On 7/5/16 9:17 AM, Jim Brown wrote:
On Tue,7/5/2016 7:22 AM, Jim Thomson wrote:
>http://k9yc.com/VerticalDipole.pdf
## Look at the PDF. Think of a dipole.. but turned vertical.
With coax going to center, and center conductor bonded to upper wire
half,
BUT, the lower wire leg, is replaced by the braid of the coax itself.
A CMC is inserted where the lower insulator would normally be placed
on a wire dipole.
Below the coax CMC... is just a continuation of the same coax... all
the way back to the xcvr.
## it’s a unique way to build a vertically polarized dipole...using
a coaxial CMC as the lower
insulator. It also places the CMC at an extreme high V / high Z
point...and Im surprised the
CMC actually survives.
Yes. This is EXACTLY why I built my first vertical dipole in 2007 -- a
worst case dissipation test for a ferrite choke wound with coax. It was
cut for 40M, and suspended in a tall redwood next to my house, and fed
with RG59 (a Belden RG59 with real copper center and copper braid). The
choke was one I had measured, 7-8 turns through a big clamp-on. It
lasted about 15 minutes in a CW QSO at 1.5 kW. Failure mode was melting
of the coax. I replaced the coax with a transmitting RG6 (two copper
braid shields and copper center with Pasternak's name on it bought at a
surplus store in Si Valley) and wound two of the same chokes on it. That
combination held the TX power just fine. Double the resistance, so half
the current, dissipation divided between the two chokes.
BTW -- this feed method could also be used to rig a dipole horizontally
from a high window or roof to a tree.
It's also been used in one of ARRL Antenna Compendiums for an
inductively loaded 4 square, except there, they ran the coax up inside
the lower half which was tubing. Same idea, though..
So you do need a fairly high Z on the outside of the shield for this.
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