Guy Olinger wrote:
I have not personally seen work to validate signal strength and prove the
mechanics of arrival at various altitudes at 50 km. ... NEC4 says that
it doesn't continue. But NEC4 also nicely predicts the 2.8 km helicopter
measurements.
This depends totally on the accuracy of the NEC model, and the analysis that
the NEC operator asks NEC to perform -- which depends on the background,
skill and experience of said operator.
A NEC far-field plot shows zero relative field (E/Emax) in the horizontal
plane, and not much more at low elevation angles above it.
A plot close to the radiator that includes the surface wave shows that
radiation from a monopole is maximum in the horizontal plane, and not much
less than that for low elevation angles above it -- which low-angle fields,
in reality, are space waves. There is nothing in physics that would
preclude those low-angle fields from propagation on to the ionosphere.
There apparently is completely zilch commercial interest or need for the
answers to our speculations.
Not so, sorry. Class A (50 kW, non-directional, 24/7) AM broadcast stations
depend on this low-angle radiation to produce their large coverage area
footprints, at night. This low-angle radiation is equally important to
hams, but apparently is not recognized by most of them.
In my college days at Berea, Ky, I would listen at night to clear channel
650 WSM in Nashville (Grand Ole Opry, etc), using a 195 degree tower,
supposedly the Cadillac for low angle transmission. In the daytime at a
distance of 165 miles, the 50 kW signal was there but very raspy to the
point of irritation at Berea. I learned not to bother turning on that
station during day.
No surprise, there, especially if you were using a Zenith all-American five
in a dorm room. Probably a good car radio even a bit beyond Berea, KY
would/does receive a noticeably better daytime signal from WSM than you
report.
The WSM daytime signal in Berea at 165 miles would be groundwave, which for
those conditions will be relatively weak. The WSM nighttime signal in/near
Berea would be mainly skywave, which might average more than 10X greater
fields than their day/night groundwave there.
R. Fry
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Topband reflector - topband@contesting.com
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