"Tam/WB2TT" wrote

You know that can't be right, because combining two antennas
gives 3 db gain.
_____________

The vector sum of the EM fields at every point in free space from identical
radiators fed by the same source depends in part on their relative physical
orientations, and their separation in wavelengths (see Kraus' ANTENNAS, 3rd
edition, chapters 5 and 6).

If all radiators in an array generate identical fields relative to each
other, then the peak directivity of an array of two radiators exceeds 3 dB
for radiator spacings of about 0.75 to 1.2 wavelengths (max of about 3.3
dB).

If they are spaced 1/2-wave apart then the peak directivity drops to about
1.9 dB, and at 1/4-wave separation it drops to about 0.5 dB (see Johnson &
Jasik ANTENNA ENGINEERING HANDBOOK, 2nd edition, Figure 3-4).

RF

Richard Fry wrote:
The vector sum of the EM fields at every point in free space from
identical radiators fed by the same source depends in part on their
relative physical orientations, and their separation in wavelengths (see
Kraus' ANTENNAS, 3rd edition, chapters 5 and 6).

It's too bad that we cannot see the interference patterns
created by two radiators. Just know that all of the
interference patterns involving visible light that we can
see with our own eyes are also possible at RF frequencies.

Who hasn't been listening to a repeater that almost
completely faded out while stopped at a red light?
Letting the vehicle move a short distance brings it
back to Q5. That old familiar "picket-fencing" that
some of us have experienced is the antenna alternately
moving through zones of destructive and constructive
interference. The same thing can be caused by an
airplane flying over during local TV reception.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote
Who hasn't been listening to a repeater that almost
completely faded out while stopped at a red light?
Letting the vehicle move a short distance brings it
back to Q5. That old familiar "picket-fencing" that
some of us have experienced is the antenna alternately
moving through zones of destructive and constructive
interference. The same thing can be caused by an
airplane flying over during local TV reception.
______________

All true, but those cancellations don't originate in the transmit array.
They are the result of reflections from surfaces in the propagation
environment that arrive at the receive antenna ~180°out of phase with the
direct signal from the transmit array.

RF