|Subject:||Re: [TowerTalk] Towers and Open Wire Feeder|
|From:||"Rob Atkinson, K5UJ" <email@example.com>|
|Date:||Thu, 28 Sep 2006 16:56:22 +0000|
In the early days of balanced rotatable antennas (quads, yagis, LPs) hams
fed these directly with open wire because coax was either not developed or
not freely available. The problem of getting around the rotation point was
of course the sticky part and I had the solution explained to me once by one
OT but unfortunately, I didn't mentally file it because I thought I'd never
need to know it.|
I would nevertheless consider feeding your antenna directly with the window line, or ladder line because it is after all, a balanced antenna, and you'll be free of baluns at the antenna, with no need for worry about mismatch and swr--just tune the shack end of the line with a balanced transmatch.
There is one important thing to remember however, and that is that your rotation will probably be limited to 180 degree arcs left and right of the point where the antenna feed point and the line are on the same side of the tower. That's okay in that you'll cover all compass points, but trying to get the line to follow the antenna for a 350 degree rotation for example, may be asking too much of the system for keeping the parallel line away from the tower.
here's one idea for getting the line around the rotator: you have your line coming up one of the tower faces on 2 or 3 foot standoffs (one every 10 feet or so) and at the top of the tower, at the last standoff, you enclose your feedline in a plastic hose with enough slack to form a 1/2 helix from the top standoff to the antenna feedpoint, when the f/p is 180 degrees opposite the top standoff. when rotated back -180 degrees, it should loop out with the hose giving the feed enough rigidity to not sag down upon itself. you'd have to get the distance from the top standoff and the length of the feed and hose right to make this work so you have the right combination of length, and a happy medium between stiffness and slack so the feed stays out away from the tower but will move with the rotator. you could close the top of the hose (or plastic duct) and leave the bottom open. you would want material that would not breakdown in sunlight and wx.
maybe someone on the reflector who has been around long enough, can recall how parallel balanced feed was routed around rotators. I'll admit my aversion for baluns may be somewhat irrational.
73, rob / k5uj
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