>Hello all,
>
>My lot (105' x 310') will accommodate up to 80' of tower guyed at 80%
>"by the book". What compensation would I have to make if I were to
>sneak it up to 90 or 100' (72 and 65%) ? Would the extra 10 or 20
>feet be worth it?
>
>The tower will be 25G with a TH7 at the apex and 40-2CD 12' above.
______________________________________________________________________
Sean, I have a 130 foot Rohn 25G in my back yard and one of the guys
could only be put at 84 feet from the base of the tower. The other
two guy anchrs are at approximately 105 feet from the base. I also
have a TH7DXX at 132 feet and a 40-2CD at 140 feet and a tall mast
that extends to 150 feet with a many pointed lightning rod projection.
It is my personal opinion (unsubstantiated) that says that Rohn has
become very liability conscious and tends to derate things and make
sure that if they err it is on the side of safety.... possibly to the
extreme..... again, my opinion. I am not a structural engineer but
I have had that Rohn 25 in the back yard now for 5 years and it has been
subjected to some very high winds. The guys are 1/4" Siemans-Martin
at the two lower levels, 32 and 64 feet, and 4,000 lb Phillystran (sp?)
at 96 and 128 feet. Each of the guy anchors are imbedded in a yard
of concrete with the bottom of the concrete at a minus 5 feet in relation
to the earth. I tensioned them to 10% of their tensile strength and
have never had to re-tighten the turnbuckles.
The theoretical optimum height of 93 feet for antennas (based on the
mean incoming/outgoing radiation angle) is so close to 100 feet that
I believe it explains why so many of the DXers and Contesters have
their antennas at a nominal 100 feet height. If I were to err slightly
on the height, I would tend to err on the high side and go to 100 feet.
I don't believe you would be sorry.
I can't recall the gentleman who made the statement but some noteworthy
DXer said something to the effect, "Every 20 feet of antenna height to
100 feet uncovers a new layer of DX". I tend to believe that!
73, Rod W5HVV
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