Paul Christensen, W9AC wrote:
The surface wave tool most be used in conjunction with the normal modeling
application to get a complete and accurate vertical profile from 0 through
90 degrees.
Agreed.
That a vertical monopole 5/8 wavelength and less in height, using a less
than perfect ground plane has a certain takeoff angle above the horizon
where radiated field is maximum is a very common (almost universal) belief
of ham radio operators This is based on the use of MoM software such as NEC
to model only its far-field elevation pattern.
The graphic below shows a different conclusion when considering the surface
wave in NEC evaluations, for the parameters shown.
The NEC far-field pattern for 0.1 km shows a maximum field intensity of 590
mV/m at an elevation angle of 23 degrees (the assumed "takeoff angle"). It
also shows that the field at an elevation angle of 5 degrees is 348 mV/m.
The NEC surface-wave pattern for 0.1 km shows that the maximum field lies in
the horizontal plane rather than at 23 degrees, and is about 890 mV/m rather
than 590 mV/m.
The surface wave analysis also shows that the field radiated toward 5-degree
elevation is about 850 mV/m, rather than the 348 mV/m shown by the far-field
analysis. Of course, the ratios of these fields are even greater for
elevation angles below 5 degrees, and infinite in the horizontal plane.
It is true that at great distances from a vertical monopole, the radiation
present at low vertical angles is much less than at higher angles. But that
does not mean that the greater radiation directed at low elevations __as
launched by the monopole__ no longer exists. The radiation toward an
elevation angle of 5 degrees shown in the surface wave plot continues in
essentially a straight line, to reach the ionosphere.
It is the radiation launched at these low elevation angles that can provide
the greatest single-hop range and fields for skywaves reaching that range,
even though its existence might be unrecognized, or disregarded.
http://s24.postimg.org/6nchfpt1h/NEC_FF_vs_NF_Calcs.jpg
R. Fry
_________________
Topband Reflector
