Establishing a Good Ground

Now that we have the SPGP connected through the wall to the outside world, we still have a lot of work to do.  Here we switch from brainpower and the challenge of getting copper strap through walls to the brute force required to establish a good ground system.  The operative word here is system, not a ground rod, but a whole network of interconnected ground rods. 

The primary purpose of the external ground system is to couple as much of the lightning energy into the earth before it rides the coax into the radio station.  No matter how hard we try, some of it will follow the coax, which is why we created the protection plan for the radio equipment.  The easier we make it for the strike energy to dissipate into the earth before it comes to the radio station, the less our equipment protection plan will be taxed.

With great diligence, hard work, no real estate restrictions, plenty of funds, and highly conductive soil; it is possible for up to 90% of the strike energy to be dissipated into the earth leaving just 10% coming toward your equipment.  This would be quite an accomplishment.  In many commercial sites it doesn’t always work out that way and rarely, if ever, does it work for the amateur radio station – there are always restrictions.  Let’s see what should be done and then adjust to the home environment restrictions.

The sketch to the right shows what has to be done.  In the center, the concentric triangles represent your tower.  Ideally, the tower is separated from the house by twenty to fifty feet.  This distance provides sufficient room for the dissipation of the magnetic fields during the strike event.  This distance also takes advantage of the natural inductance of the antenna feed lines to limit the amount of energy and to delay this energy headed to the radio equipment in order for the tower grounding system to absorb most of the strike energy.

Radiating out from the base of the tower is a set of eight radials.  While the number of radials required for a particular installation will be dependent on the soil conditions in your location, the system shown here is a reasonable start.  Each radial is a bare copper wire (preferably, strap) buried six to eighteen inches below grade.  The radials are positioned so that the energy dissipation is away form the house.

Connected to the radials are ground rods.  The ground rods are spaced approximately twice the length of a ground rod.  For an eight-foot rod, that would be sixteen-feet.  During the strike event, each ground rod has a cylindrical-shaped area of influence centered on the ground rod.  This is the area that the ground rod uses to disperse the strike energy into the soil.  If the rods are placed closer than approximately twice the length of a rod, the areas of influence begin to overlap and the efficiency of a ground rods’ ability to disperse energy is diminished.  This overlapping does not harm the ground system; however it does increase the cost since more rods will be used. 

Ideally, the connection between a radial and its ground rod should be done using an exothermic bonding process.  This connection will most likely outlast the life expectancy of the ground system and more importantly you won’t have to do annual inspections.  There are a number of manufacturers that supply the molds and fusing material for a variety of cable/strap and ground rod sizes.  Two of them are Erico Incorporated and Alltec Corporation.  If the exothermic process is not used, there are mechanical clamps that can be used to connect the radial to the ground rod.  PolyPhaser makes a clamp that will connect copper strap to a 5/8-inch ground rod.  All mechanical connections must be inspected annually or more frequently to ensure the integrity of the system.

Ground rods must be mechanically driven in to the ground.  This is the only way to ensure that the rod achieves a reasonable ‘connection’ to the earth.  Drilling a hole and the back filling the space around the rod is not acceptable.

While a very long radial/ground rod system is good; there are some electrical and economic considerations that come into play.  By analyzing the average cost of installing an additional foot of the grounding system versus the benefit of a lower impedance system, the null point is somewhere around eighty-feet.  So for practical purposes in areas with reasonably conductive soil conditions, the maximum length of a radial should be limited to approximately eighty-feet.  If the impedance of the ground system needs to be lower, then additional radials should be used as opposed to longer radials.

Some caution must be exercised when laying out the radials.  If a radial comes within four feet of a metal object, it must be bonded to the metal object.  The four-foot rule applies to object that are above, below, to the left, or to the right of the radial.  This includes metal fence posts, kids' swing sets, buried fuel tanks, etc.  Be sure to watch out for dissimilar metal properties when bonding to these objects.

Care must be exercises when connecting the radials to the tower.  Most towers are zinc-coated steel (galvanized).  Connecting a copper wire or strap directly to the tower leg will cause the zinc to be eroded allowing the base steel to oxidize (rust).  This is turn will increase the resistance of the connection and over time may threaten the mechanical strength of the tower segment.  One solution to this problem is to use a buffer layer of stainless steel between the zinc and the copper.  The PolyPhaser’s TK-series tower leg clamps will do well here.