In rec.radio.amateur.antenna Spike wrote:

Imagine a short rod vertical aerial not connected to ground, for the
(say) 160/80/60/40m bands, as might be found in a typical /M set-up, fed
with RF energy and operating over ground of average conductivity.

Three different waves will be launched from this: the sky wave, the
space wave (including the reflected ray), and the surface wave. Each of
these have their own characteristics, inasmuch as the sky wave is
launched willy-nilly even if the band isn't open for that mode, the
space wave depends on the path to the receiver, and the surface wave
depends on the electromagnetic characteristics of the air and the
surface material, although to some extent the latter affects all the
waves generated.

These "waves" are actually called skywave and surface wave and are
a propagation phenomena.

See:

http://en.wikipedia.org/wiki/Skywave
http://en.wikipedia.org/wiki/Surface_wave
http://en.wikipedia.org/wiki/Line-of-sight_propagation

for how signals propagate.



--
Jim Pennino