[Towertalk] GAO Challenger DX

[Chuck Counselman]

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