RE: NEC-LIST:helical ant. modeling

Hi Chuck,

At 9:29 AM +0200 10/4/01, Ian Roberts wrote:
>My hands-on with a 22 turn helical at 435 MHz highlighted two aspects:
>Helicals suffer severe gain saturation if constant radius/constant spacing
>is applied....
>The workaround I applied was to group turns of this helical into 10 at 1.1
>circumference for maximum gain, 6 at 1.0 circum, and the rest at .95
>circumference while maintaining the same spacing....

>?Never having tried to build a helical ant., I'm confused by this
>description. I'd like to understand:

>Are you varying the radius of the helix while maintaining the axial
spacing of turns? Which end of the helix has smaller radius?

Yes this is correct, the last turns at the feed end tend to define the
overall feed impedance of a helical. I don't think it can be reversed as one
would have a decreasing impedance characteristic as the wave approaches the
rear of the antenna (the feed end). It might be possible to feed the smaller
radius end and mount this againt the ground plane, but I don't know.
Progressively decreasing the element lambda/circumference as the wave
approaches the driven end would surely kill a yagi style antenna - it might
be OK in a helical.

>Is a picture/drawing available?

No, have a look at the illustration on the Web site mentioned previously.
There the approach regarding matching was different. The last turn was much
larger, as in a yagi, and served as a reflector.

>Tnx -Chuck

Cheers,
Ian.

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