Topband: Skywaves from Monopole Surface Waves

[Michael Tope]

On 10/9/2012 7:31 AM, Tom W8JI 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?
>
> Is the disagreement about how useful the really low angles are, or is 
> the disagreement about if a low angle measurement (groundwave) is 
> meaningful in determining changes in radiation at useful higher angles?
>
> Groundwave has no value at all for working long distances, and under 
> nearly all conditions extremely low angles have no value on 160 meters 
> for DX.
>
> On the other hand, I don't think many would dispute a groundwave 
> measurement of FS changes between various vertically polarized 
> radiators would be closely tied to FS at usable higher angles. The 
> exception would be those cases where high angle horizontal propagation 
> is a dominant mode.
>
> I have about ten pages of ABC tests from here to VK/ZL and I'm pretty 
> comfortable that angles at or below 20 - 30 degrees dominate almost 
> all of the time, with the most common exceptions only at sunrise or 
> during geomagnetic disturbances. This even compared a dipole at about 
> 280 feet effective height above ground, so there was "lowish" angle 
> horizontal polarization in the test.
>
> Groundwave is a very good way to evaluate vertical antenna efficiency, 
> but certainly not a horizontally polarized mode. I know someone who 
> measured a horizontal antenna at a modest distance and claimed he 
> improved efficiency 10-20 dB by removing his balun and altering 
> feedline length. :-)
>

Tom,

I agree that groundwave measurements provide a meaningful way to 
evaluate vertical efficiency, but not horizontal antenna efficiency.  
Also, I see no reason to dispute your findings on which angles of 
radiation are best for DX. I remember eavesdropping on some of those 
test you made when you were keeping daily skeds with VK3ZL and I've done 
no such tests myself.

My question (it is not a disagreement because I am not sure I know the 
correct answer) is whether the NEC-4 elevation patterns which include 
surface-wave (such as the one Richard Fry has linked to) are 
representative of what gets projected on to the distant ionosphere or if 
the far-field skywave pattern is a better representation. If Richard's 
assessment is correct, then a vertical over average soil should have as 
much gain at 1 or 2 degrees elevation angle as it does at 20 or 30 
degrees. Furthermore, it should only be a few dB down from a vertical 
over salt water over that same broad range of elevation angles. That 
certainly contradicts the conventional wisdom.

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