[TowerTalk] homebrew 20m ground mouted vertical... good idea??

[Jim Reid]

Some one suggested:


> If you can elevate the vertical about 10' or so and use
> 3 or 4 elevated radials...

You will be dissapointed!!  The E-field lines at the ends
of the resonant elevated radials will couple strongly to
earth at such a low height!  The result will be that both
xmit'd and rcvd signals will be down from 3 to 6 dB
from those of a horizontal dipole up 1/2 wave or so
above ground.  The strong voltage points at the radial
ends couple RF current into the soil where half or more
of your rcvd and output power will be lost to heat!!

If you put such an array up at least 1/2 wave,  or about
33 feet or so to the feedpoint of the vertical element and
the four "radials"  you will not loose so much power,  but
you also will not have the "gain" off the sides of the dipole;
but at least reasonalbe omni spreading of all of your output
power.  Also,  the radials need to be horizontal up there
at that height to avoid coupling to earth.

Out here on Kauai at WWVH,  they found they had to put
in a very extensive radial field beneath their elevated
vertical 15 mHz antennas,  which also use the down
sloping radials.  Have a look at this photo of the installation
at WWVH;  you can see the radial lines,  about 120 of
them around the antenna base.  Look under the fence
rails about the enclosure:

http://www.boulder.nist.gov/timefreq/stations/wwv.html

or a close up:

http://www.boulder.nist.gov/timefreq/images/radiostations/wwvh-large/wwvh5.j
pg

This is a 15 mHz,  1/2 wave vertical array,  with several down
sloping radials.  The ends of the radials terminate at those long
white insulators at the lower ends.  These towers are spaced
1/4 wave apart and driven 90 degrees out of phase: the
result,  a cardioid pattern favoring out at the far Western
Pacific.  Note that the feed point is up something over 1/4 wave in
height and the ends of the "radials" are up nearly an eighth
wave;  120 ground screen radials were still needed to
recover the field strength out in the Western Pacific.

73,  Jim  KH7M




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