Some may wonder why I posted the groundwave coverage contour of an AM
broadcast station as being relevant to this thread. Hams are mostly
interested in the space wave radiated by an antenna system.
The NEC4.2 plots linked below show how the space wave and ground wave fields
launched by a vertical monopole are related to each other, over a range of
r-f loss in the buried radial systems used with that monopole. Loss in that
r-f ground connection directly affects the fields a vertical monopole
radiates at all elevation angles below the zenith, including the horizontal
plane.
Space wave fields decay at a 1/r rate. Ground wave fields decay at greater
than a 1/r rate due to the added propagation loss where the radiated field
encounters a lossy ground plane (the Earth). The plots linked below show
the fields close to the radiator, where propagation loss for the ground wave
is very nearly the same as for the space wave.
For long propagation paths the magnitude of the ground wave field is so low
as to have no practical use. This is the condition shown by NEC and other
MoM software when plotting the far-field (only) patterns of a vertical
monopole over real earth, for infinitely long paths over a flat ground
plane.
This relationship between the space wave and ground wave fields is the
reason that such ground wave field analyses are (or should be) valuable to
hams.
Also note that the greatest field launched by each of the systems in the
graphic below always occurs in the horizontal plane -- not at some "takeoff
angle" above it.
R. Fry, CPBE
http://s20.postimg.org/4sp4pk6zx/Elpat_Compare.jpg
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