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# Non-integer Helical

• Subject: [amsat-bb] Non-integer Helical
• From: "Larry P. Thomas wa0gwa" <lpthomas@xxxxxxxxx>
• Date: Mon, 12 Mar 2001 16:25:28 -0600

```I've read with interest the threads on Helical antenna and studied the
various matching methods with interest but I am confused about a couple of
things.
1. Most of the RHCP antenna are an integer number of turns ... 20,24, 30,
etc.  But the short LHCP antenna typically used to illuminate a dish and
get RHCP is 2.5 turns or 3.5 turns.  Can anyone explain how the fractional
turn affects the feed point impedance, etc. of the helical.

I once read about a commercial helical antenna that terminated the end of
the helical's last turn with a 1/4 whip running from the end of the last
turn (at the circumference) radially toward the center.  The reason given
was to improve circularity of the helical.  I can't remember if this was a
long or short helical.  The 1/4 wave would add some linear component to the
net EM wave and most likely present a low impedance at the end of the last
helical turn instead of the normal high (open circuit).  I think the
current in the helical diminishes as you move away from the feed end so
possibly the added EM component is not significant.

What I am wondering is if the grounded feed system currently being looked
at by Jesse Morris (w4mvb) could be used on a short helical used to feed a
dish.

AND ....

2. I've read that the two or four helical configuration has better side
lobe properties.  I was wondering if matching and feed problems where the
only thing keeping this configuration from being preferable over the single
helical ...  Why not four 10 turn instead of say one 30 turn.
Anybody got any ideas on these two.

Thanks
Larry

- Imagine it / Achieve it - Dream it / Become it -
Larry P. Thomas    wa0gwa  voice : 1 913 888-0282
Krell Technologies           fax :
8960 Bond                  pager : 1 913 617-HELP
Overland Park, KS              www.krell.com
66214-1722    USA            lpthomas@krell.com

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