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Old April 15th 07, 08:56 PM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Mar 2007
Posts: 3,521
Default Constructive interference in radiowave propagation

Jim Kelley wrote:
As I said, Cecil, your ideas about waves 'possessing energy' need a
little work.


All it takes to prove you wrong is a
look at a typical S-Parameter equation involving
the superposition of two terms. In the following the
'@' sign is used for the angle sign. a1 and a2 are
normalized voltages. s21 is a transmission coefficient.
s22 is a reflection coefficient.

b2 = s21(a1) + s22(a2)

Given a1 = 10 @ 0 deg, a2 = 10 @ 180 deg,
s21 = 0.707 @ 0 deg, s22 = 0.707 @ 180 deg

s21(a1) = 0.707@0(10@0) = 7.07 @ 0 deg

s22(a2) = 0.707@180(10@180) = 7.07 @ 0 deg

superposing those two values gives:

b2 = 14.14 @ 0 deg

All is well and good. Multiply b2 by SQRT(Z0) to get
total forward voltage.

Now let's look at the powers in accordance with HP's
Ap Note 95-1. For that, we don't need to know the Z0.
The beauty of an S-Parameter analysis is that if one
squares the normalized voltages, one gets power.

|s21(a1)|^2 = 50 watts

|s22(a2)|^2 = 50 watts

|b2|^2 = 200 watts

Even in the S-Parameter analysis, superposing two 50W
waves in phase yields 200 watts. Constructive interference
not only makes it possible but demands it.

Jim, I challenge you to find anything wrong with this S-
Parameter analysis. It follows exactly Born and Wolf's
intensity equations for constructive interference when
the phase angle between a1 and a2 is 180 degrees and
their magnitudes are equal.
--
73, Cecil http://www.w5dxp.com
 
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