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Re: Short helix feeds.



Thanks for the note David.
If I've learned one thing from my engineering career, it's that the
assumptions and caveats are the first thing to be edited (out)...I would
not be surprised to learn that the original bibliographic references to
short helical dish feeds mentioned the lack of purity in their CP, but
by now it's been edited out.

But then, it may not necessarily matter.  The short helices I've
modelled on NEC generally come up with around 6dB axial ratio.  Although
that's not considered particularly good for a CP antenna,

[Aside:  What is axial ratio?  Basically it's the peak-to-peak envelope
response you would expect from an antenna if it were receiving from a
perfectly linear far-field source.  A perfectly circular antenna would
see no fluctuations in RX power as you spun a linear far-field source
(spun as in polarization); a perfectly linear antenna would vary from
some maximum value to zero, so the AR is (infinity)dB--the peak-to-peak
swing--or zero in non-logarithmic units.  This is somewhat simplified;
anyone wanting more let me know directly]

assuming the AO-40 antennas have around a 1dB AR (good CP antennas) the
same-sense polarization mismatch is at most -0.64dB...pretty much "who
cares?" at that point.

So...sure they work.  Field tests prove it.  Theory confirms the
polarization mismatch to be minimal.  They're cheaper and easier than
(most, if not all) other feeds.  Build Away!

PS to anyone:  Still seems unclear whether or not it will be
necessary/desireable to have switchable sense CP on the AO-40 S-band
downlink.  I have heard conflicting reports.  Long, traditionally
dimensioned helices don't swap sense in their sidelobes (not the ones
I've modelled, anyway) so I would guess that sticking to RHCP should do
it.  I would encourage all those with switchable sense capability to
continue to let the list know what their findings are!


G0MRF@aol.com wrote:
> 
> In a message dated 7/24/01 7:52:14 PM GMT Daylight Time,
> owner-AMSAT-BB-digest@AMSAT.Org writes:
> 
> << I would think that helices, being slow-wave structures, would need some
>  length to "get settled"...just like a Beverage antenna (not many 1/4wl
>  Beverages out there!).  Comments?
>   >>
> 
> I think NEC is correct.
> The short 2.5t helix has always been recognised as a compromise when it comes
> to true C.P.
> A better choice would be a patch etc, but at the time the helix represented a
> simple antenna to build.
> 
> I'll be helping run an antenna test range this weekend for 2.4G antennas. I'm
> sure we'll see some very strange results when we start looking at signals
> from dishes with those feeds.
> 
> 73
> 
> David  G0MRF
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