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Re: phase shift with coax



Hi Stan,

I used one of these for AO-10 in the mid-1980's and still have a few
elements lying around.  In fact I took them off and re-spaced them acording
to the Cushcraft 13B2 only using 10 elements to make two yagis on 11-foot
booms.  I have since removed them and they are now replaced with four M2-20
element x-yagis {for eme}.

But the reason I reply is that I kept the phasing harness with the idea of
using just the two driven elements to make a turnstile {for Leo's}.  Well
that project is still languishing, but your query did stir my memory of the
strange dimensions used in the harness.  Assuming polyethylene RG-58 and
RG-59 were used, that implies a velocity factor of 0.66.  Your formula is
correct:  L = 246*[.66]/146 = 1.112 feet = 13.345 inches.  That is a bit
long compared to the 10.625 inch RG-59 sections and the 10.5 inch RG-58
section.  The UHF barrel and Tee will not account for this discrepancy.
Nor will the spacing on the boom as the V and H elements were only offset a
couple inches.

Regarding polarity measurements, that is harder, even on a good antenna
range.  I recall that tuning the individual vertical and horizontal driven
elements for minimum swr did not result in the a good match when using the
harness; I recall having to re-adjust the matching for each after the
harness was connected.  So did Cushcraft goof? Or were there other hidden
factors in their design?

It has been my thought that Cushcraft came up with this length
experimentally.  I would just make another cable the same as they did.  The
antenna worked pretty good back in those days.

Ed
Hmm, maybe I'll get that turnstile up one day...

>From: "Stan Vandiver" <Stan@Vandiver.com>
>Hi all,
>
>This is probably a no-brainer, but I will ask anyway. I'm needing to 
>make a new phasing harness for a Cushcraft A144-20T to generate 
>circular polarization.  I have 2 of the 3 pieces that are original, and 
>they seem to be somewhat shorter than I expect (about 10-5/8").
>
>Is the "free space" formula for 1/4 wavelength (246 * velocity factor / 
>freq in MHz) = length in feet) the correct way to calculate the 
>lengths for these pieces? Do I need to factor in a little length to 
>account for the barrel connector between the parallel phasing line 
>and the delay line? Or account for the "T" connector that splits to 
>the horizontal and vertical sections of this beam?
>
>The connectors might explain why the Cushcraft pieces seem a 
>little short.  But also I do not know the velocity factor of the coax 
>that they used, so I'm kinda confused on how to replace them. I'm 
>thinking of replacing all of them, but maybe the better choice would 
>be to just make one to replace the missing piece (which is one of 
>the RG-59 75-ohm legs)?
>
>Will there be any good way for me to judge the effectiveness of the 
>circular pattern.... or is SWR the only measurement I can make?
>
>Your comments are welcome!
>
>73 de Stan/W4SV
>
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