[BC] efficiency vs height

[Richard Fry]

A PDF document has been added at the link below, comparing the elevation 
patterns of MW radiators of 90, 180, 195, 205 and 225 _electrical_ degrees 
in height, over a perfect ground plane.  This should make it easier to see 
the result of the added "efficiency" and skywave changes for the taller 
radiators plotted.

A second plot on the page shows the calculated gains in more detail.  The 
gains are given in dB above an isotropic source (dBi).

The gain shown for the 90 degree radiator is 5.15 dBi, as expected.  When 
the plotted gain in dBi is adjusted for the 2.15 dB gain of a dipole over an 
isotropic source, and the 3dB reflection from the perfect ground plane, the 
result is 0 dBd.  By definition, 0 dBd is the gain of a reference 1/2-wave 
dipole -- which is what the 90 degree MW vertical and its image form.  Note 
also that these are the gain values for this configuration given in standard 
antenna engineering texts (Kraus, Terman etc).

The gain spread between the 90 degree and the 225 degree radiators is less 
than 3 dB in the horizontal plane.  The inverse fields they can produce at 1 
km per kilowatt of input power tends to give an inflated perception of the 
differences.  A field strength change of less than 3 dB probably would not 
be noticed by most regular listeners in the groundwave coverage area. 
Management of the skywave probably is more important, at least for stations 
with nighttime service.

   http://rfry.org/Software & Misc Papers.htm

RF