On 9/30/2012 7:29 AM, John Frazier wrote:
Thank you, to all who took the time to offer good insight and experience.
Another important point, John. The DX performance of any antenna is
strongly dependent on how it interacts with the earth, and the quality
of the earth in the miles around the antenna. The vertical pattern of a
VERTICAL, especially the low angle performance, is the result of the
first reflection from the earth adding in phase to the direct radiation.
The strength of that first ground reflection depends on soil
conductivity -- the difference between very good soil and poor soil can
be as much as 6dB, and sea water adds another 6dB.
When we add radials to a vertical, we reduce the earth losses in the
NEAR FIELD -- that is, within a quarter wave or so, which launches a
stronger signal (2-3 dB for a serious ground system), which makes the
TOTAL signal stronger, but it doesn't help the SHAPE of the vertical
Horizontal antennas are very different -- height determines the SHAPE of
the vertical pattern, but soil conductivity doesn't change the STRENGTH
of the far field signal. SO -- those who say their vertical arrays are
outperforming high dipoles probably live in areas where the soil
conductivity is very good and have put very good radial systems under
them, and those whose high dipoles work better have less wonderful soil.
When we talk about "high" dipoles, we're generally talking about more
than a quarter wave above ground. Raising a dipole from one-eighth
wavelength to a quarter wave is good for only 1dB at 10 degrees
elevation, but getting it up to a half wave is good for ANOTHER 5dB.
In addition, there's the issue of terrain. Horizontally polarized waves
interact with the terrain in ways that are rather predictable, and some
ground slopes are much "nicer" than others. N6BV's HFTA (High Frequency
Terrain Analysis) program, which comes free with the ARRL Antenna Book,
is quite good at studying this effect. Those of us who live in places
where the land isn't flat have found it quite useful (and accurate) in
planning antenna heights. .
73, Jim K9YC
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