Mr. Fry supplies a monolithic, complete fill up to three degrees at some
unspecified frequency over unspecified ground out 2.8 km. The difference
between that and the NEC pattern generated for 1/4 wave over 120 0.4
radials at three degrees elevation is over seven dB. That is a LOT of
fill.
I recently modeled the WLS-AM radiator, using the dimensions and ground
system provided by my former supervisor, Warren Shulz, who recently retired
from WLS after working for more than 40 years in Chicago broadcast
engineering.
When using NEC 4 during the WLS modeling, the vertical profile also shows
(as expected) an infinite notch at zero degrees elevation over moderately
good soil. What the far field model of NEC does not show is the
significant radiation at low elevations. For that, a surface wave tool is
required like the one that comes with 4Nec2.
If we were to only believe in the NEC far field computation, AM braoadcast
would have met its death in the early 1920s -- for nobody would be able to
hear the stations except at night when the upper angle radiation could
utilize the ionosphere.
To get a complete picture of the vertical profile over soil really requires
a meshing of the two data. So far, I don't think any of the modeling
authors have expressed an interest in creating a hybrid analysis tool. NEC
is providing the raw data and its accurate. But we need a better form of
display that integrates the far field with the surface wave.
As W8JI pointed out, this is nothing new. Academics like Terman, BL&E, et.
al. were teaching it back in the early 1930s. But we've become firm
believers in the typical vertical profile field plots when the only accurate
vertical profile result from a vertical radiator is produced from a vertical
over a super-conductive surface like salt water. There's nothing wrong with
the far field plot, but we should realize its limitation in the context of
the vertical profile, especially on topband.
Paul, W9AC
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