[Elecraft] Featherweight Yagi Design Questions

[Ron D'Eau Claire]

Antenna elements, including beam elements, can be shortened by so-called
"linear loading". For a while, this was always done by winding the element
into a spiral 

(Watch out! Mention that often enough and someone who doesn't understand how
antennas work will say that makes it and RF choke and everyone knows that RF
chokes don't radiate. The problem isn't in the winding, it's in the person
saying it. That is categorically NOT true). 

Various manufacturers have made other forms of "linear loading" popular,
especially the technique of zig-zagging the element back and forth. 

I've never seen good data on exactly how the loading effects the resonant
frequency. Obviously, it lowers it, but by how much seems a matter of 'cut
and try'. DeMaw years ago published the suggestion that a 1/2 wave length of
wire wound into a linear-loaded element at least 1/16 wavelength long would
produce resonance approximately as a 1/4 wave element. That is 32 feet of
wire on a form at least 5 feet or so in length would resonate as a 1/4 wave
on 20 meters. But it's at best a rough estimate.

Several ARRL antenna books have examples of "linearly-loaded" beams for 20
that have more precise suggestions for an actual design. The renowned L.B.
Cerbik, W4RNL, devotes considerable space to the subject of small beams
on-line at: http://tinyurl.com/2rf96

In general, shortening any antenna by any means increases losses because it
decreases radiation resistance, hence increases RF currents and ohmic losses
in the conductors. Yagis already exhibit lower radiation resistances
compared to ordinary half-wave radiators due to the inductive coupling to
the parasitic elements. Still, short Yagi antennas can exhibit considerable
gain and performance, especially when they are mounted at least 1/2
wavelength above the ground.

Ron AC7AC 

-----Original Message-----
I'm thinking about the possibilities of building a 
3-element, 20-meter monoband yagi...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?...
Thanks for any advice you may have to give--it will be appreciated.

Jack

Jack H. Shrawder, KT6JS
K2 3256