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Re: Helix Match



Joe, Gang

> My curiosity was relative to how the circular polarization held up.

My understanding is that the reflector can be bigger than required - but
not less than the proper dimensions. Polarization not affected by this;
but putting the spirals too close together (laterally) may result in some
interaction (I've heard it called squint), assuming the spirals are both 
identical (for the same band).

> So if you can provide any insight on the effect of having two separate
> reflectors I'd be grateful.     How far apart the centers of the
> reflector are?    Any knowledge of the overall beam pattern.

Same as for yagis; put them side by side and the horizontal beamwidth
will be narrower but the vertical beamwidth shouldn't change, put them one
above the other and the opposite will occur. My pair of helices were built 
using three broom handles; one each for the central support and the third
one is the boom they're mounted on ... so about 3 to 4 ft apart.

> Your thoughts on the matching may lead to methods of a two band helix
> feed .     Concentric or offset helix may be considered.   

I heard somewhere about having helices for different bands on a common
central support; it works. Theres some interaction but its not a killer;
and constructed like that it doesn't result in off-pointing, just loss of
efficiency (so give the spiral an extra turn or so). I recall an article/
talk by G3RUH who experimented with nearby metalwork and its probably that 
source I'm thinking of.

> >Separates. Circles of chicken wire (about half-inch mesh).

For 23cm I'd suggest a smaller mesh.

73
Richard W L Limebear G3RWL
g3rwl@amsat.org
FOC # 1188

          So many beautiful girls ..... (sob) so little time
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