> Under certain simple circumstances the interaction between antennas in
> an array (stack) can be constructive. This in the case for a pair of
> half-wave dipoles where for separations in the vicinity of 0.7 waves
> the mutual impedance is real and negative. The negative impedance
> lowers the radiation resistance of the elements in the array resulting
> in additional gain.
In my opinion it is both confusing and misleading to consider "gain"
occurring from decreased radiation resistance. Pattern is what
controls the directive gain.
For a given applied power, and a given loss in the elements, power
gain is strictly a function of pattern shape. With an identical pattern
and power loss (as heat) power gain will be identical regardless of
radiation resistance.
Decreased radiation resistance for an array of elements of a given
size is the direct result of radiation of two antennas cancelling in
many directions (an increase in directivity). The "squeezing" of the
pattern causes the radiation resistance to decrease, because the
antenna is fighting itself with destructive interference between the
multiple sources of radiation.
For example, a small loop antenna has much lower radiation than a
similar size small dipole, because radiation from every area of the
loop is cancelled by radiation from other sides. Because we apply
fixed power, this causes current to increase. The small dipole is
just as directive, and yet has much higher radiation resistance.
Because of the lower current (higher radiation resistance) the small
dipole is generally much more efficient than a small loop when
reasonable Q loading is used.
73, Tom W8JI
W8JI@contesting.com
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