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Lightning REXBURG REGION TRAINING 23 JULY 2015 Sources Three-part QST article Lightning Protection for the Amateur Radio Station Ron Block KB2UYT http://www.arrl.org/lightning-protection http://www.arrl.org/files/file/Technology/tis/info/pdf/0206056.pdf http://www.arrl.org/files/file/Technology/tis/info/pdf/0207048.pdf http://www.arrl.org/files/file/Technology/tis/info/pdf/0208053.pdf Lightning Characteristics 1. Lots of moist air rising from ground level to a few thousand feet 2. Cooler air above with little to no wind 3. 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, Lightning 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. LIGHTNING STRIKE! Thunder Several large pulses of energy > 50,000 °F in 1 microsecond With no time to expand, creates pressure Compresses air around the channel Shock wave faster than sound for about 10 yards Then an ordinary sound wave producing thunder! Generated RF 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 μS. That translates into a radiated RF signal at 139 kHz. Rise times can vary from a very fast 0.25 μS to a very slow 12 μS, 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 nS. 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. Current 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. 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 mS. What to protect Transmitting and Receiving Equipment Antenna Tuner Linear Amplifier Terminal Node Controller Computer Antenna Rotor Transmission Line Construct a Block Diagram of System Protect Each I/O Line Each I/O line is a potential source for lightning strike energy. The best protection is in series with the equipment. They limit the amount of lightning strike energy your equipment will receive. Let-through energy ratings are in milli- or microjoules. Select protection devices with the least let-through energy that meets all the requirements for the connection. Coaxial Cable Protectors Not increase system SWR Induce no signal loss Broad range of frequencies Support receive and transmit power levels Each coax line needs its own protection Protection mounted to a common plate Common plate connected to external ground system AC Power Protection Inexpensive Some depend on safety ground wire Some use ferrite core inductors Plastic housings Printed circuit boards AC Power Protection – NOT! Inexpensive Some depend on safety ground wire Some use ferrite core inductors Plastic housings Printed circuit boards AC Power Protection Matches your voltage and current requirements Each AC line exiting the circle should aggregate to a single line Telephone Inline device is most effective Avoid modular connectors. They are a fragile connector and common amounts of surge energy are very likely to destroy the connector by welding it or fusing it open. In addition, there are also issues of flammable plastic housings, ground wire characteristics, and printed circuit boards that allow arcs to the equipment side. Control Circuits Protection for external devices Especially tower mounted Rotor Shunt-type protection is usually sufficient. Miscellaneous Network Connections Television Satellite dishes GPS feed lines Grounding (in the shack) All system grounds and protection devices MUST be connected to a single “ground” point in the shack. Ground Bus to External Ground 0.5” wide, #26 AWG (0.0159 inch) copper strap has less inductance than #4/0 AWG wire As straight and direct as possible Width of strap must be wider than total circumference of coax coming into the shack. Outside Ground A good ground “system” Not a single ground rod—but a system of interconnected ground rods Purpose: Disperse as much lightning energy as possible before it follows a feed line into the radio system. The BEST systems can dissipate 90% of the strike energy into the earth. Aerial View of Grounding System Cross-Section of Grounding System Operating safety No connection is as good as “no connection”! No matter how good your lightning protection system is, you cannot be in contact with the equipment during a strike. You are the path of least resistance! Consider a storm warning device. Leave the shack when the warning sounds.
