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Re: Disparaging Helical Antennas for AO-40...some notes



A good question Jerry.  There's another paper out there (Trueman et.
al., "Modelling Helix Antennas with NEC-4", 1997 IEEE Int. Symp. on Ant.
and Prop., p. 1584) who went about validating one of the King and Wong
helices.  They modelled a 10-turn job including the cup.  At first NEC-4
overpredicted, but then they slowed down the wave velocity on the helix
by adding a dielectric "sheath" to the helix wire and got very good
(within 0.3dB versus the entire frequency bandwidth) agreement in gain. 
The authors attributed this to the fact that K+W built their helices on
a styrofoam form which has a non-unity dielectric constant.

On the "user-unfriendly" NEC code versions there's a flag in the RP card
to calculate normalized directive gain; it's a method to double-check
the gain calculation and adjust it down/up depending on the actual
pattern distribution.  Does NEC4Win95 have any such option?  (Sorry I am
in-the-dark on "modern" NEC tools...I learned on punch card NEC data
entry and will stay with it until the final SK :-)

In spite of all this I'm building a 2x2 helical array, N=18.  Should be
20.6dBic according to K+W plus some array theory calculations, all in
less than a square foot (but a square foot that's two feet long hi). 
Why?  Because I Can.

I'll be on AO-40 by the end of the summer...I'm treading water as fast
as I can!

 
K5OE@aol.com wrote:

> 
> > I should also point out that the King and Wong measurements used helixes
> > that were backed not by a flat ground-plane, but a "cup" or cavity.
> > Other investigators (references not handy at the moment) have found that
> > the "cup" gives about 2dB additional forward gain.
> 
> I have wondered if the discrepency between King & Wong and Emerson was due to that.  Kraus discusses the semi-parabolic reflector.  I note my NEC4Win95 models report numbers much closer to Kraus, so I don't know who to believe!  NEC does confirm the cupped reflector reduces the 45 degree side lobes.  A similar design was used in RV3TH's helix I tested last year:
> http://home.swbell.net/k5oe/rv3th_69.jpg
>
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