Jim Kelley wrote:


Cecil Moore wrote:

Jim Kelley wrote:

I don't think it matters where it is, or how much space is involved.
For a single source to create an interferernce pattern, there must be
a reflector somewhere. An antenna tuner for example. Interference
is the result of the overlap of waves.


Do you consider a dipole in free space to be
a "single source"? There is no reflector yet
there is plenty of interference.

If there is no reflector and no splitter to redirect a portion of the
ratiated energy, but an interference pattern still results, then there
must be more than a single source. It should be fairly obvious that
both parts of the antenna must be present in order to generate the
dipole-type interference pattern.

73, ac6xg

A dipole is by no means a single (point) source. Each tiny part of the
dipole creates a field proportional to the current at that tiny part, so
in effect it behaves like an infinite number of sources which are spread
out in space along the dipole conductors. Interference of all those
various fields is what creates the familiar dipole radiation pattern. In
fact, moment method antenna analysis programs such as NEC and EZNEC
break the antenna into a finite number of segments and calculate the
current (magnitude and phase) on each segment. It then calculates the
field produced by each segment according to its length, orientation, and
current. The sum of the fields is shown as the radiation pattern. This
process can be done manually as well.

The hypothetical isotropic radiator is a point source. It has a
perfectly spherical pattern because there's no interference.

Roy Lewallen, W7EL