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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

Hello
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
Seattle



Hi Bob.
 
The answer would seem to be yes, plastic tube formers do affect  the 
performance.
 
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.
 
73
 
David   G0MRF
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