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Re: JavaScript Axial-Mode Helix Calculator



 "Edward R. Cole" writes:

> It tuned perfectly without adjustment.  I use a quarter wave copper strip
> line from the connector to the beginning turn.  It was very close to the
> ground plane [a 15 inch pizza pan].  The helix was stepped down in diameter
> to approximate a taper.

I built a couple of 70cm helices a long time ago. The boom was a broom
handle; the spiral was coaxial cable (inner and outer shorted together);
Impedance matching had me stumped for a while but then ohms law hit me:
two times 140 ohms in parallel equals 70 ohms so I built a second helix
and mounted it a few feet to the side of the first one. Parallelled the
feeds and fed it with 75 ohm feeder. The only tricky bit was making 140
ohm feedline between the two booms - in the end the calculations in the 
ARRL Antenna handbook led me to (carefully) remove the inner conductor 
from some (I think) RG-214 and replace it with a thinner conductor. (They 
hadn't invented computers at that time.)

73
Richard W L Limebear G3RWL
g3rwl@amsat.org
FOC # 1188

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