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[TowerTalk] Re: 40M wire beam (long)

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Subject: [TowerTalk] Re: 40M wire beam (long)
From: (Martin Ellis)
Date: Wed, 26 Nov 1997 08:35:41 +1300
At the Zl1AA club field-day contest site, I have been using
a 3 element 40m wire beam with outstanding results.  We have won
the Northern section for each of the last five years here in ZL.

I did not use a program, but relied on the old Handbook formulas.
Use 468/f = 66feet 8in for the dipole radiator, but make site
adjustments for end and ground effects.

1.      Erect a sky wire using non-conductive nylon rope between
two towers or masts or trees. Remember this is the boom, and should
point in the direction of propagation. 

2.      Mark out the spacing for the elements, keeping elements at
each end away from the metal towers.  I suggest 19 feet spacing
between each element. So a 3 el version has 19 feet from ref to ra,
and 19 feet from ra to de.  Remember that wire is a narrow
conductor, and also it is difficult to get the alignment of the
elements precisely the same.
For these reasons the mutual conductance between the elements is
less that for aluminum elements, and therefore close-spacing
will work better than wide spacing.

3.      Attach an inverted V dipole as the radiator.
I used a standard dipole center fitting with an SO-239.
You will need to reduce weight on the sky-wire (boom) so feed
with RG-58, not RG8 type.
After the RG-58 drops to the ground, larger feedlines
can be connected for the run to the shack if necessary.
Support the ends of the inverted V dipole with poles or bamboo
or tie off around the yard using light garden twine.
The ends should be at least 20 feet above ground.
Trim the ends of the dipole for resonance in the CW section
around 7.025. Wrap ends back above the egg insulators for
12 inches or so in case adjustments are needed when the other
elements are in place.

4.      Attach the reflector, cutting it  +5% longer than
the actual length used in the radiator.

5.      Attach the directors.  I would suggest that you
use -3% for the first, and -5% (of the radiator length)
for the second and third directors if any.

6.      Check the alignment of each inverted V element,
and tighten the sky-wire to reduce sag.
We use bamboo poles for the ends of each element and tie
two light nylon guys off to support the pole.

7.      Recheck the SWR and if needed make final adjustemnts
to the length of the radiator.  We have not found this necessary.

The antenna is easy to dismantle and store away for future contests.

Good luck with the project.
Martin ZL1ANJ

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