How Do You Ground a ROCK??
A Tower Grounding Project
Having discovered that my tower would have to be installed on top of bedrock - the rock starts just 6 inches under
the dirt - I had to figure out how to install an effective ground system. Colorado has a good number of high powered thunderstorms, and I didn't want to
take a chance of the tower being hit without SOME sort of lightning grounding system to divert a stroke to ground, rather than have it show up inside the
house and do major damage.
Providing Lightning Protection is a Challenge
With Bedrock Just Inches Below the Surface
Not being able to drive a ground rod into solid bedrock, I had to make some hard choices - pun intended.
I checked with some old timers in the area who had experience installing grounding systems in telecommunications sites up in the mountains. They told me that
the methodology they used was to install a perimeter ground, then run copper grounding straps out across the rocks, without actually burying them. The straps
bleed off the lightning strokes fairly effectively. That made my decision easier, and relieved some of my concern. I decided to purchase some large copper
tubing, and install it as close as possible to bedrock in a circle around the base of the tower.
Using Copper Tubing to Help Protect the Tower
Lightning, I learned many years ago, tends to travel on the surface of a conductor - the larger the conductor, the more
easily it travels, and is diffused to ground, in this case. At the beginning of this project, I attached a large ground wire to the rebar frame I built for my "above
ground" tower mounting system. This serves as a "Ufer" ground, named after the gentleman who discovered it and put it to use in military and commercial
communications systems. This heavy, solid copper wire comes out through the top of the 20,000 pound cement "block" and is attached to the tower leg. I
used a piece of stainless steel as an interface between the galvanized tower leg and the copper wire to prevent degradation due to dissimilar metals coming
in contact with each other. Clamping the copper wire to the tower leg, but first contacting the stainless steel, precludes that from happening.
"Ufer" Ground and Copper Wire Help Dissipate Lightning Strikes
I used cadmium plated bolts, nuts and washers to connect the ends of the flattened ends of the copper tubing together. I drilled holes through the tubing
(after compressing a small section into a flat spot with some vice grips) and installed an inch and a half cadmium bolt through the hole to connect a copper
ground wire. I did this in 8 locations around the tower base, to hopefully give lightning a better chance to bleed off. Each of these #6 solid copper wires
were then connected to the Ufer ground wire, which was attached to the tower leg, using electrical connectors designed for connecting heavy copper wiring.
They are available at your local electrical shop.
The next step is to connect copper wires or flashing to the copper "ring" around the base, and run them out about 50 feet, ensuring they are in contact with
the ground as much as possible.
To protect my equipment as much as possible, I also installed a NEMA box at the tower base, with an ICE 303 surge protector for the coax that leads into the house. The box
is a fiberglass model made by the Belding Division of Robroy Industries, model number J1816HPL. I drilled holes through the bottom for coax cable ingress/egress, then sealed
around the cables with a liquid rubber product called "Plasti Dip", which is easily removable and keeps the bugs out.
Interior view of the NEMA box with the ICE protector hard at work. Also visible is the Array Solutions 6 Pak antenna relay system, and the common mode chokes on the
coaxial cable which significantly reduce electrical noise, especially on the low bands. I have those chokes on the SPG end of the coax as well.
That completed the grounding project at the tower base.
Copper Single Point Ground Plate
With I.C.E. Model 303 Coaxial Cable Protector and
Model 348 Control Cable Protectors
The final step of lightning protection is to install bypass devices on the coaxial cable and the control lines for the rotator and the SteppIR 4 element beam.
I did manage to drive a ground rod into the dirt near the house foundation. That will serve as the grounding connection for the single point ground that will
protect my equipment against a lightning strike. The devices I chose are manufactured by I.C.E., and protect coaxial cables and control cables from
lightning by diverting the surge to ground within nanoseconds. It is important that the grounds on ALL electronic systems in your "shack'" be connected to
this single point ground, so that electrical surges do not flow through the equipment to ground, but go directly to ground - at the single point.
Copper Single Point Ground Plate Mounted on House
The cables entering the house are first connected to the protection devices mounted to the single point
ground plate. The plate is connected to the ground rod with a short run of heavy copper wire and 5/8 inch copper tubing - nothing like overkill! I mounted the plate in a junction box on the wall on the side
of the house to protect it from weather, and provide the shortest run to the ground rod for the connecting copper wire. From there, the cables feed through
the house to my radio room
That's it - I feel much better prepared. Lightning is a powerful and mysterious thing - my system may not protect completely against a lightning strike, but
it should do far less damage, should that event ever occur.
Disclaimer
This article is an illustration of how I had to work around some challenges I faced - please consult the
experts at I.C.E. or, Polyphaser for technical guidance and
references on how to protect your systems against lightning. The ARRL also has some great articles on lightning
protection available on their web site. Every installation is different.
|
