Guy Olinger wrote:
So far NEC4 has predicted any real measurements, regardless
of operator skill.
Presumably the above text was meant to read "So far NEC4 has _NOT_ predicted
any real measurements, regardless of operator skill."
Some may not recall the comparison of groundwave fields calculated by NEC to
those measured with an accurate, calibrated field intensity meter by a
broadcast consulting engineer, posted here some months ago (link below).
Best-fit earth conductivity for the measured path was about 6 mS/m.
The fields calculated by NEC are shown on the top half of the page. Note
the close agreement between the data shown by the blue line there, and the
solid line below the inverse distance field in the consultant's data plotted
at the bottom of that page. This is at at least one case where NEC and
real-world data agree.
Another important observation to be made from that NEC data is that
space-wave radiation from elevation angles below ~2 degrees equals the
radiation in the groundwave at 1,300 meters downrange. For further
distances downrange the space wave exceeds the groundwave, as the space wave
is decaying at a 1/r rate, and the groundwave decays at greater than 1/r,
due to earth losses and eventually, earth curvature. This is shown by the
green line on the NEC chart starting to exceed the blue line at an
h-distance of 8,000 meters.
For long enough point-point paths along the surface the earth, the
groundwave essentially is zero, and the much greater space wave radiation
from such low elevation angles can produce nighttime skywave coverage over
the longest, single hop paths.
This type of performance also applies to the monopoles of 5/8WL and less
used by ham operators on Topband and the HF bands (regardless of earth
conductivity at/near the monopole site).
The first thing would be to prove that NEC4's null at 50 km is a fantasy.
The fields in the groundwave itself do not provide coverage by ionospheric
skip, and there is little point in examining the groundwave 50 km downrange
(or anywhere else) to prove or disprove this.
NEC properly calculates zero field for its far-field plot of the elevation
pattern of a monopole over a non-perfect ground plane, and not much more at
vertical angles below 5 degrees or so. And a NEC near-field plot properly
does _not_ show this "notch" for distances sufficiently close to the
radiator. These calculations are not in conflict when properly understood.
The fields providing the greatest single-hop skip range are produced by the
elevation plane fields within a few wavelengths of the radiator that are
directed toward the lowest elevation angles. Those space-wave fields are
much greater than the groundwave for sufficiently long paths along the
surface of the earth. And those fields exist at much lower elevation angles
than shown by a NEC far-field plot, alone.
NEC calculates all of these fields accurately. It is only their
(mis)understanding and incomplete use that leads to the concept of "takeoff
angle" for the elevation patterns of monopoles over a non-perfect ground
plane.
http://i62.photobucket.com/albums/h85/rfry-100/Measured_vs_NEC2D_Fields2.jpg
R. Fry
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Topband reflector - topband@contesting.com
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