Just back from a hamfest long weekend and was interested to read all the
comments about 80m yagis and associated issues. I have had a 4 element full
size yagi up here since Oct 1998. It has not been without problems, but I
have never worried about it coming down. The tower is large - 40" face -
and well up to supporting the yagi.
It went up the "old fashioned" way with gin pole and snatch block, playing
leap frog with the 2 sets of guy levels, before it reached home at 160 feet.
It was taken down 2 years later to replace a bent reflector and to revamp
the rotor system. It was just down for a short time and then went back up
to 150 feet, an easier reach for the available crane.
I am very interested in hearing about other 80m yagis, especially full size,
and the problems they have run into. I have asked before about such
designs, but with no replies. ON4UN's Low Band Dxing (2nd edition) shows
fewer examples (and none that appear to be currently in the air) than in his
first edition - that is not a good sign?
Some pictures at http://narc.net/ve6jy/ Please, all take turns (no
pileups!) looking at the pictures or the web provider (tnx to Mitch VE6JTM)
will experience a big blip in the usage stats and will know I've talked
about the web site on some reflector......
My two most significant "I know better now" slip ups were:
A: believing a large element could be designed using "static" stress
analysis. Large, heavy elements can build up momentum and if a gust comes
along at just the wrong moment, i.e. to push an already heavily stressed
element, it may fail. Mine only bent one side of the reflector inwards by
about 20 deg or more, about a year later it was bent perfectly straight
again. In the next big wind it was bent again. I believe the material I had
on hand for this (failing) section was a softer aluminum, but even if it had
been the good stuff, it was still close to the edge. Yet the other half of
the reflector (exact same material) survived 2 years. They have all been
reinforced and now look quite comfortable in a wind.
B: not worrying enough about wind balance. Initially putting it up with
the gin pole, I had to worry about weight balance (even installed a large
chunk of heavy pipe on a trolley that ran along the inside of the boom -
allowing it to be balanced perfectly as it was adjustable from the center of
the yagi. The other constraint was that I wanted to reach the DE from the
tower (remember this was in 1998 BC - Before Crane) This resulted in a
pretty unbalanced design, and even with some extra wind loading on the rear,
analysis (tnx VE6AQ) showed it may have a 7500 foot-lb unbalance in a 75 mph
wind.
The current project this summer/fall is to extend the mast another 140'
downwards so when the rotor breaks again, I can work at ground level. After
I get that rotation matter tamed, I wonder what else will fail?
73 Don
VE6JY
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