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Re: S-Band Antenna Comparisons, etc.



Yes, it would be very time-consuming to model the parabolic reflector as
a wire grid, and you would probably exceed the program's segment limit. 
However, it might be sufficient to model the reflector as a series of
concentric circles, since the helix is providing circular polarization. 
That's lots less modelling.  I might play around with that.
73,
Bob


K5OE@aol.com wrote:
> 
> kd1vv@mediaone.net writes:
> 
> > I did some quick helix antenna runs using NEC.  These runs use an
> >  infinite backplane, so they are only approximations, but may be useful
> >  for comparisons.  The 7-turn helix you describe (with 44 mm dia. i.e.
> >  1.106 wavelength circumference; and 37 mm spacing, i.e. 15 degree pitch)
> >  has 11.9 dBi gain.  The 16-turn helix (I assume it is built to nominal 1
> >  wavelength circumference and 12 deg pitch) has 14.8 dBi gain.  The
> >  parabolic reflector on the 7-turner may be enhancing it's gain somewhat.
> 
> Bob,
> The designer/builder (Oleg, RV3TH) tested the antenna and tuned the feedpoint
> using commercial test gear (he works for a radio station in Nizhny Novgorod).
>  He documented 14.5 dBi gain with 25 dB F/B.  He used a tuning strip near the
> feedpoint and I noticed several other variations:  The first turn is about
> 0.35 of the "spacing", the second turn is about 0.75 "spacing," and the
> remaining 5 turns are consistant "spacing" at about 37 mm (+/- 2 mm).
> 
> I have previously modeled some helices in NEC4Win with both decreasing
> diameter and increasing spacing, but have not tried just the latter.
> Modeling a parabolic reflector will take a LOT of work.  It will take me less
> time to build an antenna to test than it would to build up the wire matrix
> for the reflector :-)
> 73,
> Jerry, K5OE
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