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R: Helix phase center



Hi  John,

For 2400 MHz i am using 1,2 meter aluminum dish with F/D=0,45 and a feed
made with 3 turns elix LHCP to get RHCP on reflection from the dish.

3 turns are correct to  illuminate the  edge of  a 0,45 F/D dish at
approximately -10 dB

I was not able to get any theory about the position of the phase center even
reading in to the "Antennas" by John Kraus

I decided to made experiments with sun noise and i found the best position
for the focal point to be at 25 mm (about one inch) from the elix reflector
so that the phase center seams to be inside the elix.

Also i have directly connected  my low noise 0,6 dB preamplifier to the elix
via an SMA connector protecting it into a plastic box.

It works and i get 5 dB of  sun noise with the actual sfu (Solar Flux Units)
and i hear very well the  AO40 S2 transponder noise.

73 de i8CVS Domenico

----- Original Message -----
From: John Wright <g4dmf@AMSAT.Org>
To: <amsat-bb@AMSAT.Org>
Sent: Friday, May 18, 2001 10:57 AM
Subject: [amsat-bb] Helix phase center


> Is there any definitive definition of where the phase center of helix
> should lie? I am currently trying to setup a 60cm offset dish for Mode S,
I
> have made a 3.5 turn helix, that has about the right beam-width to feed
the
> dish, but would like to have a rough idea as to where the helix should be
> in relation to where the Ku band LNB was. I think that the phase center
for
> the LNB is probably inside the mouth of the horn, the feed point is
> probably within the area covered by the original mounting clamp, so its a
> bit difficult deciding where to put the helix. I've tried to find data on
> Ku band LNB's to see if I can find where the phase center is on them, but
> have been unsuccessful. Perhaps it's not really that critical? Any help
> appreciated.
> John
>
> ____________________________
>
>       G4DMF  QTHR   IO93HA
>       Amsat-UK Member 5372
>           g4dmf@amsat.org
>       http://www.g4dmf.co.uk
> ____________________________
>
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