|[AMRadio] Tower feed point and mechanical question|
jtml at losalamos.com
Wed Jun 14 12:49:45 EDT 2006
I was about to say this, but Don C. already did it. Shunt feed is improved when the tower has top loading.
If you already have a lowband yagi or tribander or some VHF/UHF antennas on the tower, (plus a few other ornaments like i have a ch 55 UHF yagi pointed at Sandia peak) it will be difficult to series feed as you need to have the feed cables to those other antennas brought across the insulated base. Bringing them all through a coiled piece of large flexible pipe with enough turns to present higher Z to 160 meters is one possibility. Shunt feeding the tower eliminates this problem, and you can just let the cables all go to ground at the base.
Once I was at party in Quincy, Illinois, up on a hill. The house was bought by a friend, and he didn't tell me much about the previous owner. There was a commercial tower beside the home, with a big insulated base, just like a MW broadcaster. It looked like 160 meters. I inquired, and the previous owner was the late Parker Gates, i.e., Gates Radio CEO. I would have died and gone to heaven to have bought that place, it had all sorts of ecoutraments (?) as needed to bring coax out of the house, with great ground radials around the tower.
I picked up a Tristao tower last summer for nothing, had to take it down from a SK's place. It is a 50 foot crank up, but instead of using guys, it had steel posts which go up to about the 30 foot level, where the first stationary section ends. These were each anchored in concrete, of unknown size, as I just cut the bottom fasteners off with a cutoff wheel when i dropped the tower. So I have never found information as to what size those anchor blocks need to be under the three supporting poles. I figure the main tower base will be a cubic yard, similar to what I have for my 30 foot freestanding alum tower already. IT has a fold over hinge base plate and I will reuse all that. I looked at Tristao catalogs from the 1970s and didn't find this particular model, and the company that took over that line (Tri-Ex?) didn't know either. Anyone here have experience with this, I am not a mechanical engineer or a structural engineer or PE.
I hope to install that tower near the center point of my wire dipoles, so that I can elevate them as well as use it as a vertical radiator, with a tuner at the bottom.
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