©Copyright 2002, LLoyd W. Phillips
Introduction:
Compare lightning to a giant Peltier Device in reverse. You put atmospheric differentials in, and get lightning out.
Deserts, volcanoes, diesel engines, etc., add particulate matter (dust) to the atmosphere. The dust, moisture, and differential atmospheric pressures (isobars) and temperatures cause electrical charges to form by "stirring" this atmospheric "soup." This is the "junction." The sun supplies heat and (the thinning ozone layer) causes even greater ionization and atmospheric turbulence, resulting in more heat in one mass (earth or another cloud), while colder air fronts and frigid high altitude temperatures supply the cold side. Cloud masses are basically water in various states with particulate matter. There must be a collision of atmospheric fronts, with moderate to high temperature differences, for lightning to occur.
Cloud to cloud and intra-cloud lightning attempts to "equalize" upper atmosphere potentials, while earth-sky lightning attempts to equalize potentials between the earth and upper atmospheric masses.
Note: As of May 3, 2002, the National Weather Service has no way of monitoring particulate mater, and checking with Stephen Lord at the Environmental Modeling Center, no weather prediction models factor in the heat contained by matter in atmospheric suspension.
RADIO FREQUENCY (RF) RADIATION CAUSED BY PUBLIC UTILITIES
An important consideration in the search for Radio Frequency sources that block communications and make garbage out of Broadcast, Short Wave, DTV and HDTV, and signals well into the microwave bands, is the RF induced on power lines by faulty connections, insulators, LAs (lightning arrestors (surge protectors)), etc. The transmission lines and vertical grounds then act like a giant WINDOM antenna, capable of radiating many frequencies, some with many times more Effective Radiated Power (ERP) than a tuned dipole or 1/4 wave vertical antenna.
DEFINITIONS / FAQs:
Lightning is an electrical discharge through the atmosphere, which attempts to bring (oppositely) charged masses back into electrical equilibrium.
Lightning Channel is the path that lightning takes through the atmosphere between charged masses. There can be many branches, but usually only one lightning channel. Lightning travels in 30-50 meter increments, called "step leaders."
Voltage in a lightning channel can exceed 1,000,000 volts
Current in a lightning channel can exceed 200,000 Amps ( 200 kA ), with slew rates in excess of 400,000 Amps per microsecond. A typical 30 foot vertical #6 copper wire "ground" on a utility pole has a relatively high impedance at this frequency, and serves more as a "pointer," rather than a road, much like a painted line on a highway. Because lightning has a "slew" component (change in electrical properties over time [ kA/microsecond ]), it has Alternating Current ( AC ) characteristics, and AC laws can be applied.
Temperature in a lightning channel can reach 50,000 degrees Fahrenheit.
Skin Effect is the delay at the inside of a conductor compared to the outside surface of the same conductor as frequency increases. With lightning, the existing magnetic field tends to push a newly applied charge to the outside. A 4 inch wide, flat, bare copper ribbon only 0.030 inches thick provides a better path to ground for lightning than a solid 4 aught copper conductor with insulation. It's the outside surface area that counts, not the inner circular mils. Painting a copper ground strap reduces it's efficiency.
SUMMARY:
Rather than bore you with long-winded technical jargon, charts, and details, lets go right to the summary.
- Use a DISSIPATER above the highest equipment on your tower.
- Lightning does not like sharp bends in ground (bonding) wires or flashing to towers, poles, or other structures. Bends in vertical grounds should NEVER exceed 45 degrees, and ideally be no more than 20 degrees from tower ground to earth ground. If you are dealing with a lightning channel, don't expect to drastically alter its course.
- Decouple a potential lightning channel many feet ahead of a tower base or guy anchor to prevent blasting away concrete.
- Each horizontal messenger in a utility system should have its own separate ground to avoid a delay at each messenger tie level. This delay is caused by the impedance of the vertical ground wire when coupled to the capacitance of the horizontal messenger or cable to be grounded, and may actually DIVERT a portion of the lightning channel into the messenger and equipment attached to it.
- If you must tie horizontal messengers or cables to a common vertical pole ground, clamp the ground from the horizontal messenger to a point at least one foot BELOW the messenger, at a 45 degree angle to both sides of the horizontal messenger or cable span.
- NEVER tie a tower or pole ground to a point at or above the lowest point of your equipment, messenger wire, or surge (lightning) protector.
- If your antenna system or other equipment is at the top of a tower, pole, or other structure, add a separate sharp-tipped lightning rod or a dissipater. This device must extend many feet ABOVE the equipment to be protected. It is advisable to offset or side mount the antenna or equipment, and keep the lightning rod or dissipater in vertical alignment with the supporting structure.
- If you must offset the protector, use at least 3" wide UNPAINTED heavy copper flashing to the vertical ground, at no more than a 20 degree angle from true vertical. Painting copper increases impedance.
- To help protect structure-mounted equipment, clamp any wiring harness or antenna line to the tower or structure at a point BELOW the item to be protected, and then coil all inputs (wiring harnesses, Heliax, etc., in several large diameter loops in open air, between the tower and equipment. The space between each loop be about 1/3 the diameter of the coil. Use fiberglass rods or small diameter, heavy-walled UVA/UVB resistant PVC pipes to retain the coil's shape . This presents a higher impedance to the electronics or electrical equipment downstream when a 400 kA/microsecond lightning pulse is applied.
- Decouple a potential lightning channel many feet BEFORE it enters a Transmitter building or other structure by means of a grounded bulkhead.
- Unpainted, heavy gauge, 4" Copper flashing should be clamped to all vertical antenna and conduit runs (for AC power and strobe lights) close to the base of the tower, BEFORE they make their bends to enter the transmitter or equipment buildings. This ground should be directly bonded to the ground radial system for the tower, not the equipment or building ground.
- DO NOT USE BRAIDED CABLE for Lightning Channel Grounds. When we use a flat braid in certain LNA installations, it is for the purpose of providing inductance (delay) for that particular installation. BRAIDED CABLE IS AN INDUCTOR.
- Carbon based and other soil additives and other technology is available to decrease ground resistance in rocky, mountainous, sandy, or other "difficult to ground' areas.
IMPORTANT CONSIDERATIONS FOR EQUIPMENT and JACKETED HELIAX RUNS:
If you have a 2000' tower, and run a 2000' Heliax line to your antenna, with the Heliax insulated from the tower, you can usually expect lightning damage to equipment at both ends.
Lightning hitting the leg of a tower is dissipated to other legs through cross-members, and the legs act as inductors, while the cross-members and braces have both capacitive and and inductive characteristics. This changes the velocity of the path. The insulated, jacked Heliax has a straight shot down the tower, and to your equipment.
You need to keep the power in both paths at an equal speed and potential by bonding the outer conductor of the Heliax to the leg of the tower every fifty feet or so. In communications equipment rooms inside buildings, mount your equipment on isolation pads that extend about 1 foot beyond the base of the floor mounted equipment cabinets. Suspend all power, RF, and Telco lines in overhead troughs. Run heavy copper flashing grounds in all troughs. When lightning hits, your equipment should be at or near the same potential as all incoming lines. DO NOT GROUND THE EQUIPMENT CABINETS SEPARATELY to any other ground, except the overhead trough ground.. Your goal is to keep them isolated from the floor and other ground paths.
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