Topband: Monopole Radiation Patterns, takeoff angles etc

[Richard Fry]

>What I am saying is that ground loss must increase the higher we go in 
>frequency, attenuating the surface wave more and more the higher and higher 
>we go.

>But Rich is also talking about the radiation at zero degrees bouncing off 
>the ionosphere and returning to the earth at some distant point.

Groundwave propagation losses increase directly with frequency and inversely 
with earth conductivity.  However my statements in this thread, and the 
graphics I have linked to do not support the conclusion that the radiation 
at zero degrees elevation is the source of the low-angle fields launched by 
a monopole.

A review of the comparison linked next below (again) shows in the NEC plots 
that it is the space wave radiation from the monopole at elevation angles 
above about 1 degree that are responsible for the skywave fields existing 
100 meters above the ground plane -- not the groundwave field radiated at 
zero degrees.  Nothing prevents those fields at 100 m AGL from traveling on 
to the ionosphere.

http://i62.photobucket.com/albums/h85/rfry-100/Measured_vs_NEC2D_Fields2.jpg

>The far field pattern will accurately capture what is going to reach the 
>ionosphere. A surface wave table will accurately capture your groundwave 
>range.  If you were to try to do a plot as far as the distance to the 
>ionosphere at a low angle, the above plot would be misleading because the 
>earth has dropped away significantly, taking the strongest part of the 
>earth-hugging surface wave with it....
The NEC surface wave includes low-angle fields well above zero degrees 
elevation that do not appear in a NEC far-field plot, and they are in fact 
space waves (see link below).  Those fields can propagate to the ionosphere 
and back if the paths are unobstructed.

The surface wave field in this plot is that for a 50 kW broadcast station on 
760 kHz, over a 145.3 km "groundwave" path with a conductivity of 8 mS/m.

http://i62.photobucket.com/albums/h85/rfry-100/WJRElevationField.gif