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Re: S band helix

In a message dated 16/06/2005 07:08:43 GMT Standard Time,  
owner-AMSAT-BB-digest@amsat.org writes:

Subject:  [amsat-bb] S Band Helix

I'm doing some "stuff" with 2.4 ghz  helix'.  I have seen some designs
that use a plastic type pipe   (pvc) for a form.  I bought a 1.2 ghz 
helix on ebay that is a  commercial unit and is enclosed in a large pvc
type pipe.  Do these  forms have a significant impact on the performance
of these antennas?   I have read conflicting information about this.  I
want to establish a  general consensus of what is "right".

73 Bob  W7LRD

Hi Bob.
The answer would seem to be yes, plastic tube formers do affect  the 
At the AMSAT-UK meeting at Surrey each year we try to run an antenna  testing 
range. This can measure gain of an antenna against a standard gain  horn.
The results are consistant and can be broadly summerised as:
Winding the standard size helix on a tube results in lower gain.
Supporting the helix on one side scews the angle of the beam. ( apx 15-20  
degrees measured) and may reduce the gain.
A helix of less then 4 turns does not seem to be long enough to  generate 
genuine circular polarisation......but probably good enough.
The most reliable method is a central support with non conductive spacers  on 
2 or all 4 sides to hold the helix in place. A common choice is half inch or  
slightly smaller (10mm) square section aluminium.
The matching scheme of soldering a piece of copper foil to the first  quarter 
turn and then running this close to the 'reflector' appers to match to  50 
ohms very well.
Hope that helps.
David   G0MRF
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