[Antennas] vertical dipoles

2002年7月12日 18:16:54 -1000

Terry wrote, in part:
> I 'd like some info on vertical dipoles, and don't seem to be able to
> find a good source.
>> Do they need a good ground system (I'd guess not, ....

Yes, they do need a good radial system; otherwise
you will lost up to 6 dB of signal (-75% of rig output power!).
Best example is right here on Kauai where I live. Have
a look at the WWVH site info:
"The 5, 10, and 15 MHz antennas are phased array vertical
dipoles. They consist of two half-wave vertical dipoles that
are separated by a quarter-wavelength and driven 90 degrees
out of phase. These antennas radiate a cardioid pattern with
the maximum gain pointed toward the west."
And here is a link to a photo of the 15 MHz array of half wave,
vertical dipoles at WWVH:
http://www.boulder.nist.gov/timefreq/images/radiostations/wwvh-large/wwvh5.j
pg
Now, note the many radials, on the ground, going under the
fence in the foreground of the photo. Note the center insulator
of the Rohn towers of the verticals. These actually might be
called sleeve verticals, since the coax runs up the center of
the tower, but is shielded from the lower half of the dipole
by the many sloping "elevated radials" down from the feedpoint
to the ground (these are the lower half of the dipole).
The radials were found to be absolutely needed to regain the field
strength out in the far Western Pacific to which the WWVH siganl
is directed by these vertical dipole phased arrays.
Have fun.
73, Jim KH7M
On the Garden Island of Kauai