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Re: Wimo Helix for 23 cm



Hi Joe, KK0SD

I am looking in to "ANTENNAS" by John Kraus edition 1950 Chapter
The Helical Antenna page-197 diagram Fig-7-22

Considering a lenght of one turn equal to one wavelenght and a pitch
angle of 12.5° between turns the half power beam width is as follows:

10 turns = 37°
20 turns = 25°
40 turns = 18°

In this diagram Kraus is quoting the directivity wich increases with the
numbar of turns but for good reason he do not mention the gain in dB.

If we express the directivity in dB than the gain in dB equals to
directivity only if the efficiency of the antenna is 100%  i.e. no power
is lost in the ohmic resistance of the turns.

If you wind up two identical helices using for one copper wire and
for the other one steel wire than the measured beam width and
directivity are the same but the gain for the helix made of  steel wire
is less.

Increasing the numbar of turns increases the losses in the wire but the
gain seams to increase apparently because it is only the directivity to
increase and this is why it is not recommended to go over 15 to 20 turns
with an helical antenna.

On the other side if you made an array of 4 helices with the attemp to get
6 dB increase of gain another problem arises because the beginning and
the end of each helix must be oriented simmetrically to the same axial
position in the space.

The above mechanical situation produces that the feed lines for tho helices
arriving at a single N connector are longher than the other two so that
signals are not radiated or received in phase and distortion of the pattern
and reduction of gain occurs.

For the above reason I have built only single helical antennas with no more
than 10 to 15 turns.

Best 73" de

i8CVS Domenico


----- Original Message -----
From: "Gary "Joe" Mayfield" <gary_mayfield@hotmail.com>
To: <amsat-bb@amsat.org>
Sent: Tuesday, March 07, 2006 2:00 PM
Subject: [amsat-bb] Wimo Helix for 23 cm


> Spring is coming --- and I have gathered almost everything for an L Band
> uplink.  Does anyone have any experience with the Wimo helix for 23 cm?
It
> appears they have 3 models with 10, 20, and 40 turns.  Is there any point
to a
> 40 turn helix?  I thought after 15 to 20 turns they didn't improve.
>
> Thanks and 73,
> Joe
> kk0sd
> AMSAT # 21357
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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