Protect Your Site From Lightning

Part 1

Feed Line

by
W.C. Alexander

We usually think of thunderstorms and lightning during the spring and summer, when the big storms wreak havoc at our transmitter sites and studios. After a damaging strike has occurred is often the time that we look around to see how we could have better protected our site and equipment.

The fall is an excellent time to address lightning protection issues. There are few storms around to cause problems, the weather is usually very good, and many of us are working on preparing our sites for winter. If we can get the proper lightning protection in during the fall, we will be ready when the early storms come next spring.

The type of lightning that does damage to broadcast installations is the discharge of energy from an electrically charged cloud to the ground. Cloud-to-cloud discharges seldom cause damage on the ground. A "typical" lightning strike has a peak amplitude of 20,000 amps and lasts 40 microseconds to half amplitude. Some lightning pulses can reach four times that current value of more. The rise time of a typical strike is about 5 microseconds to peak amplitude.

The current path in a lightning strike is from the cloud to what we call "ground". A perfect ground connection, however, does not really exist, and any real ground connection will have a finite impedance of from several ohms to several hundred ohms. Applying Ohm’s Law, you can see that a large potential can be developed from a ground connection to "real" ground. One million peak volts or more can easily be developed in such a situation.

In a typical broadcast transmitter or tower site, there is a ground at the tower base and a number of other ground points. The current from a lightning strike will see several parallel paths to ground. For example, the ground rod(s) at the tower base will be one path, the outer jacket of the transmission lines through the equipment cabinets to the transmitter building ground will be another, and the AC safety ground wiring to the distribution panel ground on the tower light wiring still another path. If you can imagine an equivalent circuit of these several resistive paths in parallel, you will be able to grasp the idea that even with a solid ground at the base of the tower, large and damaging potentials can be developed across the other paths. In addition, the fast rise time currents that will flow in all these paths will produce large magnetic fields that will induce significant unwanted currents in nearby conductors such as AC wiring, control cables, audio lines and the like. These are sometimes the most damaging by-products of a lightning strike.

Ground Rods

The most important principle of lightning protection, as most of already know, is to provide the best, lowest impedance local ground connection possible as close to the tower base as possible. This is usually best achieved by using an array of at least four ground rods driven around the tower base pier and tied together with a large copper conductor. The rods should be separated by at least twice their length, and ideally they should penetrate below the deepest frost level into the water table. I recommend cad-welding 1/0 or larger bare copper wire to the rods, making a ring connecting all the rods and then connecting each rod with a separate length of wire to the tower base. The tower connection should also be cad-welded. A wire connected to the tower by way of a lug or using a bolt and washer will have a lot higher resistance than a cad-welded joint.

In some areas where the soil is particularly dry and non-conductive (such as a mountaintop with no water table and little top soil), there are chemical ground rods available to lower the impedance of the ground connection. These rods contain a chemical paste that over the life of the rod seeps into the soil into which the rod is driven through weep holes in the rod. Once the chemical paste has been exhausted, the rod must be replaced. The service time of the various chemical rods is listed in their specifications.

Those of us with AM towers should not be fooled into thinking that the ground screen and radial system provides a good lightning ground. In some areas with very conductive soil, this may be true, but in many locations it is not. A set of rods should be installed at the tower base and connected with 1/0 cable or larger to the ground side of all the arc gaps. Unless you are absolutely certain that your soil is conductive enough to make the screen and radial system an adequate lightning ground, better safe than sorry is the rule.

AM tower bases should also have their antenna tuning unit (ATU) chassis connected to the tower base ground rod array. Even with the best ground rod array, some portion of the current is going to flow in the parallel path presented by the tower feed tubing to the ATU. Once it hits the ATU chassis, it needs a low-impedance path to ground to prevent it from flowing through ATU components and into the transmission line. Most modern ATUs have a horn or ball arc gap right at the point where the tower feed tubing leaves the chassis. The ground side of this gap needs to be tied into the ground rod array. If your ATU does not have an arc gap at this point, you can purchase one inexpensively from Kintronic Laboratories, Phasetek and other manufacturers.

