Topband: Skywaves from Monopole Surface Waves

[Michael Tope]

On 10/7/2012 3:42 AM, Richard Fry wrote:
>
>
> It will be seen from the data that no "notch" exists in the fields 
> radiated by the monopole at elevation angles of 3 degrees and less, as 
> expected by some when considering only the far-field patterns shown by 
> MoM (NEC) software, and in antenna textbooks.
>
> That low-angle radiation can reach the ionosphere to produce a skywave,
> under the right conditions.   That skywave can be very useful to hams
> using vertical monopoles, even though its existence may not be
> recognized.

Everyone seems to agree that at moderately short distances from the 
vertical radiator (a few miles), there will be pattern fill-in at very 
low-angles and that this fill-in is not predicted by the far-field 
pattern equations. What everyone seems to be dancing around (Dick hints 
at it above) is whether or not any of this low-angle ground-wave energy 
ever reaches the ionosphere and if so, how? Clearly if you keep 
increasing the distance from the vertical radiator, the E-field at zero 
elevation angle drops faster than 1/r (except in the case of infinitely 
conductive ground) so eventually the "notch" in the pattern predicted by 
the far-field equations appears.  One can image taking the helicopter 
used by Dick's BC engineer friend and repeating E-field versus height 
measurements at increasing distance intervals out to many miles. In 
fact, one of the NEC-4 surface-wave plots Dick posted a few days ago 
shows that for average soil a fairly deep low-angle notch (~ 20dB) has 
already appeared in the elevation pattern at just 20 miles distance from 
the vertical radiator:

http://i62.photobucket.com/albums/h85/rfry-100/Surface_Wave_Flds.jpg

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?

73, Mike W4EF......................