[Elecraft] Featherweight Yagi Design Questions

[Jack Shrawder]

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