I live in Bulverde about the same distance due north of San Antonio. I
suspect we have the same basic soil/rock configuration. It is mostly
Limestone rock and as such, it will absorb water. This makes it vulnerable
to shifting if the terrain is level where some water will stand.
I installed a 48ft. American tower in 1997, which supports a HyGain TH6DXX
HF Beam and a 14el 2 Meter beam. I have very little top soil similar to your
area. However mine is on a slight slope, so water does not accumulate near
the tower. I dug a bell bottom hole about 4 ft deep and about 3ft across the
top. I then drove several 7/8 in rebar about 12-18 in into the rock, at an
angle into the bottom of this hole. I attached these to the base framework.
You can use a jack hammer with a pipe sleeve around the pick to keep the
re-bar in place while you drive it into the limestone rock. I framed the top
above ground using 2x6's. That gives me 5 1/2 in above the surface. I have
three guy anchors which are mounted in concrete about 3 ft deep and 12 in or
so diameter bell bottom holes in the rock. I have guy wires at the top and
near the middle to keep the tower stable. My three upper guy wires are
insulated and used for sloper antennas. My Prop Pitch rotor is mounted about
4 ft from the bottom of the tower. So it does not figure into the
weight/wind loading factors. Its also easier to service.
This is not an engineered installation, however it has withstood some pretty
high winds during the hurricane seasons. This is the third (and final)
location for this tower in the San Antonio area and I have never had a
problem with the bases. The base was in limestone rock at all three QTH.
Jim - W5IFP
[mailto:firstname.lastname@example.org]On Behalf Of Michael Goins
Sent: Friday, February 12, 2010 8:54 AM
Subject: [TowerTalk] tower installation HG52SS
Guys, I'm needing some help here.
I am installing a HG52SS about 16 miles north of San Antonio in an
area that is nearly all solid rock and I have two sets of factory
specs on installation instructions from HyGain. One says (and I'm
going from memory here, but I'm pretty sure I am correct) 30X30X42"
for the hole and the other says 42"X42"X5.5 feet for the hole. The
issue is that under an inch or so of soil here it is virtually all
solid rock. We are now down about 3 1/2 feet into solid rock on all
four sides, and it has required a large jackhammer and many hours to
get there. Assuming the base hole is to be designed for both
compressive and lateral movement, there is no way it is going
anywhere, either way.
This location is where I plan to live the remainder of my life, so I
want it right. The tower will have a 2 el quad on it, and likely
nothing every bigger than a 3 or maybe 4 el monobander as I have never
owned a commercial antenna and have always built my own. With it being
a crank-up, one antenna at a time (especially the 2 el quad) is
sufficient for me.
My question is: Is this sufficient, considering the ground materials?
I am highly considering the hole as is, wth a pad one to two feet tall
around it above ground and tied into the factory rebar base (which I
will have to cut down and slightly re-do in order to make it fit the
current hole). With a sufficiently tied-in top pad, the tower cannot
move in any direction as the above ground part would try to push down
onto the rock around it, which it cannot do (as the rock 1" and less
underground is solid).
An engineer is out of the questions as I am an average guy, teaching
college for what often feels like minimum wage. I am also at least 16
miles form the nearest possible engineer which would add to the cost
Again, this is an amateur tower installation. The load will never be a
lot as the tower is not rated for a lot of load. It cranks up and
down, and I monitor the weather here, so it would be lowered when
conditions suggested that it would be prudent.
I could sure use some opinions.
Michael Goins, k5wmg
Pipe Creek, Texas
Fast cars, slow boats, big dogs, and summers off to write
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