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Station ground lightning and safety
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Ground resistance measurements RF ground resistance measurements on small 160 meter antenna
Small antennas and radiation resistance radiation resistance
Basic Ground System Functions
A ground system provides four primary functions:
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To help disperse or divert energy from lightning strikes | |
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To provide safety in case some problem or fault energizes the cabinet or chassis of equipment with dangerous voltages | |
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To provide a controlled RF return path for end-fed or poorly configured or designed transmission line fed antennas | |
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To provide a highly conductive path for induced radio-frequency currents or fields rather than having them flow in lossy soil |
While all of the functions above are distinctly unique, some ground systems can serve two or more functions quite effectively.
Lightning Ground
Lightning is a high energy stepped waveform pulse. Rapidly changing steps in voltage contain high frequency energy. This energy has a peak in the dozens or hundreds of kilohertz, with the bulk of energy ranging from low AC frequencies to perhaps 1MHz. Damaging energy extends to hundreds of megahertz. Lightning should be considered a dc to VHF energy source with the bulk of energy at lower frequencies. Current is massive, thousands of amperes can flow in a lightning strike. A good ground must have a very low impedance over a very wide frequency range. This rules out thin wires, and loosely woven braided conductors should be avoided. The very best ground leads are solid wide smooth surfaces, although braiding sometimes must be used in areas that require flexibility.
Most of the time damage comes from lightning strikes on power lines. Lightning most often follows the utility lines to the house, through house wiring, and to ground in antenna systems. The real danger is lightning flowing through the equipment and the house to seek a ground.
With taller towers, lightning can be a frequent unwelcome visitor. Tall structures often require a large area ground with low impedance wide smooth copper flashing surrounding critical areas. Tall towers need a ground that rapidly and evenly spreads charges out over a wide area. The goal is to prevent objects near the structure from rising significantly faster in voltage than other objects located near the tower. Very high currents can flow between things near the tower, it is important to provide a low impedance path for these currents.
Lightning grounds should always provide a common low impedance path between everything conductive entering a building. This means power lines, telephone lines, TV antennas, and metallic conduits or pipes should all share a common ground connection buss that has very low impedance. Normally the lowest impedance connection is provided by a wide smooth surface copper flashing, although very heavy round copper can be used. Round copper has lower RF resistance per unit length for a given surface area, but flat wide copper has less reactance and lower overall impedance. This is because fewer magnetic flux lines encircle any given area of wide strip than enclose the surface area of a compact conductor. In effect the magnetic field is "spread out", reducing inductance.
There are many sites detailing ground connections, Polyphaser being the most accurate overall.
A good lightning ground is generally a good equipment fault safety ground, but it might not be a good RF ground! RF grounding sometimes requires shielding of the earth from radio frequency fields, or moving the RF energy out of the lossy soil. Lightning generally requires we connect to the earth, and freely move energy into or out of lossy soil. Certain RF ground applications require minimizing RF current flowing into lossy soil.
By the way, despite having two 300 ft tall towers, four 130 ft verticals in a four square, a 220 ft tower, a 200 foot rotating tower, and miles of receiving feedlines covering distances up to 3/4 mile... I don't use any coaxial lightning surge protectors. I have bulkhead entrance panels and use common point grounds and a few MOV's on power lines, but none of my feedlines have surge suppression devices. All of my feed and control cables stay connected during lightning storms, last count that was about 50 cables. My equipment stays connected to power mains through a main disconnect switch.
I fabricate my own copper grounding plate for use at cable entrances:
Every cable enters through bulkhead connectors attached to a plate like those above. This plate is tied into the common ground at my station entrance. That ground is common with power line and utility entrance ground to the house.
Despite multiple hits every year on my tall tower, I never suffer equipment damage. This is all due to my use of proper grounding protocol. Everything entering my operating room, including power line safety grounds, bonds to a single point at the entrance point of my operating tables.
Safety Grounds
Radios that operate from power mains stand a chance of having the power line accidentally fault to the chassis. Worse yet, a linear amplifier with high voltage power transformer might develop a secondary to primary short, and that short might cause the chassis to rise to peak secondary voltage plus peak primary voltage! An amplifier with a 2400 volt RMS transformer operating on 120 or 240 volt USA power mains might have a chassis voltage as high as 3600 volts from a secondary to primary failure inside the transformer.
Any advice saying we don't need a safety ground is very bad advice. Line operated amateur radio gear, especially devices with HV inside, require a safety ground.
The safety ground only requires a connection back to the power mains service entrance ground though a very heavy conductor, although it is a good idea to augment this safety ground connection with, at minimum, a few extra ground rods. As with lightning, the safety ground should tie right back to the utility entrance ground. A good lightning entrance ground also makes a good safety ground, provided it is carried back into the operating area so equipment can be bonded to the ground.
RF Return-path Ground
An antenna system using properly installed and connected coaxial or balanced lines would never require a RF station ground. All currents flowing out to the antenna would be perfectly matched by equal currents flowing back on the second conductor, be it a shield or the second identical conductor of a balanced line.
The problem is many antenna feedpoint or feed systems are poorly designed. It is the abundance of poorly designed systems that cause problems. These flawed systems are behind notions that good Ham shack RF grounds are required to reduce TVI, prevent RF in the shack, or improve transmitting or receiving ability. The troublesome currents are called common-mode currents, because they are not normal push-pull transmission line currents found in two-conductor transmission lines, like coaxial or ladder lines.
A few examples of antennas producing excessive RF in-shack ground currents:
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End-fed halfwaves, including end-fed "dipoles" of all types | |
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Zepp antennas, especially those where the feedline is not an odd 1/4 wl and the antenna not a multiple of 1/2 wl) | |
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Center fed dipole antennas without a balun or suitable feedline length that minimizes common-mode | |
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Verticals with poor grounds or somewhat sparse grounds, including "half-wave" verticals with small or no radial systems. | |
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End-fed longwires or Windom antennas |
It can be truthfully said if we use a two-conductor feedline of any type and have RF in the shack problems, our antenna feed system is poorly designed or constructed. An RF ground in the shack is absolutely NOT required unless something is wrong with our antenna system.
The sole exception to this is a single wire feeder, like a longwire, brought directly into the shack.
The type of ground required for this is one with low RF impedance that spreads current around over a large area. The idea is to use many small conductors radiating out from a central connection point. These conductors do not need to contact earth, they function simply by providing "electrical mass", or a low RF impedance, for the antenna system to push against. It is not a question of surface area or capacitance, it is a question of distributing charges efficiently over a large spatial area in terms of the operating wavelength.
Induced Ground Currents
All efficient antennas, including loops, dipoles, verticals, and beam antennas, are surrounded by very strong electric and magnetic fields. If the antenna is close to earth in terms of the operating wavelength, considerable current can flow in the lossy soil. This causes power loss, even though the antenna has no direct earth connection. It is especially problematic when dipoles are placed at small fractions of a wavelength, or when verticals are mounted near the earth.
Currents like are minimized by covering a large area of earth surrounding the antenna with many closely-spaced conductors. The conductors do not need to be any particular length, they only requirement is they extend beyond the area where field density is high. If the conductors are less than .05 wavelength apart, they can be considered a large single conductor covering the entire area. Much wider spacing than .05wl, and they allow the lossy media between the conductors to be exposed to strong fields.
The diameter of the conductors used isn't important, but maximum spacing and overall length is!