[TowerTalk] Tower Base - Design Change Question

[Dan Levin]

Learned friends,

I am about to install a used Hygain HG70HD 70 foot heavy duty (relatively
:-) crank up tower.  I plan to install a 6 sq. foot 4 element TET tribander
just above the top of the tower, and some kind of ~5 sq. foot 2 element 40
meter yagi about 10 feet above that.  My soil is very heavy clay, with a
rather solid sandstone below that.  I don't know whether my tower base will
reach into the sandstone or not, and I won't know until I dig the hole.

The HG70HD base is designed with three "feet", made of steel bar stock
something like 1/2" x 2" x 8".  These "feet" have 3 3/4" holes drilled in
them.  The design is to bury matching pieces of stock in the concrete, with
~10" sticking up into the air, and then mate each pair together (the piece
that is welded to the tower and the piece that is buried in the concrete)
and connect them with 3 3/4" bolts.

This seems like a perfectly reasonable plan, but it does not allow for any
leveling of the tower after the concrete is poured.  The "feet" sticking out
of the base have to be plumb - if they are not that is tough luck.

Now, I am an engineer by training, and I hate designs that lack mechanisms
for adjustment.  I have seen the U.S. Towers base design, and it is the same
with one small but critical difference.  Instead of bolting the feet
directly to mates buried in the concrete, the U.S. Towers bases bolt the
feet onto a steel plate, which is in turn suspended on three anchor bolts
that stick out of the concrete.  This allows full adjustment for plumb,
since the plate can be raised or lowered relative to any of the three bolts
by simply turning the nut that the plate rests on.  This seems like a much
better design to me.

So, I have a couple of questions.

1) Can anyone explain to me why the original Hygain design, which lacks
adjustability, it better than the U.S. Towers design?

2) If not, can anyone help me to calculate the size of the base plate and
the anchor bolts that I would need to use a U.S. Tower style base for this
tower?

3) If not, can anyone tell me the size of the base plate and anchor bolts
used on the U.S. Towers TX-472 or HDX-572?  These towers are of similar
size, weight and height to my Hygain, so it seems reasonable to use the same
size base components (no?).

Thanks much,

            ***dan
            Dan Levin, N6BZA


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