My summer project besides upgrading my K2-100 with the latest modes and
maybe DSP, I'm thinking about the possibilities of building a
3-element, 20-meter monoband yagi with the design emphasis on light
weight to enable use of a 40-ft steel, telescoping, TV-type mast and
rotor which I can afford and not raise and building permit issues. This
idea inspired by PA3HBB article about such a yagi in the July 2001 QST.
The availability of relatively cheap (under $10 USD) 20-ft telescoping
fishing poles for elements and composite tubing for booms make this
design seem plausible.
The QST design involves attaching #12 solid copper wire to the fishing
poles to construct the elements. Can anyone tell me if the elements can
be practically shortened if the wire was spiral wound around the pole
elements instead of laid straight. How can one calculate the effect of
a spiral winding and predict the shortened length? Would such a spiral
design denigrate gain, or SWR bandwidth performance to the point it
would not be advisable?
Also, would matching antenna to feedline (RG-8) using an off-center
driven element approach to match impedance be a practical solution? If
not, what might be the best method to reduce weight?
Any comments or recommendations on the best element spacing parameter
to ensure 1.5 SWR from CW to SSB segments of 20 meters. I see .2
wavelength even and off-set element spacing designs in the literature.
Can anyone recommend one or the other approaches? ( I understand that
off set elements will induce a higher wind torque load on a rotor
unless compensated for. Any other considerations such as element or
boom guys?
Thanks for any advice you may have to give--it will be appreciated.
Jack
Jack H. Shrawder, KT6JS
K2 3256
"Success is the only option."
--Dave Borges, DC
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