Walter Maxwell wrote:
On Fri, 06 Apr 2007 23:03:42 GMT, Cecil Moore wrote:

MRW wrote:
Any comments? Really, what I'm trying to understand here is: if
constructive interference does any good in radiowave propagation. I
was thinking that with an increase in amplitude the signal would be
able to travel a little further, but the signal received may not be
accurate in terms of the information it is conveying.
Antenna gain over isotropic is an application of
constructive interference. The constructive
interference must be balanced by an equal amount
of destructive interference elsewhere to avoid
violating the conservation of energy principle.

This is what I've been trying to persuade the 'anti's' that whenthe radiation fields from two vertical dipoles
superpose at some point in space, where their magnitudes are equal and are 180° out of phase, the wave
cancellation resulting from destructive interference produces a null in a predetermined direction, and thus
prevents those fields from propagating any further in that direction. At the precise instant the null is
produced, the constructive interference following the principle of energy conservation yields an increase in
the field strength in directions away from the null direction. This explains the concept of antenna-pattern
modification, and contradicts the notion that the two fields just plow through each other with no effect on
either.

Keep in mind that the two fields are coherent because they were developed simultaneously from the same source.
It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Walt, W2D

Walt,

Your observation is "correct" only in the case that most people consider
for practical reasons. The calculation showing the null behavior is
almost invariably performed at infinite distant from the sources, i.e.,
far field condition. The path from each source to the observation point
is considered to be exactly parallel.

As you know, there are usually three or more linear dimensions that
enter into radiation calculations. In the case of two sources there are
four:

Wavelength
Size of each source
Distance between sources
Distance to the observation point

In the typical "null" presentation, such as that shown in the ARRL
publications, the distance to the observation point in always large.

Lets take another case, however. Suppose the distance between the
sources is some what larger than the wavelength. Make it large enough so
there is a region between the sources that would be considered far field
from each of the sources. Now calculate the phase differences along some
direction from the center point between the sources that eventually
points to a null region in the infinite distance. Don't pick an
obviously symmetric direction, such as broadside or end-fire, as that
would be a special case.

What you will find is that when looking at the phase difference along
the ultimate null direction is that there is no such null much closer to
the sources. The paths from the individual sources are not parallel in
this case. The null "line" is actually a curve. The waves pass right
through each other in the closer region. The "passing waves" then go on
to form nulls in the infinite distance. The nulls in the closer region
are not in the same directions as the nulls in the far field.

Again, the ground rules:

Totally coherent, monochromatic sources
Fixed phase difference
Far field conditions for each source

There are no "tricks" here; this is just a matter of simple geometry.
However, it shows that the null you believe demonstrates some permanent
interaction and annihilation of EM waves is simply a special case.

In classical, non-cosmic, non-relativistic conditions EM waves do not
interact in free space. This condition is so widely understood in the
scientific world that it becomes a prime candidate for argument on RRAA.

8-)

73,
Gene
W4SZ