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Re: Topband: Skywaves from Monopole Surface Waves

To: topband@contesting.com
Subject: Re: Topband: Skywaves from Monopole Surface Waves
From: Michael Tope <W4EF@dellroy.com>
Date: Sat, 13 Oct 2012 20:58:57 -0700
List-post: <topband@contesting.com">mailto:topband@contesting.com>
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.........





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