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5132Re: [loopantennas] Re: Fm question

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  • Jim Dunstan
    Apr 1, 2008
      At 11:29 PM 3/31/2008 -0400, you wrote:

      >Nice Jim,
      >Size wise the quads are hard to beat. I love the pipe wrench
      >rotator! After my heart there. I am awaiting some nice weather
      >to see what I can come up with for antennas. I have lots of TV
      >mast and 1000' of polyester rope to hold things steady. Never
      >built an HF quad to date, but plan on that too. I have toyed with
      >them on VHF though. Nice.
      >By high maintenance was that due to the bamboo or wire breakage?
      >Was not too much of an issue with VHF, but a lot less flexure there.


      As with most things when you go big everything is magnified .... a quad for
      20m is approx. 17' per side. I arranged the spreaders so they would be on
      a 45 deg angle to either the vertical or horizontal plane. The distance
      from the center to each corner is exactly 12'. I tied on small metal
      fittings to the ends and spread thin flexible braided type copper wire
      around the 4 corners (approx. 68' total) ... I fed the center of the bottom
      horizontal portion with 72 ohm balanced line. The 72 ohm balanced line
      (they don't make it any more as far as I know) came down to the shack and a
      1:1 voltage balun (made by Heathkit from air-dux coils).

      It was a perfect match ... and the antenna will take any legal level of
      power you would wish to put to it.

      I used approx 6' to 8' for an aluminium boom. I used a second loop as a
      reflector ... and tuned it with a piece of open wire line for maximum front
      to back ratio. I later mounted a second set of loops within the first loop
      for 15 m and fed it from the same feed point with the same feed line. I
      never noted any deterioration in performance.

      The only maintenance problem was ICE .... the antenna was very gud in the
      wind, but if there should be freezing rain with a build up of ice load the
      whole thing will droop like a wilted flower. The other problem was general
      weathering of the bamboo ... eg getting dry and brittle over time. The
      whole thing is quite light.

      The spreaders in this arrangement are not under much tension. I used home
      brew flanges at each end of the boom to clamp the 4 bamboo poles for each
      loop. The tricky part is building this thing and then getting it up
      there. My 30' tv mast was made of 3 pieces that would nest together. I
      could build and adjust the antenna on the top of the 10' piece then push it
      up by hand. the bottom of the mast sat in a smooth cup on the base ... and
      the guys were attached to rings that allowed the mast to turn. so the only
      thing holding the antenna from turning was the friction in the cup at the
      base. The 2' pipe wrench was more than enough to rotate the whole
      mast/antenna assembly.

      The quad worked very well on 20m at 30' height. A friend of mine had a
      quad on a 60 ft tower (a commercial quad ... GEM quad) and it was exceptional.

      Jim, VE3CI
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