The efficiency of a dipole is nearly 100 per cent or 1.0. Depending on
construction, it might be 99 per cent, (0,99, 0,98, etc.) It is as
others stated a statement of power minus losses, giving what is radiated.
A half wave dipole converts almost all the available power to radiation
is another way of looking at it.
Coupling factor is something else, usually applied in my experience to
relations between tuned circuit inductors.
As a dipole driven element is coupled to parasitic elements, it's
radiation resistance becomes lower. It is no longer a 75 ohm feedpoint
impedance, but rather something much lower. This is why driven elements
of yagis tend to have gamma matches, or other means of matching the low
impedance of the driven element to 50 ohm coax. As the radiation
resistance gets lower, the ratio of radiation resistance to ohmic loss
resistance gets lower. The ohmic resistance of the conductor becomes a
more significant part of the total impedance, so the efficiency goes
down compared to dipole not coupled to parasitic elements.
One method of achieving a higher feed point impedance on the driven
element of a parasitic array is to use a folded dipole instead of a
plain dipole.
Remember TV antennas feed with 300 ohm ribbon transmission line? Many of
those used folded dipole driven elements.
DE N6KB
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