...When negative meets positive
Lightning can be a serious hazard depending where you live in the world. Chances are that if you living in an area where lightning strikes are common and you have a communications antenna, it may be attracted to you...
It is said that there are more than a thousand thunderstorms around the Earth causing some 6,000 flashes of lightning every minute and each large thunderstorm has more total energy that an atomic bomb. Even an individual lightning charge contains 30 million volts at 100,000 amperes and can cause serious damage.
However, lightning does not occur uniformly around the globe. In equatorial regions, the warm and humid weather conditions lead to the highest incidence of lightning. As you travel further away from the tropics, the number of lightning storms decreases to as low as less than one day per year in polar regions. Recorded extremes have been as high as 242 thunderstorm days one year in Uganda to l day in 10 years at the poles.
World Thunderstorm Zones
While these statistics give an indication as to lightning risk, they do not reflect lightning intensity or take account of varying topography, which can create quite different conditions near bodies of water, mountains and other non-uniform surface features.
Nor do they differentiate between atmospheric and cloud-to-earth lightning flashes and seasonal variation. Lightning activity is normally at its peak during summer when temperatures are hottest. It also varies with time of days, with 70% of lightning strikes occurring in the afternoon, between noon and 6pm, due to increases in temperature and moisture evaporation.
In a lightning strike, when negative charges reach a sufficiently high level within a cloud they send out a stepped leader which seeks an easy path to the positively charged ground below. A return stroke reaches up from the ground to the clouds resulting in lightning. Most strikes occur in less than half a second and each charge contains 30 million volts at 100,000 amperes. In a complete discharge there can be a sequence of strokes following the same path and lasting up to one second or more. These high voltages can cause intense heating if they pass through a bad joint in a metal conductor or poor insulators like trees or brickwork, and can result in fire.
Lightning is usually attracted to the highest object in the immediate area. Whether you are on land or on the water, the antenna often provides the most attractive path to ground due to its height. The electrical charge takes the most direct route to ground or water, where it dissipates in all directions. You may be part of that path - especially at sea where you are frequently in contact with metal and wet, salty surfaces, or even graphite fishing rods which are good electrical conductors. Your valuable electronic equipment may also be in its path.
More often, however, damage to equipment is caused by voltage or transient currents in antennas and mains wiring and by static charges rather than by direct strikes, as electrical and magnetic fields generated from lightning kilometres away can be induced into power lines and cause equipment damage.
There are a number of ways in which protection can be provided to prevent or minimise damage from lightning strikes and induced currents,. These vary from disconnecting the antenna and power when not in use, which is not always practical, to installing a full lightning protection system. Antennas installed with a change over earthing switch are not protected as the switch is not likely to be capable of carrying lightning currents. Neither are some antennas.
Obviously fitting surge suppression devices on all primary conductors attached to communications equipment and antennas is a simple precaution that can prevent serious damage, whether it be from induced currents or from someone digging up a mains cable nearby. AC power lines, antenna feed lines and rotators, if you use them, should all have transient protection.
Efficient ground systems are essential for optimum performance at HF frequencies and below where antennas usually operate in the quarter wave grounded mode, as well as for lightning protection. The connection to ground is usually made with driven rods or a wire radial system for land installations and at sea by the metal hull of a vessel or in the case of wood or fibreglass vessels, by the installation of a separate earth plate , usually copper, or, where electrolysis may occur, aluminium.
While all-metal ships provide large earths to dissipate electrical strikes, wooden and fibreglass vessels, etc., are very vulnerable. In particular, special attention needs to be made to the earthing of masts as this is not always carried out, masts being prime targets for strikes - especially where they are constructed of conductive material!
Whether you decide to have full lightning protection or not, earth resistance should be as low as possible. In some countries the recommendation is that it should not exceed 10 ohms, while in others (eg USA) recommendations are based on soil type, as soil conductivity varies considerably. Complexities arise where soils are mixed and from variations in soil depth, moisture content, and transfer resistance from the grounding rod to the earth. Sea water is an excellent electrical conductor and provides a better earth than any soil type
In making earth connections, it is not actual size that counts, but rather total surface area. Importantly, marine earth plates must have no high impedance joints and be installed in locations where they are below the water line at all times for the connection to be made. This is especially relevant where speed boats are concerned. And naturally they need to be able to withstand being immersed in salt water year after year.
