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R: Quadrafiliar helix feed for 2.4 GHz?



Hi  Mike,

I have buit a quadrifilar elix for 435 MHz wich works very well .
In order to use a quadrifilar elix  as a feed for 2.4 MHz consider that the
half power beamwidth (-3 dB) is 114° and the -10 dB power beamwidth
is exacly 180°

Remember that you need to illuminate the edge of your dish at a level of
 -10 dB and so the quadrifilar elix must be placed exactly in the plane of
the mouth of your dish .

A dish having a focal point in the plane of its mouth has an F/D ratio of
only 0,25 and  dish with F/D=0,25 are not readily available

In  order to get RHCP from the dish the small bifilar and the large bifilar
of the elix must be wound to radiate LHCP to the dish and the sense of the
 bifilar must be the reverse of that you normally see in Fig 8-17 page 8-16
of the ARRL " The Satellite Experimenter Handbook" by Martin Davidoff,
 K3UBC

The bandwidth of the quadrifilar elix is 2% and you can calculate the
dimensions for any frequency  from table 8-6 of the above mentione book.

Unless you have a dish with F/D=0,25 and the time to cut and try i suggest
do not use a quadrifilar elix as a feed.

73 de i8CVS Domenico

----- Original Message -----
From: Mike Murphree <n4cnw@knology.net>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, May 20, 2001 5:16 AM
Subject: [amsat-bb] Quadrafiliar helix feed for 2.4 GHz?


>
> Has anyone worked out the dimensions for a quadrafiliar helix
> that could be used for a S-band dish feed?
>
> I have some surplus .070 semirigid coax to work with...
>
> What kind of bandwidth could I expect from it?
>
> Thanks,
> Mike N4CNW
> ----
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>





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