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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