GOOD QUESTION!
The radiation resistance of a Dipole Antenna changes with height above
ground.
(See ARRL Handbook or Antenna Book). It goes through a minimum between
5/8 and 3/4 wavelengths (82 to 105 feet on 40 meters).
Power = I squared R - For a constant power output, current (and
gain) is maximized when the radiation resistance is the least.
40M dipoles between 80 and 100 ft have proven to be very effective DX
antennas.
For local communtication up to 600-800 miles, 40 to 50 ft works quite
well.
BTW, for a 20M Yagi on a 24 ft boom, the boom is self resonant near 7.5
MHz.
If you mount a 40M dipole above such an antenna, parallel to the boom,
the
Yagi boom will act as a parasitic director, pointing straight DOWN. It
is better
to mount the dipole parallel to the elements or match the Yagi boom as a
40M
end-loaded dipole.
de Tom - N4KG
:
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