At 7:40 PM -0700 10/6/02, Jim Shaw wrote:
> >for DX (as opposed to short-range ground-wave work) a horizontal dipole
> >beats a vertical antenna of the same height.
>
>Hmmm, wonder if that is true for a vertical 'dipole'?
I simulated vertical dipoles with NEC-4. See below.
>Always thought at some heights, the vertical 'dipole' might have an
>edge. Likewise, at some heights, seem to recall that an inverted
>vee (30 degree droop below horizontal) might have the edge over a
>horizontal dipole....
The heights I looked at with NEC-4 did not exceed one-half
wavelength, so the length of the vertical dipole was constrained not
to exceed one-half wavelength. I varied the height of the bottom end
of the vertical dipole above ground. For comparison I used
horizontal half-wave dipole at the same height at the top of the
vertical. I did not consider inverted vees. I assumed poor ground,
typical of where I live.
As is well known, a horizontal dipole has low (poor) gain off its
end; but broadside, or within 45 degrees of azimuth of broadside, and
for the low elevation angles (say, between 5 and 10 degrees) that are
typical for DX, a dipole wins.
The exception, of course, is with near-perfect "ground," e.g., sea
water. In this case a vertical wins.
It's not possible to present numbers for many cases here, but if
someone gives me parameters for one or two cases, I will rerun NEC-4
and post the results for the given case(s). Give me:
* the frequency;
* the heights above ground of the top and the bottom of a vertical
antenna, which I will center-feed in "dipole" mode (for realism,
limit the top height to a half-wavelength);
* the ground parameters (or I will assume epsilon over
epsilon-sub-zero equal to 5 and sigma equal to 0.0015 S/m, which
are the actually-measured values for my New England QTH); and
* the elevation angle (between 5 and 10 degrees) at which you'd
like to know the power gains of the vertical and of a half-wave
horizontal dipole having height equal to the top height of the
vertical.
Here's a case that I've previously run:
Inputs:
* frequency = 7.0 MHz;
* vertical top 20 m and bottom 2 m above ground;
* my actual ground parameters; and
* elevation 7.5 degrees.
Horiz. is
Power gains calculated by NEC-4: (dBi) better by
* for the vertical at any azimuth: -4.5 (dB)
* for the dipole, broadside: -0.3 4.2 <--
* for the dipole, 45 deg from b'side: -4.0 0.5
* for the dipole, off the end: -14.2 -9.7 (worse).
Warning: I won't say "Don't try this at home," but be warned that
MININEC, NEC-2, and programs based these can not accurately model an
antenna that is within a small fraction of a wavelength of realistic
ground. NEC-4 can.
73 de Chuck, W1HIS
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