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For those that do not have a firm understanding of what the chart of
dipole height over ground shows, I offer the following explanation.

The charts show, for a dipole antenna at various heights in wavelengths
over perfect, very good, average, and extremely poor ground, the gain
and elevation angle of the antenna main lobe.

The main lobe is where the majority of the energy is radiated.

To understand what the charts mean in the real world, first you have
to understand a little bit about propagation of RF.

For a dipole antenna, there are two modes of propagation that are relevant,
and those are NVIS (Near Vertical Incidence Skywave) and skywave which
is sometimes called skip.

Both modes depend on the RF being reflected or refracted back toward
Earth by the ionosphere.

For NVIS mode, the RF is directed straight up, that is an elevation angle
close to 90 degrees is desired. The range of NVIS communications is on
the order of 50 - 650 km, depending on the state of the ionosphere. The
amateur bands where this is effective is limited primarily to the 160M
to 40M band, again depending on the state of the ionosphere. It is not
impossible to have NVIS communications on the higher bands, just much
less probable to happen.

For skywave mode, a low elevation mode is desired. Most of the literature
recommends angles of 30 degees or less. In this mode the RF "bounces"
at more obtuse angles, and with good conditions in the ionosphere, more
than once, providing communication over global distances. Skywave
depends heavily on the condition of the ionosphere and during sunspot
peaks often occurs well past 10M.

Now since a dipole with a main lobe at 90 degrees still has some gain
at low angles, though it can be 20 to 60 dB down from the main lobe,
when conditions are very good some stations can still be heard by
skywave mode, though it is a rarity and can not be depended on.

Conversely a dipole with a low elevation angle of the main lobe has some
gain at very high angles and can occasionly hear stations by NVIS mode,
but again it is a rarity.

The bottom line of all this is that if you desire NVIS communications,
you should mount your dipole at a height where the elevation angle is
close to 90 degrees while if you desire long distance communications
you should mount your dipole at a height where the elevation angle is
less than 30 degrees, or higher if possible.

If the required height is impractical at your location, then the
alternative is a ground mounted vertical or a close to ground mounted
ground plane antenna, which will have an elevation angle in the 20
degree range.

Along the lines of a "testimonial"...
I once lived in the center of a state that had an active 75 meter net. At
one point I was asked to be one of the net control stations because of my
consistent strong signals within the net.

The secret? A 75 meter dipole at 20 feet with 100 watts.
On longer paths, of course, the "big boys" kicked my butt big time.