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

To: <topband@contesting.com>
Subject: Re: Topband: More anecdotal "stories" to cause one to stop and....
From: "Richard Fry" <rfry@adams.net>
Reply-to: Richard Fry <rfry@adams.net>
Date: Thu, 12 Sep 2013 09:35:01 -0500
List-post: <topband@contesting.com">mailto:topband@contesting.com>
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
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