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

[AJ4SQ]

I will preface my question by saying that I haven't worked through a problem like this in 20 years, so it's quite possible that I'm forgetting some obvious details.

I'm puzzled by how this surface wave can reach the ionosphere. I believe you are saying that the standard MoM solution ignores the surface wave because, by construction, it includes only real outgoing (up) fields. Then it would seem that what you call the surface wave must be the remaining part of the complete solution, and so it must decay exponentially in the vertical direction. While this part of the total field undoubtedly  makes a large contribution near the surface and relatively close to the source, a decaying solution can't be projected in a straight line and assumed to reach the ionosphere. Perhaps I am misunderstanding your terminology.

Jack
WS3N

On Sep 12, 2013, at 9:35 AM, "Richard Fry" <rfry at adams.net> wrote:

> 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