Tower Grounding (T0B08)
The 2022-2026 Technician License question pool asks about proper methods of grounding an antenna tower to protect against lightning:
T0B08: Which is a proper grounding method for a tower?
A. A single four-foot ground rod, driven into the ground no more than 12 inches from the base
B. A ferrite-core RF choke connected between the tower and ground
C. A connection between the tower base and a cold water pipe
D. Separate eight-foot ground rods for each tower leg, bonded to the tower and each other
A typical lightning bolt may carry current of between 20,000 and 40,000 amps, half a billion to 500 billion joules of energy, and an electric potential of millions to hundreds of millions of volts. According to various sources a single strike could (with no claim of accuracy or testing of any of these):
Toast over 100,000 slices of bread
Light up 150,000 light bulbs
Power a home for a week
Electrocute a quarter million people
Power 3 million HT radio contacts, each one minute long
Calling upon the water analogy of electricity, a direct strike of lightning is sort of like having all of the current of Niagara Falls splashing down through a one inch diameter pipe under unimaginably extreme pressure. When it exits that pipe it is going to run and drive along every possible path, seeking low resistance routes to ground level voltage. It is prudent to not be in its path!
So, any way you slice it – toast, bulbs, HT contacts, or waterfalls – that’s a lot of juice to handle. A radio tower needs to give all that energy a convenient path to ground in the case that Mother Nature comes to call with a friendly bolt or two. That’s what this question is getting at.
With a direct lightning strike there is almost no way to avoid some damage to structure, cable, electronics, and almost anything nearby that is a conductor. Metals, such as option B’s ferrite-core RF choke, will melt like butter under extreme heat, and even the best lightning protection tubes will still allow a big spike of current to pass. Lightning is also fond of jumping from conductor to conductor, arcing through the air 4 feet or more, especially if some resistance is encountered in its path. So, you want to provide a short, direct, very low resistance path to ground that the bulk of the strike energy can travel along. A connection to water pipes, as suggested by option C, is sometimes OK for RF grounding of your station, but for lightning mitigation that approach could potentially conduct the energy from the tower right into nearby manmade structures. We need something better.
While option A’s single four-foot ground rod is a nice thought, it is completely insufficient for Electric Niagara. As a minimum for a tower, separate eight-foot long ground rods for each tower leg are needed, bonded to the tower and to each other as described in option D. These deep ground rods will help to dissipate the lightning energy into the ground. Even better is an array of multiple eight-foot ground rods for each leg, bonded together for good conduction. With one or only a few rods, the earth in the vicinity around the rods can become saturated with charge and offer impedance, encouraging other paths or jumping of the lightning surge. Place ground rods so they have a free radius equivalent to their depth – about 8 feet – or a minimum of 16 feet between any two rods. This will provide sufficient dissipation of the surge without saturating the earth.
A direct strike is dangerous and damaging, but most of the energy can be shunted into the earth with a properly designed mitigation system including rods as described here, and short, straight, wide strap conductors bonded among the tower and rods to create a dissipative network for the energy surge to travel away from the tower.
Bonus Note 1: Nearby lightning strikes are more common and less damaging than direct strikes, often creating waves of energy that roll outward from the strike zone through the ground near the surface. Imagine this like the circular ripples in a pool, spreading outward from the strike, weakening as they travel along. A ring of buried rods around the shack, interconnected with buried strap conductors just a few inches beneath the surface, can greatly reduce the effects of such ground waves entering the structure by routing the energy along the ring and into the deeply sunk rods to be dissipated. Such a construct should also be integrated with any rods/straps specific to the station’s tower or antenna. Granted, this may not be practical for many ham shack situations (and ham wallets), but it is very effective protection against Mother Nature’s occasional rages.
Bonus Note 2: Nearby strikes also produce a strong surge of RF emissions -- the strike is accelerated electrical charges, after all, similar to the accelerated charges that produce RF electromagnetic waves from your station antenna, only of much greater magnitude. Further, your antenna is designed to receive a portion of the RF emissions from a strike and route the resultant currents right into your station. It is these types of surge currents that lightning arresters (AKA "lightning tubes") placed into the antenna feedline are intended to stop, acting as a rapid fuse with a low-impedance path to shunt the energy surge to earth ground instead of into your shack.
The answer to Technician question T0B08, “Which is a proper grounding method for a tower?” is D. Separate eight-foot long ground rods for each tower leg, bonded to the tower and each other.
-- Stu WØSTU