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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.
   >73
   >Richard W L Limebear G3RWL
   >g3rwl@amsat.org


 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.

73
Terry

terryb@bmts.com

Net-Tamer V 1.11.2 - Test Drive

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