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Re: S-Band Antenna Comparisons, etc.

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.

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 :-)
Jerry, K5OE

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