Find That Low-Impedance Path to Ground
This is the second in a three-part series. Part 1 is archived under Tech Tips at radioworld.com.
Eliminating parallel ground paths and providing a single, low-impedance path to ground for lightning energy is the key to mitigating damage from strikes. A key component in this low-Z ground path is the ground electrode or ground rod.
There are a number of ways of creating a low-impedance ground electrode, but 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.
| An example of damage from a direct hit to an FM antenna bay.|
Exothermic (welded, not clamped) 1/0 or larger bare copper wire should be used to tie the rods together, making a ring connecting all the rods. Then a separate wire from each rod is run to the tower base. The tower connection should also be 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 welded joint, and that resistance will increase over time with weathering and corrosion.
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 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.
The ground rod array and tower connection provides the first line of defense against lightning strike damage. It gives lightning current a place to go that is not through equipment or transmission lines.
Like water, lightning current seeks the path of least resistance. A well-engineered installation gives it a place to go that is far away from equipment.
AM is different
AM towers often are insulated, and this presents a special case.
Insulated AM towers employ a spark gap formed by two steel balls suspended side by side with an air gap between them. When lightning hits the tower, the air between the balls is ionized and current flows through the ionized path from the tower-potential ball to the ground-potential ball. This ionized path has a very low impedance, provided the gap is not too great.
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 0.02 inches. As altitude increases, the breakdown voltage decreases. A good rule of thumb is to reduce the breakdown voltage by 20 percent for every 5,000 feet AMSL (we pay attention to this in Colorado where I live!).
The peak modulated RF voltage across the base of an AM tower can be calculated by the following formula:
VPEAK = 3.182 x ZA x IA
ZA = antenna impedance in ohms
IA = antenna current in RMS amps
The minimum gap spacing can thus be calculated by utilizing the above formula to determine the peak modulated RF voltage across the gap and then multiplying .020 inches per peak kV, adjusted for altitude.
We do not, of course, want the gap set right at the spacing indicated by this exercise because towers themselves exhibit static buildup as the result of wind blowing through their structure or nearby convective activity.
The proper gap spacing is thus determined by starting at the minimum spacing and then adjusting that out to eliminate nuisance arcs in normal conditions (i.e., without thunderstorms in the area).
AM installations employ a buried radial ground system and either interspersed short radials or a ground screen near the tower base. Readers with AM towers should not be fooled into thinking that this 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. More than one broadcast engineer, when investigating altered AM antenna system parameters after an electrical storm, has found his ground radials (typically #12 copper wires) burned off the ground ring at the tower base. This type of conductor, even with many in parallel, is not capable of carrying the kinds of current present in a lightning strike.
AM tower bases should also have their antenna tuning unit 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.
Another measure that AM engineers can and should take is winding at least one turn in the feed tubing between the tower and the antenna tuning unit.
This will produce a series inductance that presents an elevated impedance to fast rise-time lightning current, (hopefully) making the path across the arc gap to ground through the ground electrode more attractive.
Don't forget the lines
In most cases, the tower at a broadcast transmitter site is located some distance from the transmitter building. Whether this is 20 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.
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 bonded properly 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 lightning discharge 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.
Transmission line manufacturers offer grounding kits for their various lines that provide a secure, weatherproof ground connection to the outer conductor.
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 point of the ground array for the transmitter building (more on that later). Shorter lines, where the tower is within 10 or 15 feet of the transmitter building, do not need the additional ground.
Some engineers advocate winding one or more turns into smaller transmission lines (7/8 inch or smaller) in the horizontal run between the tower and transmitter building. This produces the same effect as a single turn in the RF feed to AM towers, producing an inductance that is a high impedance to fast rise-time lightning current.
A caveat: this can produce a "transformer effect." The copper in the turn of transmission line will be cut by the lines of magnetic flux from the lightning current flowing in the tower steel and induce a significant voltage.
This is why it's important in such cases to provide another ground connection on the transmitter building side of this turn in the transmission line.
We'll start our discussion next time by looking at a central ground or "star" scheme diagram.
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