Topband: Monopole Radiation Patterns, takeoff angles etc

[Richard Fry]

A common use for correctly defined NEC models shows the electrical 
characteristics of the radiator system itself.  But NEC also will show the 
field intensities that system will produce at a given distance for a given 
applied power, frequency, and earth characteristics -- and do so quite 
accurately.
The link below leads to a comparison of the groundwave field measured from a 
real-world AM broadcast system by a broadcast consulting engineering firm 
vs. the NEC-2D output using those same system parameters.

Included in the NEC analysis is the value of the space wave field at an 
elevation of 100 meters above the (level) ground plane, for this 
installation -- which should help better understand the points of the 
opening post in this thread.

Note that the value of the space wave at 100m elevation and 100m downrange 
is lower than the groundwave 100m downrange, and increases as the range 
increases.  This is as expected, because the relative field (E/Emax) of the 
elevation pattern "launched" by this 1/4-wave monopole reduces as the 
elevation angle increases.

Hope this graphic and explanation are useful.

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