Re: NEC-LIST:helical ant. modeling

Thank you George,

That is exactly what we are doing.
We started with the Kilgus paper and build some prototypes that worked very well.
Now we we plan to simplify the manufacturing aspect and look into a few variants,
i.e. feed from the bottom and not from the top and open arms instead of the
"closed loops". First results seem to suggest that it will work..
All our work so far was based on copper wires, we want to replace with printed
strip on a rolled tube. That is why I was looking at a way of modelling the strip.
The general consensus suggests that a wire diameter 1/4 of the strip width should
be a good approximation.

Cornel

"ghagn_at_erols.com" wrote:

> All:
>
> Several years ago, I had a commercial client (Orbital Sciences' ORBCOMM) for
> the design and prototype development and testing of an antenna for use in the
> little LEO satellite band at 150 MHz. The best structure to satisfy all of the
> requirements was a quad helix. I modeled it with NEC-3 (and then NEC-4) and
> had good agreement with the measurements (done in anechoic chambers developed
> for stealth work) and the NEC predictions--both patterns and impedance vs
> frequency. The antenna designed is now in orbit! It was a real challenge, and
> the SRI team I pulled together did a fantastic job of the almost impossible in
> the trade-offs of weight budget, coverage, axial ratio, etc. Some of the
> history of ORBCOM is available in the book, "Silicon Sky."
>
> The bottom line is that NEC accurately predicted the measured performance, and
> was invaluable as a design tool.
>
> We used the classical paper by Kilgus to get us into the "zone" for good
> performance. Kilgus was hired by ORBCOMM to come and give them a tutorial on
> the quad helix. SRI had proven that the design, used for the first two birds
> launched, would never meet the spec set down by ORBCOMM. That was a
> disappintment to the management of ORBCOMM, since they were on their 3rd
> antenna contractor prior to hiring me and my guys! But SRI designed the
> antennas for the constellation that is now in orbit, and those antennas met
> the spec. It was a satisfying project.
>
> So check out Kilgus' paper for some guidance on getting into the "zone", or
> sweet spot, on the initial design. He did some great work! And trust NEC, but
> verify (as Reagan said), with measurements.
>
> Hope this helps.
>
> George
>
> Original Message:
> -----------------
> From: Ian Roberts ITR_at_nanoteq.com
> Date: Thu, 4 Oct 2001 17:02:49 +0200
> To: ccc_at_space.mit.edu, nec-list_at_gweep.ca
> Subject: 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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