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

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.

--
The NEC-List mailing list <nec-list_at_gweep.ca>
http://www.gweep.ca/mailman/listinfo.cgi/nec-list
--------------------------------------------------------------------
mail2web - Check your email from the web at
http://mail2web.com/ .
-- 
The NEC-List mailing list <nec-list_at_gweep.ca>
http://www.gweep.ca/mailman/listinfo.cgi/nec-list