[Towertalk] A new tower, extending an existing one, some questions.

[Martin Sole]

I am starting to think about towers having recently moved to a new QTH.
After a little thought I am considering something about 70 to 80 feet,
it will have to be self supporting given the available space. What I
have right now is a 30 foot tower that was previously installed on the
roof of the 14 storey apartment building that was the previous QTH. This
with a 12 foot steel mast supported a lightning bolt quad, turned by a
Yaesu G800SDX rotator.

My current thinking is to use this existing tower as the top part of a
larger one. The original local manufacturer will hopefully be willing to
undertake building of four additional 10 foot sections giving 70 feet
overall or with the addition of a third parallel sided top section maybe
75 feet.

Pictures of the existing tower sections can be seen at
http://uk.photos.yahoo.com/bc/g4uqf

The top two sections have parallel legs with a face of 15 1/2 inches,
leg diameter is 1 3/8 inches. The rotator shelf is mounted at the bottom
of the top section, about 5 feet below the bearing. On the four lower
sections the leg diameter is 1 3/4 inches and the face tapers out from
15 1/2 inches to 27 inches at the bottom.

Other hardware will be initially a refurbished T2X turning a Cushcraft
X9. At some later date I hope to move the X9 down and mount 2el 40m at
the top. I will replace the galvanised steel tube mast with something
more substantial in any case.

Okay, now to the questions.

First, is this a sensible way to go?
How can I calculate how big the tower will need to be at the base? I'm
thinking about 48 inches face width.
How can I determine what dimension of foundation will be needed to stop
all this falling over?
Would it wise to move the rotator lower in the tower?

73's
Martin G4UQF
Bangkok
Thailand