How about the center-pin base plate. More better to inspect for rust. I had
issues, inside legs of buried section, never seen until it was taken down after
20 years.
Sent from my iPhone
On Jan 4, 2016, at 12:59, Jim Brown <jim@audiosystemsgroup.com> wrote:
> On Mon,1/4/2016 8:58 AM, Douglas Ruz (CO8DM) wrote:
> Hi,
>
> I am almost ready to build a Buried section base for a 25G tower.
How tall? What will be on it?
> The hole is 2 x 2 ft and almost 3 ft deep...is it ok for a guyed tower?...it
> is almost half cubic yard...and about 2000 pounds of concrete.
Rohn recommends more. 3 ft x 3 ft, 4 ft x 4 ft. More mass in the ground is
better when the wind blows.
> ..no steel bars inside.
That's BAD -- steel bars help hold the concrete together. VERY important.
> I need some ideas about the lightning protection...How can i do a good
> connection to the tower legs to the ground rod ?...
Bond the steel bars together, also put three pieces of copper strap in the
concrete, bring all out and bond to the tower legs and to ground rods. The
standard recommendation for rods is to drive them so that only enough is above
ground to make a connection. Start with a rod by each tower leg, than add a rod
radially (that is, more distant from the tower) that is spaced equal to the
length of the rod. The concrete works as a ground when connected as above. So
if you're using three 8 ft rods per leg, one would be 8 ft from the tower, the
next 16 ft, the third 24 ft. Those distances are minimums, and a bit more is
better. Bond the rods together with big copper (we use #4 in the US, but you
may not have access to something that big).
For connections to the tower, a special fitting is needed to prevent
electrolysis from the connection of dissimilar metals. DX Engineering sells
suitable fittings. As I recall, they place stainless steel between the copper
and the tower leg.
Install coax and rotator cable inside the tower, and bond the coax shield to
the tower at top and bottom. Use wraps of very good tape to hold these cables
to a tower leg. Don't use TyWraps or other cable ties, which will degrade from
UV.
Ground the coax shields again where they come into your house, with ground rods
spaced like for the tower.
Here's my take on grounding and bonding for ham radio.
http://k9yc.com/GroundingAndAudio.pdf
73, Jim K9YC
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