Topband: Monopole Radiation Patterns, takeoff angles etc

[Richard Fry]

All vertical monopoles of 5/8-wavelength __and less__ radiate (launch) their
maximum relative field (E/Emax) in the horizontal plane.  This is true no
matter what the loss in the r-f ground connection they use.

A lossy ground connection will reduce the gain of the antenna system, but it
will not change the relative fields they radiate.  IOW, their pattern shapes
remain the same regardless of the loss in the ground connection, be that to
salt water, or dry sand.

The link below leads to a plot of the radiation patterns and directivities 
of
several monopoles.  These are the shapes of the radiation patterns leaving
the monopole as they exist at the beginning of the far field of the 
radiator.

These patterns were calculated for two ohms of loss in the r-f ground
connection - which is about the loss that 120 x 1/4-wave buried radials
provides even in poor soil.  If fewer/shorter radials are used, then loss
increases and the directivities (gains) of these patterns would be
reduced -- but the radiation pattern shapes would remain the same.

Many amateur radio operators consider only the far-field pattern of a
monopole antenna as shown by NEC and in textbooks, without realizing that
this is not the shape of the radiation leaving the monopole.  It leads to
the concept of a "takeoff angle" where radiation apparently was maximum
from that monopole.

However the elevation field radiated by a monopole always is maximum in the
horizontal plane, and always is less than that at the elevation of an
assumed takeoff angle.  A NEC analysis including the surface wave from the
monopole will show this.

Some of that low-angle radiation can reach the ionosphere and produce
skywave service, even though according to a NEC far-field analysis, the
fields are approaching zero at those low angles.

This doesn't mean that radiation at and near the "takeoff angle" does not
provide significant skywave service, but it does mean that significant
skywave service can be generated by radiation at much lower angles than
commonly believed.

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