On 10/7/2012 3:42 AM, Richard Fry wrote:
It will be seen from the data that no "notch" exists in the fields
radiated by the monopole at elevation angles of 3 degrees and less, as
expected by some when considering only the far-field patterns shown by
MoM (NEC) software, and in antenna textbooks.
That low-angle radiation can reach the ionosphere to produce a skywave,
under the right conditions. That skywave can be very useful to hams
using vertical monopoles, even though its existence may not be
recognized.
Everyone seems to agree that at moderately short distances from the
vertical radiator (a few miles), there will be pattern fill-in at very
low-angles and that this fill-in is not predicted by the far-field
pattern equations. What everyone seems to be dancing around (Dick hints
at it above) is whether or not any of this low-angle ground-wave energy
ever reaches the ionosphere and if so, how? Clearly if you keep
increasing the distance from the vertical radiator, the E-field at zero
elevation angle drops faster than 1/r (except in the case of infinitely
conductive ground) so eventually the "notch" in the pattern predicted by
the far-field equations appears. One can image taking the helicopter
used by Dick's BC engineer friend and repeating E-field versus height
measurements at increasing distance intervals out to many miles. In
fact, one of the NEC-4 surface-wave plots Dick posted a few days ago
shows that for average soil a fairly deep low-angle notch (~ 20dB) has
already appeared in the elevation pattern at just 20 miles distance from
the vertical radiator:
http://i62.photobucket.com/albums/h85/rfry-100/Surface_Wave_Flds.jpg
So again my question - if this low-angle ground-wave (aka surface-wave)
energy dies off so quickly (e.g. down 20dB at just 20 miles), how does
any of it get to the ionosphere where it can be useful for topband DX?
73, Mike W4EF......................
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