Topband: More anecdotal "stories" to cause one to stop and....

[Richard Fry]

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