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Re: 1/4"3m ribbon helix reply to replies



Howard,
My own testing, using a 2.4 Ghz bridge to observe the match, indicates:

> o Helices in contact with any former (PVC or otherwise) perform badly.

The PVC form lowers the apparent frequency of the antenna by a considerable amount.  I found concurrently reducing the size of the helix (diameter) by about 10%, compensated for the affects (of the inductive reactance) from the PVC former.  Once reduced in diameter to achieve a good feedpoint impedance match, I could not discern any difference in pattern behavior between an air-core helix of nominal Kraus dimensions and a PVC-core model of 85-90% nominal Kraus dimensions.

> o Helices with an off-centre boom not only have mediocre performance, but
> also their pattern is way, way off centre (15 or 20 degrees).

Yes!  I observed very, very significant deleterious affects from an off center metallic boom.  If the metallic boom is centered exactly, it seems benign.  If you move the helix just a small amount, however, the feedpoint impedance reacts wildly and the pattern skews considerably.  If you need a central metallic boom, it can work just fine, but you must find a way to secure the helix from "wandering" around it.  I did not test non-metallic center supports, but would assume the affect is similar but perhaps not as significant.

I wonder if the organizers for AMSAT-NA in November are planning a test range?  There certainly are lots of people with the skills and equipment up there in the DFW area (North Texas Microwave Society's back yard).  I have a garage full of dishes I would love to haul up there and test :-)
73,
Jerry, K5OE
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