[K3PZN-List] Tower Grounding

[Peter Morton]

Thanks to WA3MEJ, WX3F, KJ4GU and NS3X for their inputs on tower grounding.

First, in my defense, I based the grounding system on the ARRL Antenna Book, 22nd Edition, Paragraph 26.4.7.  There is states, "Connect each tower leg and each fan of metal guy wires to a separate ground rod.  Space the rods at least 6 feet apart.  Bond the leg ground rods together with #6 AWG or larger copper bonding conductor (form a ring around the tower base, see Figure 26.25).  Connect a continuous bonding conductor between the tower ring ground and the entrance panel.  Make all connections with fittings approved for grounding applications.  Do not use solder for these connections.  Solder will be destroyed in the heat of a lightning strike."  Figure 26.25 states that the bonding conductors should be buried at least 12 inches deep.

My tower is adjacent to the house so a ring around the tower was not possible.  So, I spaced two of the ground rods half the recommended 6-foot spacing from the house foundation and from the center of the tower with the third rod spaced 6 feet from the other two rods.  I connected each leg of the tower to individual rods and connected the three rods together using using #4 AWG solid bare copper wire.  All clamps on the tower legs and ground rods were bronze with stainless-steel screws and all are rated for direct-burial applications to meet the National Electrical Code.

So, now to the comments:

Exothermal bonding of the bonding conductors to the ground rods.  This is a much more reliable connection underground as the junctions are welded and not subject to the galvanic reaction between metals and to the corrosive affects of the bare earth. I will replace the clamps on the rods with Caldwell Plus One Shot exothermic welds available from The RF Connection, and lower the ground rods to the wire level (the wires attach to the top of the rods).

Use Petrox or equivalent on the tower leg clamps.  Okay, will do.  Have Burndy Petrox.

Rod spacing.  The rods are now spaced in accordance with ARRL recommendations at 6 feet.  However, per the literature linked by NS3X and the handout to club members a number of years ago at a club meeting by an engineer specializing in lightening protection, the rods are spaced well inside the "Sphere of Influence" or the "Interfacing Hemispheres" of each rod.  The two articles state that the rods should be spaced 2 times and 2.2 times the length of the rod, or for 8-foot rods, 16 and 17.6 feet.  This places each rod outside of the "Sphere of Influence" of the others.  This maximizes the effectiveness of each rod and provides a lower combined ground resistance.  This spacing is confirmed by Revision G of the TIA/EIA 222 standard for antenna supporting structures and antennas effective January 1, 2006 which specifies 5/8-inch 10-foot rods spaced at 20 feet, 3 rods around the base of guyed towers plus one at each guy anchor,  and 6 rods around the base of self-supporting towers. (Also requires the bonding conductor to be 2/0 minimum.)  In my case, I have one rod next to my deck at 3 feet from the basement wall and 3 feet from the tower center, so there is not much room on that side to expand.  I'll probably live with what I have for now unless someone can give a substantial reason to re-space the rods.

The article referenced by NS3X and one other reference (I believe it was Caldwell) also state that the top of the rods and conductors should be buried 18 inches.  This will require that I do more ditch digging.  Comments please.

Should not have a rod right next to the tower base.  I agree and will remove the fourth old re-used rod that is next to the tower base.  This rod was left over from a pipe mast I replaced with the new tower.

To meet code, I must connect this ground system to the service-entry ground.  Unfortunately, the service entrance is at the diagonal far corner of the house at the corner of the garage.  To avoid cutting through 40 feet of parking pad in front of the garage, I will have to run this conductor on the "long path" around the house with about 160 feet of #4 bare copper wire at $1.49 per foot or $238 (200 ft. spool is available for $220) and must be buried 18 inches (I think).  So, I need to investigate trenchers or wire-laying devices capable of burying stiff #4 wire at 18 inches.  Any recommendations?

Thanks for all your support.  It has been a good learning experience.

-Pete, W3GVX