[amsat-bb] Re: Helical array

[Richard@xxxxxxxxxxxxxxxxx (Richard W L Limebear)]

Gang

> Terry Bendell wrote:
> >  as I have a readily available supply of 1/8 and 1/4 inch
> >  hard drawn copper line stock available.

Did you know that you don't have to use solid metal or tubing ?

I have a NASA Tech Brief (from my Oscar-6 days when I was 8P6DR) which
says that its ok to use a spiral of coaxial cable supported on wooden
spreaders. Short inner and outer together. Broom handle along the middle.
*Thick* coax for the 2m one, thinner for the pair I used on 70cm. The
spiral sagged a bit on the 2m one with spreaders at 120 degrees round the
pole; 90 degree spreaders would be better for 2m but 120 deg is fine for
the smaller 70cm spirals.

and KF4FDJ writes:
> Have you ever seen a 2 meter helix?  Bigger than a bus was my
> proclamation, the first time I saw one.

My 6-turn 2m helix sure was; I had to walk it through the streets one time, 
got some real strange looks :) 

> >  How about feeding that Helical antenna with 75 ohm T.V. Coaxial cable?

I fed the 70cm pair with 75 ohm feedline from the shack and matched on the 
boom. Feed impedance of a helix is 150 ohms; parallel two and 75 ohms is 
fine. Matching stub (100 ohms) made of 50 ohm cable with the inner pulled 
out and replaced with a thinner wire; 75 ohm feedline connected to the 
middle of the stub which ran between the two helices.

One thing I found out about helices: they don't like nearby metalwork.
Keep them as clear from neighboring objects as possible.

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

          So many beautiful girls ..... (sob) so little time
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