from [Tom W8JI]:
The issue here is one of measurement distance and the assumed flat earth,
and how much the ground wave contributes to low angle radiation. We
obviously know radiation at zero degrees is not zero, as distant models
show.
As to measured data, below with his permission is a quote from a broadcast
consultant acquaintance of mine who has made helicopter measurements of the
elevation patterns of MW monopoles. These data below were taken using a
helicopter at a horizontal distance of 2.8 km from the monopole, showing the
fields existing at elevation angles of zero to about 3 degrees above the
horizontal plane.
This path length is short enough so that earth conductivity has little
bearing on the measured fields, but not so short that the intrinsic,
~cosine-shaped elevation pattern of a 1/4-wave monopole will significantly
affect the fields at those measured elevations.
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 is a space wave, not a ground wave, and given the
right conditions it can reach the ionosphere to produce a sky wave. That
sky wave can be very useful to hams using vertical monopoles, even though
its existence may not be recognized.
QUOTE
Helicopter Vertical Calibration
To check for variations in field intensity due to elevation above ground, we
hovered over one location and rose from the ground at 50-foot intervals to
500 feet. Location #12 on the 230° radial was used (non-directional mode),
and the data as presented below shows no significant effect. Measurements
taken for this report were done at an average elevation of 500 feet and the
calibration of the meter takes the vertical factor into account.
Height above ground Field Intensity Variation from 0 feet
0 56 mV/m (Ref.)
50 56 +0.0 mV/m
100 56 +0.0
150 55 -1.0
200 55 -1.0
250 55 -1.0
300 55 -1.0
350 54 -2.0
400 54 -2.0
450 54 -2.0
500 54 -2.0
Averages: 54.91 mV/m -1.09 mV/m (-1.99%)
END QUOTE
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Topband reflector - topband@contesting.com
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