Old Ed wrote:
"---trees could be used to support a monopole."

Yes, but in supporting a monopole, the tree is more or less parallel,
near, and lossy. Horizontal dipoles are somewhat perpendicular to trees
which are likely to be in the nulls of the dipole pattern.

Shortwaves are propagated effectively by bounce at the correct angle off
the ionosphere. Propagation of shortwaves along the earth`s surface is
seriously attenuated.

Vertical antennas up to 5/8-wavelength launch their maximum signal along
the earth`s surface, not good for skywave propagation in most cases. It
works at sea where conductivity is excellent and over much greater
distance than over land, so that not all shortwave tangential energy is
necessarily wasted.

Horizontal antennas produce no tangential aignal along highly conducting
earth. The reflection at near zero elevation is equal and opposite in
polarity and it cancels.

High 1/2-wave dipoles (between 1/4-wave and 1/2-wave high) have gains at
some angle between zero and 90-degrees as compared with the same dipole
in free-space.

A 1/2-wave dipole between 1/4-wave and 1/2-wave above good ground can
have a dB or two gain at some elevation angle between 15 and 30-degrees.
It`s not much, but it`s not a loss. Even over poor ground, the
free-space 1/2-wave dipole characteristic is likely available at the
useful elevation angle.

I worked for years in shortwave broadcasting and have yet to see any
facility which employed vertical transmitting antennas. I`ve heard there
are some verticals, but when serious investment is professionally made
in shortwave transmitting antennas it is nearly always made in the
horizontally polarized variety.

Best regards, Richard Harrison, KB5WZI