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RE: S Band Helix test results

In case anyone is interested I once built a 2.4gig helix on PVC.  I also had
it tested at a professional testing range (Good to have the right friends in
the right place sometimes).  You can see the results here and draw your own
conclusions.  Did I use it on AO-40? No, I never got around to trying it.
Sorry! I do remember many trying the helix's on AO-40 with limited results.
However, for a LEO I am not sure? I have not had a chance to try it yet
since in process of getting another house and such.




----------- Original Message -------------------

Message: 10
Date: Thu, 16 Jun 2005 06:48:42 -0800
From: "Edward R. Cole" <al7eb@acsalaska.net>
Subject: Re: [amsat-bb] Re: S band helix


I built this in 2002 and just relating from memory, so the dimensions are
approximate.  BTW pipe in the US is spec'd by I.D. so if it is 1-inch the
outside diam is more like 1.25 (this is heavy wall PVC).  The the helix is
made from 0.125 diameter copper wire used for ground wire busses.  It was
wound around the PVC as a form so it is slighly more diam than the PVC O.D.
 approx 1.5 inches in diameter on its extremity (again from memory).

I can go measure it exactly and report back.  My target was for a 1-lamba
circumference helix which would be 12.5 cm (diam 3.98 cm).  That is close
to your number (4.3 cm).  My experience with helix antennas is that they
are extremely forgiving in dimensions due to the fact that they exhibit
very wide band width (at least 240-MHz at 2400 working frequency).
Theoretically, one should get nearly 720-MHz BW but this is dependent on
wire size somewhat.

All I was saying is that I have not seen the effects that you have.  I have
not done the measurements either.  My experience is built up a helix and
you will get expected receiving performance without undue concerns about
matching, resonance, etc.  If your helix is to be used for transmitting
then one probably should be concerned more about characteristics of SWR and
effects on gain.

Most folks were disappointed with their use of helical beams on AO-40 (they
realised less signal gain than expected).  Most found that the requirements
dictated use of a dish and bigger than 30-inches was better (3-foot to
really have good reception).  For reception of AO-51 much less gain is
needed and I will not give advice on that since I have not worked mode-s on
it yet.

If you plan to use a helix from your AO-40 dish, beware that it should have
been wound LHCP and should not work well on AO-51 if it is RHCP.  Best to
just make up a new helix...they are simple "creatures" to make.  I hear
that a simple patch antenna is workable so a 3-turn helix ought to be
equivalent in gain/beamwidth.

73's Ed

At 03:38 PM 6/16/2005 +0200, OZ1MY wrote:
>Hello everyone,
>There is one thing that strikes me in Ed's mail. That
>is the diameter of 1 inch.
>All the helix antennas for 2,4 GHz I have made have a diameter
>of approximately 43 mm - that is much more. They are
>center supported all of them. No PVC pipe :-)
>73 OZ1MY

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