On Tue, 09 Aug 2005 13:15:18 -0700, Jim Kelley
wrote:


Identical wave-interference phenomena establishing a short
circuit also occur in free space in the same manner as in guided-wave
propagation along transmission lines. For example, when the fields
emanating from two radiators in an array of antennas are of equal
magnitude and 180° out of phase at a point in space, a virtual short
circuit is established by destructive wave interference, resulting in
a null in the radiation pattern at that point. Following Poynting's
Theorem, the energy in the combined fields propagating is reversed in
direction at that point; and with the constructive interference that
follows, that energy adds to that in the fields propagating in the
opposite direction, thus achieving gain in the that direction.

Walt, W2DU

Hi Walt,

I think that is all well and good, Walt - except for your statement
regarding the Poynting theorem. I can find no support in the literature
for a claim that interference changes the direction of the Poynting
vector - or for that matter, is a cause for any other phenomenon to
occur. I once asked you for a reference, but your books weren't handy
at the time. Let me know if it turns up. I am very interested to take
a look at it.

Thanks,

Jim Kelley

Well, Jim, did you really grasp the paragraph above? How do you
suppose that had it not been for wave interferencethe energy would
have continued on in a particular radial direction, but due to the
interference the result was a null in that direction and an increased
amount of energy in the opposite, or forward direction, achieving gain
in that direction? The energy certainly changed direction to achieve
this condition. And you don't need references to understand this
phenomenon either. There are some concepts that are understandable
intuitively.

Please Jim, don't tell us that the energy isn't changing direction in
the rearrangement of the power distribution in the antenna pattern
that is wholly caused by wave interference and nothing else.

Walt