per unit time. However, once
we leave basic electricity and move on to circuit analysis, the term
"current flow" is used almost exclusively--and
yet we know exactly what is meant.
The same problem exists with the term "power flow." Engineering
textbooks define power as the "quantity of energy passing a point per
unit time." Thus, power does not flow--energy flows. However, except
when reciting the definition of power, textbooks and journals on wave
propagation use the term "power flow" almost exclusively, with only an
occasional use of "energy flow." As with "current flow," we know what
is meant because of the common usage which generally overshadows the
strict definition."
Perhaps this explanation will satisfy Jim, but perhaps not. We'll see.
Walt, W2DU
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