Transmission Lines

In most cases, the tower at a broadcast transmitter site is located some distance from the transmitter building. Whether this is twenty feet or several hundred, the transmission line outer conductor needs to be firmly connected to the ground rod array at the point where the line leaves the tower in FM and grounded-base AM towers. Transmission line manufacturers offer grounding kits for their various lines that provide a secure, weatherproof ground connection to the outer conductor.

A component of the current from a lightning strike that hits the top of a tower with one or more transmission lines will flow down the tower structure and a portion will flow down the parallel path presented by the transmission line outer conductors. If the transmission line outer conductors are not properly bonded to the tower structure at the top and bottom (and at the manufacturer’s recommended interval along the length of the lines for long runs), large potentials can develop between the lines and the tower structure. When the potential exceeds the breakdown voltage of the outer jackets, it will arc through. Such an arc can be sufficiently hot to actually create a pinhole in the outer conductors, making a way for pressure to leak out and water to get in.

This parallel current in the transmission line outer conductor needs a place to jump off to ground before it travels into the transmitter building and into your equipment. This is why the outer conductor of every line leaving the tower needs to be bonded to the ground rod array in addition to being bonded to the tower structure.

For long horizontal transmission line runs, it is a good idea to provide one additional grounding point for the outer conductor just outside the transmitter building. This ground should be the central ground array for the transmitter building, which we will discuss in the next part of this series. Shorter lines, where the tower is within ten or fifteen feet of the transmitter building, do not need the additional ground.

Tower RF Feeds

A piece of copper tubing is usually employed to carry the RF current from the AM antenna tuning unit to the tower itself. As we already noted, this tubing presents yet another parallel current path for lightning currents.

While we can provide a path to ground for this current by way of an arc gap at the output of the ATU, we can create what is in essence a "pi" low-pass filter with the tower feed and the tower and ATU arc gaps, making this parallel current path very unattractive for lightning currents. We do this very simply by winding one or two turns into the tubing on a 12 or so inch diameter to form a series inductor. This series inductance will present a high impedance to the fast rise time lightning current while the arc gaps at either end present a very low impedance to ground. Adding a couple of turns to the feed tubing will often require a slight retuning of the output leg of the ATU network, but the payoff can be greatly reduced potential for ATU damage due to lightning strikes.

Arc Gap Spacing

We mentioned arc gaps earlier. These devices consist of two conductors spaced a certain distance apart with an air space between then. They give lightning current a path to ground on an insulated tower. When the potential between the two conductors exceeds the breakdown voltage of the dielectric (air), ionization occurs and a very low-impedance path between the conductors develops.

Commonly seen in AM installations are ball gaps, which are common at tower bases, and horn gaps, which are more commonly seen in phasing/coupling equipment and transmitters. Many times, transmitters feature gas-discharge gaps with a specified voltage rating across the RF output terminals.

Proper spacing of air gaps makes the difference between a gap providing the proper level of protection and having little effect in terms of lightning protection. At sea level, the breakdown potential of air is about 5 peak kV per 0.1 inch, or 1 peak kV per .020 inches. As altitude increases, the breakdown voltage decreases. A good rule of thumb is to reduce the breakdown voltage by 20% for every 5,000 feet AMSL.

The peak modulated RF voltage across the base of an AM tower can be calculated by the following formula: V_PEAK = 3.182 x Z_A x I_A

Z_A = antenna impedance in ohms

I_A = antenna current in RMS amps

Once the peak modulated RF voltage is known, multiply the voltage in kV by 0.020 to determine the proper ball gap spacing. Horn gap spacing can be calculated using the same method, but the sharper points on a horn gap may require slightly wider spacing.

On a practical level, the optimum spacing for an arc gap is that which is just wider than the point which produces arcs during normal full-power modulated operation. Remember that the wider the spacing, the greater the potential which must develop across the gap before the air ionizes and the gap conducts. This translates to higher voltages applied to ATU components, isocouplers and the like, which beyond a certain point will cause serious damage and result in down time.

In the next part of this series, we will look at lightning protection techniques that we can apply in the transmitter building.