The radiation toward an elevation angle of 5 degrees shown in the surface
wave plot continues in essentially a straight line, to reach the
ionosphere."
I'm still puzzled by these statements.
Its clear that a NEC far-field analysis over a real earth path omits a
significant amount of low angle radiation produced by vertical monopoles.
Such an analysis always shows zero radiation in the horizontal plane, and
not much more than zero at very low elevation angles.
But if that pattern was correct, then MW broadcast stations would have no
daytime or nighttime groundwave coverage -- which obviously they do.
However the NEC near-field analysis used to calculate the surface wave does
show that low angle radiation.
BOTH the NEC far-field and near-field analyses are required to describe and
understand the total radiation envelope of a monopole over real earth.
For background, I contacted Gerry Burke in January, 2012 when I was
researching the basis for the comments I have been posting here. Probably
most will recognize Gerry Burke as the co-author of NEC software, working at
Lawrence Livermore National Laboratory.
I sent him the NEC surface wave plots linked below, and asked him,
"...would you expect the fields at elevation angles of 1 to 10 degrees in
these
plots to continue on to the ionosphere, and under the right conditions
be reflected back to the earth as skywaves?
His reply was (quoted with his permission): "The low angle 1/R fields should
reach the ionosphere, although perhaps not accurately predicted by NEC,
since it does not include the effects of earth curvature and the
ionosphere."
G. Burke's reply should be conclusive on this subject.
BTW, the 2.46 V/m groundwave field shown at 1 km from the WLS tower for 8
mS/m earth in the NEC plots linked below is almost exactly the value
measured at 1 km by the newly-retired chief engineer of WLS, who is an
acquaintance of mine.
http://s10.postimg.org/xq4ngg4hl/WLS_Surface_Wave.jpg
RF
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Topband Reflector
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