[TowerTalk] Ground rod

[Donald Chester]

I agree that NOTHING should penetrate the concrete/soil interface. At a guy anchor, make absolutely sure the anchor rod does not come anywhere near the re-bar inside the concrete. It's not a matter of moisture creeping in, steam cracking the concrete during a strike, etc. The problem is that if the re-bar makes direct electrical contact with the soil, or indirectly through a copper wire or other conductor, galvanic action sets up. IOW, a crude battery is formed with the re-bar acting as one of the electrodes and the soil the electrolyte. This causes corrosion of the re-bar; the corroded re-bar swells and has nowhere to go but to crack the concrete as it expands.

I once watched a documentary on PBS about a problem with the restoration of the Parthenon in Athens, completed early in the 20th century. A few decades later, some of the stones began to mysteriously crack. They figured out that the problem was steel anchor pins that were used when they re-stacked the stone blocks. Marble is porous enough to allow moisture to creep into the stone and in contact with the steel pins, which rusted, and the resultant swelling caused the stones to crack. They had to go over the entire structure with an x-ray machine, and remove all the steel pins they could find in order to arrest the deterioration. 

I have seen the effect of electrolysis at the guy anchor at a broadcast tower; the anchor rod literally rusted in two right at the exit point from the concrete pier and caused the top half of the tower to fail. (This was a great benefit to me because I inherited the old base insulator when they replaced the tower, but I'm sure their insurance company felt differently).

Lightning pulses have an extremely short rise time, so they should be treated as RF, not the same as DC or 60 Hz a.c. The best lightning ground is not several rods driven into the ground, but a radial system similar to what you would use with a vertical antenna. A dozen or so 20-ft. buried radials using #8 wire or larger would be far superior to one or more 8' ground rods. Ground rods make a good a.c. safety ground, but the best safety ground does not necessarily make the best lightning ground, and an excellent lightning ground might not pass a NEC inspection for a.c. power service.

My 160m vertical, 127' of Rohn 25, is mounted on that base insulator I got from the BC station. I have 120 quarter-wave radials (133' each) made of #12 bare soft-drawn copper wire, buried a couple of inches below the surface. I know this was overkill, but I had the wire and at the time (30 years ago) I thought this was absolutely necessary for minimum ground losses, although since then I have read enough on the subject to realise I could have used half that many radials and the additional loss would have negligible. I could have used the rest of the wire for another radial system for another tower, or additional verticals for 80 and 40m. I did sink 3 or 4 ground rods around the base pier to protect the tower until I was able get the radial system in the ground. In 30 years since then, the tower has sustained several direct hits, and the only damage has been some busted guy wire insulators and destruction of the end insulators of an 80m dipole I have near the top of the tower. No damage whatever to the tuning units at the base of the tower, or any of the equipment in the shack, which is 140' away. I use the original BC tower lightning gap across the base insulator and ground directly to the tower, via a knife switch, the feed line that runs up to the dipole, whenever I leave the place unattended or I suspect there may be a threat of a thunderstorm.

Incidentally, I have periodically inspected the radials, and after 30 years buried in the soil, they show very little deterioration of the copper wire.

Don k4kyv