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Re: S-Band Antenna Comparisons, etc.

A couple months back I modeled a 3-turn helix dish feed using NEC4WIN and
it ate several days of time [on the old P100] doing the over a dozen
iterative designs before I had a good one.  The helix was designed for 1420

Although it takes time to model the reflector grid it is easily within the
program's capability.  And, yes, it only took four hours to build ;-)  I
did find that the field pattern/impedance using a reflector grid model in
free space, varied significantly from using a perfect ground in place of
the reflector.

Now that I have it on a fast computer [P3-866] with lots of memory NEC4WIN
runs really fast!  I have the virtual memory version of the sw so the HD
can be used if necessary for large models.

NEC4WIN comes with a sample helix model which I expanded.  It consists of
eight line segments per turn.  I chose 10 segments per wire, so the 3-turn
helix has 240 segments.  I modeled the reflector grid with a rectangular
grid with .125 wavelength spacing between wires.  Each wire was 6 segments.
 The reflector was .6 WL square.

These models are great way to waste time and not get anything built :-)


>From: Bob Snyder <kd1vv@mediaone.net>
>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.
>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
>> using commercial test gear (he works for a radio station in Nizhny
>>  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
>> 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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