The raw gain figures you are seeing are roughly correct. A dipole at 30'
up has a good gain, but notice the high angle of radiation. Most DX
requires a lower angle to match the skip angles. You will also notice
that there is considerable radiation, even if not max, at the lower
angles. Hence, a dipole will work DX.
However, there is another question that leads some folks to select a
vertical, especially for the lower HF bands. (Vertical here means any
antenna with vertically polarized radiation mostly, and it includes not
only quarter-wave verticals, but vertical dipoles, half-squares, bobtails,
deltas [when side fed], etc.) Many of these antennas, when well designed
and placed, have very little gain at higher elevation angles. Hence,
although they may have less gain, they enjoy better signal-to-noise ratios
for DX, with high angle QRM and QRN being attenuated.
The far-field radiation for a vertical also depends on the quality of the
surrounding territory at some distance from the antenna. Any vertically
polarized antenna on a small island surrounded by saltwater has
considerably more gain and at lower radiation angles than the same antenna
in a desert, no matter how many ground radials we install.
Nonetheless, a vertically-polarized antenna, especially on the low bands
or for confined yards or for portable operation can do an amazingly good
job--no, not compared to a monoband beam at 200' up, but certainly
compared to simple horizontal antennas with heights less than 1/2
wavelength. Gain is not the only factor to consider in estimating the
probable performance of any antenna.
I hope these notes assist your efforts to interpret the results you are
getting from EZNEC (or any other comparable antenna modeling program).
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || firstname.lastname@example.org
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