WEA-104 Lightening Protection 4


Extracted from ARRL’s QST Magazine
“Light Protection For The Amateur Radio Station”
June, July, August 2002 articles
by Ron Block KB2UYT

The Challenge
The amateur is challenged to assemble the best radio station possible, to enjoy the benefits of the hobby and have the station operable during times of need. This can be a significant challenge especially considering the awesome capabilities of Mother Nature’s lightning strikes. While she may have the upper hand as far as when and how much energy she delivers, you have the ability to influence how that energy is diverted into the earth. Said another way, you can implement a lightning protection plan that will protect your Amateur Radio station, even from a direct strike!…

Lightning Characteristics
The conditions necessary for an old-fashioned summer afternoon thunderstorm are lots of moist air from ground level to a few thousand feet, cooler air above with little to no wind and plenty of sun to heat the air mass near the ground. As the warm, moist air is heated, it rises quickly to heights where the temperature is below freezing, eventually forming a thundercloud…

Within the thundercloud, the constant collisions among ice particles driven by the rising air causes a static charge to build up. Eventually the static charge becomes sufficiently large to cause the electrical breakdown of the air—a lightning strike.

The average thunderstorm is approximately six miles wide and travels at approximately 25 mph. The anvil shape of the cloud is due to a combination of thermal layer (Tropopause) and upper high velocity winds that cause the top of the cloud to mushroom and be pushed forward. The area of imminent danger is the area up to 10 miles in front of the leading edge of the cloud but may extend much further depending on conditions.

When a lightning strike does occur, the return stroke rapidly deposits several large pulses of energy along the leader channel. That channel is heated by the energy to above 50,000ºF in only a microsecond and hence has no time to expand while it is being heated thus creating extremely high pressure. The high pressure channel rapidly expands into the surrounding air and compresses it. This disturbance of the air propagates outward in all directions. For the first 10 yards or so it propagates as a shock wave moving faster than the speed of sound and after that as an ordinary sound wave—the thunder we hear.

During a lightning strike your equipment is subjected to several huge impulses of energy. The majority of the energy is pulsed dc with a substantial amount of RF energy created by the fast rise time of the pulses. A typical lightning strike rise time is 1.8 microseconds. That translates into a radiated RF signal at 139 kHz. Rise times can vary from a very fast 0.25 microseconds to a very slow 12 microseconds yielding an RF range from 1 MHz down to 20 kHz. However, the attachment point for a direct lightning strike has a time as fast as 10 nanoseconds. This RF content of the strike will have a major effect on the design of the protection plan. In addition to the strike pulses, the antennas and feed lines form tuned circuits that will ring when the pulses hit. This is much like striking a tuning fork in that ringing is created from the lightning’s pulsed energy. Average peak current for the first strike is approximately 18 kA (98% of the strikes fall between 3 kA to 140 kA). For the second and subsequent impulses, the current will be about half the initial peak. Yes, there is usually more than one impulse. The reason that we perceive a lightning strike to flicker is that it is composed of an average 3 to 4 impulses per lightning strike. The typical interval between impulses is approximately 50 microseconds.

Frequently, amateurs provide an inducement for Mother Nature to find us. For good long distance communications, we put our antennas on the top of towers and place the towers so that they protrude above the surrounding buildings or countryside. While this provides for great signal coverage, it also makes it easier for Mother Nature to find a shorter, conductive path to ground.

The probability of having your tower struck by lightning is governed primarily by where you are located and the height of the tower… The other significant factor that affects the probability of being struck is the height of the tower above the average ground level. As you might suspect, the higher your tower, the higher the probability of being struck…

Identify What is to be Protected
The goal of the planning process is to establish a “zone of protection” within the radio room, as opposed to the whole house or building. Additional zones may be considered separately. The first step in the process is to identify what you want to protect. The immediate answer is, well, everything. While you can come close, you may run out of money, time or energy. So let’s create a priority list and work the list from high to low priority. Probably first on the list are the more expensive items associated with your radio station, usually the transmitting and receiving equipment. What follows on the list depends on just how you enjoy the hobby—the antenna tuner, linear amplifier, terminal node controller or computer. Further down the list might be the antenna, rotor and transmission line. Each person’s list and priority ordering will be different.

The first step is to construct a complete block diagram of the equipment in your radio room starting with the top priority item. You will make a separate plan for other areas needing protection. This is usually simple and straightforward. In some installations, it may be necessary to look behind the equipment to determine precisely the connections between each element. The accuracy of the diagram is important in determining the nature and effectiveness of the protection plan. If you have multiples of an item, e.g. two dual band mobile radios, then they are probably listed in order of value. These are the heart of your radio station so make them the starting point of your protection plan which will in turn examine and diagram each element of the station.

This training was presented in condensed form to peak an interest in Lightning Protection. As you will surmise, this is just the first step in protecting your investment and your hobby. This material was extracted from an extensive discussion found in a series of 3 ARRL’s QST Magazine articles:

That concludes tonight’s training. Are there any questions, comments or suggested additions to this material?

Thanks, this is (callsign) clear to net control.