RE: Re: NEC-LIST:helical ant. modeling

Cornel:

I used the 1/4 approximation and it worked for me. We had thin copper ribbon
on a dielectric substrate that had almost no weight for the space application.
We tried both open and closed on the ends and I will let you learn what is
best by doing. But you might get a hint from John Kraus on that.

Good luck.

George

P.S. The proof of the modeling pudding is in the comparison of the model
results when compared to the measured results for gain, pattern, input
impedance and axial ratio at the required angle(s) off axis.

Original Message:
-----------------
From: Cornel Gazdaru cornel_gazdaru_at_botcorp.com
Date: Sat, 06 Oct 2001 17:07:27 -0400
To: nec-list_at_gweep.ca, ghagn_at_erols.com
Subject: 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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