Richard Fry wrote:
"Richard Clark might have allowed for systems operating above 30 MHz.,
which include several ham bands, I believe."

I believe so too.

From page 810 of Terman`s 1955 edition:
"Space-wave Propagation. At frequencies above about 30 MHz, the
ionosphere is not able to refract energy to earth, while the ground wave
attenuates to negligible amplitude in a relatively few hundred feet."

On the preceding page, 809, Terman presents Fig. 22-4 which gives among
other things the magnitude of the reflection coefficient for a
vertically polarized wave versus the incident and reflected angle
(symbol = psi, and they are identical). At zero-degrees, the coefficient
is 1.0. It falls to about 0.17 at 10-degrees, which is the minimum or
Pseudo Brewster Angle. From there, it rises to about 0.5 at 20-degrees,
and about 0.7 at 70-degrees incidence. The nip in vertically polarized
radiation does not exist at zero-degrees, the takeoff for ground wave
propagation. The ground wave is interactive with the surface all along
its path.

The space-wave does not depend upon interaction with the earth all along
its path, as the ground wave does. The wave along the earth at 30 MHz
and above "attenuates to negligible amplitude in a relatively few
hundred feet." It depnds upon reflection from the ionosphere or or
something else for popagation beyond the horizon.

Best regards, Richard Harrison, KB5WZI