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Re: Re: S band helix



Hey,

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
>Ib
>----- Original Message ----- 
>From: "Howard Long" <howard@howardlong.com>
>To: "Edward R. Cole" <al7eb@acsalaska.net>; <G0MRF@aol.com>;
><amsat-bb@amsat.org>; <w7lrd@juno.com>
>Cc: <k5oe@aol.com>
>Sent: Thursday, June 16, 2005 2:53 PM
>Subject: RE: [amsat-bb] Re: S band helix
>
>
>> Hi Ed
>>
>> > 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.
>>
>> Although I admit David's a neighbour of mine, I independently came to
>> exactly the same conclusions as him. True, a helix is inherently
>broadband,
>> but using a piece of PVC tubing as a permanent support and former has a
>very
>> significant effect, and these effects must be taken into account when
>> designing and constructing the helix.
>>
>> ISTR that I had another similar conversation with Jerry K5OE who
>> coincidentally is now an even closer neighbour of mine as M/K5OE. From
>> recollection I believe he took the PVC's delictric effect into account
>when
>> designing and making his helices.
>>
>> There are some very useful hints in Ed Krome's Mode S - The Book from
>> AMSAT-NA.
>>
>> Also I have some practical hints of my own (with others' help) on helices
>at
>> http://www.g6lvb.com/60cm.htm and
>http://www.g6lvb.com/quadruple_helix.htm.
>>
>> 73, Howard GW6LVB/P
>> ----
>> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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>----
>Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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