While just grounding the braid on coaxial antenna feed lines will eliminate static build-up, installing a low impedance single point ground system through a ground panel (bus) for all external conductors gives much better protection. The aim is to eliminate possible transient paths for lightning strikes. The suppressors may also be mounted on this grounded panel. A further ground is necessary between the transceiver case and the ground panel. As most ATUs provide little resistance to high reverse voltages, it is wise to fit a suppressor in the coaxial cable between the ATU and the transmitter in HF installations.
While hand held radio rubber ducky antennas donít need protection at the antenna connector, it is usually wise to provide protection if you are using a larger external antenna. Antenna towers/masts require a single point ground system to which each tower leg is connected. The coaxial cable shield is often grounded to the base of the tower leg.
When selecting a suppressor for the antenna port of a transceiver, you need to calculate the clamping voltage, as the device must be able to let normal RF signals through. The required voltage level is influenced by transmit RF power output, antenna SWR and operating frequency, and can be easily calculated using the formula:
where V= peak voltage across the cable, P= transceiver peak power output (watts), Z= coaxial cable impedance (ohms), and SWR= antenna SWR at required frequency.
Multiplying the square root by a factor of 3 gives the lowest practical clamping voltage for the suppression device without hindering normal transmissions. Therefore if P= 100W (power output of the transceiver), Z= 52 ohms (cable impedance), SWR= 1.5:1 , the resulting clamping voltage is 374.7V.
There are various types of surge suppressors, using spark gaps, gas tubes, metal oxide varistors (MOVs), and silicon avalanche devices. Spark gaps,,such as the Moonraker type LPU , have a reasonably fast response and can be used for high energy applications.
Gas tubes, as used in the Moonraker type CSS coaxial surge suppressor, are similar to spark gaps but can handle both lower and higher voltages, having very high current capacity and low clamping voltages. Firing voltage depends on surge rise time.
With MOVs, used for low voltage applications, energy is absorbed during surge conditions. As a result, the clamping time depends on the surge wave front, high capacitance is exhibited and there is a limited surge life expectancy.
Silicone avalanche devices can handle large surge currents but may be damaged by them, and while they have high clamping speeds, this speed is greatly affected by lead lengths. Naturally the clamping speed of the device must be fast enough to prevent damage and the device itself must be able to withstand lightning surges. Within buildings it is quite possible to experience surges of 2,000 volts due to residual energy from an external event or generated within the building itself.
Lightning protection systems attempt to provide a safe conducting path for lightning to ground. The aim is to capture the lightning strike, conduct it safely to ground, dissipate the energy into the earth through a low impedance ground system and protect all electronic systems both from direct lightning strike and from induced transient currents.
The antenna often forms part of the system. However, unless it is well designed with low system losses and of sufficient diameter, it can increase the hazard.
For the current to be conducted safely to ground, there must be adequate path conductivity and sufficiently low earth resistance, ie a large low loss surface area, such as provided by Moonraker whips. Conductors need to be of low impedance material, make a minimum number of bends, and direct the charge in as direct a path as possible. HF antenna ground planes should not be connected to this earth system.
Any bends in the conductor should have a minimum radius of 20cm (8in). Major metal components on board ship within 1.8m (6ft) of the conductor should be interconnected with the lightning protection system.
Other potential conductors need to be either isolated from it or bonded to it and people and animals must be kept away from the vicinity of the earth termination. Ship engines should be grounded direct to earth.
Materials used are normally copper or aluminium and should be non corrosive grades. Sharp stainless steel tips on rods, as in Moonraker type LPS Rods , can increase the intercept capability of the lightning leader in the charged field below a thunderstorm.
Lightning protection rods can be used singly or ,when placed around the upper perimeter of a defined area, like large roof tops or decks on ships, can protect the area within.
In deciding the level of lightning protection you need, obviously whether you live in a lightning prone area or not is important. However, if what you are protecting is mobile, like a boat, one day you may want to take your boat into such an area, even if you donít live there. Whatís more you donít need to live in a lightning prone area to suffer a lightning strike. Installing a good earthing system from the start makes good sense and, if you are using HF frequencies or below, youíll get the best in performance too.