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Re: Helix antennas & EME



I've never tried it-- but there are three theoretical problems
with this-- one is that upon reflection from the moon the rotation sense 
will change-- to LHCP from a RHCP signal and vice versa.

Therefore, you might need two sets of EME helix arrays, one RHCP and one 
LHCP to operate.  This isn't true if the station your QSOing is linear 
polarized-- and would be an advantage in such a circumatance IF and only IF 
you can take the 3dB hit (and 3 db extra gain means tighter pointing, larger 
antennas).

The other is that helixes are physically large for the amount of gain they 
produce.  They are a lower "Q" antenna than a Yagi is, and hence you get 
greater bandwidth (which is why they are so fault tolerant and easy to 
build)-- the tradeoff here is that since a yagi is narrower bandwidth 
antenna is possible to get more gain with less physical structure.  Purists 
would argue that this is simplistic, as other antennas might be suitable for 
EME-- dishes certainly are, and they are low Q [But again make up for this 
by a large physical structure]-- but at a straight comparsion of Yagi vs. 
Helix, the yagi (array) will have higher gain pound-for-pound (kg-for-kg?) 
than the helix (array).

BUT, sure, it is possible to use helices, but you might be better off with 
yagis...

W0FMS

>From: Claudio Martins <ctpm@rnl.ist.utl.pt>
>To: amsat-bb@AMSAT.Org
>Subject: [amsat-bb] Helix antennas & EME
>Date: Tue, 9 Nov 1999 20:15:08 +0000 (WET)
>
>
>Hi!
>
>    Although it may be a little off topic I would like to know if someone
>has tried to use Helix antennas for Moonbounce work.
>    They are good simple antennas for satellite, and I have been trying
>with several designs and aparently one can obtain high gains with them.
>Could a stack of helix antennas be used for EME (perhaps at 432MHz and
>up), in order to help solve the polariation rotation problem? Of course,
>the array gain would have to be higher in order to compensate for
>receiving a linear polarized signal with a circular polarized antenna,
>right?
>
>Thanks for the attention... 73 de CT3KD
>
>   C. Martins
>
>
>
>----
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>

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