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

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[Ron D'Eau Claire-2]

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


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[Stuart Rohre]

Jack,
Yes, the beam could be shortened with helical wound elements of wire.   This
was a beam shown in the ARRL Handbook way back, not sure if 1957 or when.
You could even make the elements of ham sticks!

You can fed one side with a monopole approach to get impedance matching, see
the Kent Brittain designs of cheap VHF beams in CQ VHF magazine.  He may
also have them on the web, all wire elements there, too.

The Gotham vertical and beams of the 1950's all used thin Aluminium tubes
for elements and gamma matches for the beams.

GL and 72,
Stuart
K5KVH


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