Dear Crew:
What may have been lost in this discussion - but often provided by Roy -
is the difference between a true surface wave and a sky wave (from the same
vertical antenna).

An excellent ground screen at the base of a vertical antenna reduces the
losses and thus increases the amplitude of both the surface and sky wave.

A few to several hundred wavelengths from the vertical antenna, the
amplitude of the received surface wave is highly dependent on the
conductivity of the intervening earth. One could hedge this a bit for
frequencies above something like 5 MHz.

At distances from the antenna where the surface wave has become weak,
the sky wave dominates. As Roy has pointed out several times: the amplitude
of the sky wave is mostly determined by ground properties distant from the
antenna - much farther away than any reasonable ground screen will extend.

The sky wave effect may be demonstrated in NEC by effecting a 0.25 WL
vertical with a good set of radials over mediocre ground and then changing
the ground, at some distance from the antenna, to salt water. (Visualize
the antenna being in the center of a circular island and change the diameter
of the island.)

Broadcasters at MF, with some exceptions, are interested in maximum
surface wave and minimum sky wave. Radio amateurs, for the most part, are
interested in maximizing the sky wave. A good radial system assists the
radio amateur's goal by improving the overall efficiency. It is the ground
distant from the vertical antenna that determines the sky wave.

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
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