Roy Lewallen wrote:
At the points where the fields from the
two antennas completely reinforce, the sum of the fields is 0.7071 +
0.7071 = 1.4142 times the field produced by the original antenna. This
is a field strength gain of 3 dB compared to the original antenna, and
it's a field strength gain, as Antonio says, of 2 (6 dB) compared to the
field produced by each of the two antennas.

That's an example of constructive interference.

At the points where the fields from the two antennas
are equal and 180 degrees out of phase, the sum of the
fields is zero. That's the necessary destructive
interference on the flip side of the coin that satisfies
the conservation of energy principle.

Essentially the same thing happens at a Z0-match in
a transmission line.
--
73, Cecil http://www.w5dxp.com