Topband: Skywaves from Monopole Surface Waves

[Richard Fry]

Mike W4EF wrote:
>So again my question - if this low-angle ground-wave (aka surface-wave) 
>energy dies off so quickly (e.g. down 20dB at just 20 miles), how does any 
>of it get to the ionosphere where it can be useful for topband DX?

The field over real earth that exists within a few kilometers of a monopole 
is not very much less for an elevation angle of 10 degrees than it is for 
the peak radiated field from that monopole.

This is shown in the link in my Topband post earlier today, which I'll 
repeat here:
http://i62.photobucket.com/albums/h85/rfry-100/NEC_FF_with_Surf_Wv.jpg

In this case the field existing 2.8 km from the monopole at an elevation 
angle of ~10 degrees is 86% or about -1.3 dB of the maximum field (at 67 
degrees).  Earth conductivity in the NEC model was set to 5 mS/m, d.c.13 
(about average).

It is such low angle fields existing close to the monopole that continue on 
to the ionosphere to produce useful skywave service under the right 
conditions -- not the groundwave field 20 miles downrange.