Normally, putting ANY concrete under a properly house bracketed tower is
a mistake. There used to be a Rohn letter out about it. Had to do with two
immovable objects, that is, the house and the concrete base and the motion
of the tower between them. Towers do flex in the wind and change dimensions
with temperature. Rohn said that house bracketing any tower on a concrete
base compromised it's strength. This is not an issue with guyed towers not
using a house bracket.
Back in the 1960's as a poor college student one of my part time jobs
was with a company who did TV towers installs. (This was when we sold Rohn
25 straight sections for $7.50 each.) The company did literally hundreds of
Rohn 25 installs to typically thirty to fifty feet. The rule was to go
twenty feet above the house bracket. None were guyed.
We never used any concrete. The house bracket did the work and was
installed first into something substantial, usually a backing board or plate
in the attic. A plumb line was dropped from the house bracket and a hole was
dug about two feet deep. We then put in a couple of inches of pea gravel for
drainage and two bricks for the tower to set on. The tower up to the house
bracket was then installed and plumbed. The hole was filled and tamped. Then
the tower sections above the house bracket and the antenna, rotor, twinlead,
etc. were installed.
We never had a house bracket or tower failure using these methods. Some
of these towers from forty years ago are still around although most are gone
because of cable or rust, but none ever failed because of the installation
method.
I have two towers installed this way here at my home, one 50 feet and
one 40 feet, that have been up since 1979. Other than needing a little
paint, they are still perfect.
Al, K9SI
