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Re: Re: S band helix

At 05:19 AM 6/16/2005 -0400, G0MRF@aol.com wrote:
>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
>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
>2 or all 4 sides to hold the helix in place. A common choice is half inch
>slightly smaller (10mm) square section aluminium.
>The matching scheme of soldering a piece of copper foil to the first
>turn and then running this close to the 'reflector' appers to match to  50 
>ohms very well.
>Hope that helps.
>David   G0MRF

I would question the effects if mounted on a piece of tubing (symetrical,
non-conducting plastic).  Helix antennas are inherantly wide-bandwidth
antennas with 15-30% BW.  As such, dielectric shift in resonance should
have minimal effect on resonant frequency (and gain).  I have a 5-1/4 turn
helix feed for my dish wound around a 1-inch diameter PVC pipe with a
3/8-inch bolt running inside as support.  I use a short piece of brass
flashing on the first 1/4-turn for matching.  It worked quite well on AO-40.

You can see photos of it on my satellite webpage:

Perhaps on longer helix beams using a PVC radome there is some effect.  BTW
AF9Y constructed a 23-turn 437-MHz helical beam using a large plastic pipe
as a form for his "MGS" antenna:  http://www.af9y.com/helix.htm

Ed - KL7UW 
BP40iq,  Nikiski, AK      http://www.qsl.net/al7eb
Amsat #3212
Modes: V - U - L - S
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