[amsat-bb] Non-integer Helical

["Larry P. Thomas wa0gwa" <lpthomas@xxxxxxxxx>]

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

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