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Re: Helical array

On 1999-05-22 Richard@g3rwl.demon.co.uk said:

   >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:
   >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.
   >Richard W L Limebear G3RWL

 Great Info Richard!
 I was wondering about ice build up in the winter on a helical antenna?
 thats why I was thinking about the copper. The array will be about 10
 feet off the school roof so hopefully that will be enough clearance
 from the stones over metal roof.



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