Keith Dysart wrote:
On Apr 6, 11:03 pm, Walter Maxwell wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Can one not change the location of the nulls by changing
the phase relationship of the two sources?

If so, it would seem to me that two non-coherent fields are
simply fields without a constant phase relationship and as
such, the nulls are constantly moving; still present, but
not stationary.

...Keith

Andy writes:
Correct. One example is a television signal that is received from
two
sources : 1) a direct line to the transmitting tower and 2) a
reflection
from an airplane flying .

Even tho both received signals are generated from the same source,
the reflected signal will be changing in amplitude and phase as the
reflector,
the airplane, moves along it's flight path.

The two signals combine at the receiving antenna and the
resultant signal into the receiver will rise and fall, depending on
the resultant amplitude and phase. The maximum can be several
db above the direct signal and the null can be many many db
BELOW the direct signal.

Hence, you see the image come and go for several seconds on
your screen. After several seconds the plane will have moved to a
position
such that the reflection doesn't hit your antenna anymore, and the
problem goes away. We've all seen this. In fact, 70 years ago, this
effect (on radio signals) was what inspired the development of
radar.....

Andy W4OAH