After Class: Grounds, Grounds, and Grounded
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Ground is ground the world around. That's a saying that I have often heard Ham radio operators say aloud and in writing. In a general sense, it's true, but on a local level grounds can vary widely from location to location, even within a few hundred feet. It is true both for direct current and low frequencies and for frequencies in to the GHz regions. It has to do with the conductivity of the soil and/or rock in the area as well as the amount of moisture and other elements in the ground. Antenna guys like to run conductive (usually copper) "radials" out from the mounting pole or tower in order to create a sufficient local reference ground, and electric power distribution engineers often need to salt the ground around substations with ionic compounds in order to create sufficient conductivity to provide a safe grounding system. This article from the August 1959 edition of Popular Electronics is a good primer on the subject. After Class: Ground, Grounds, and Grounded
WINXL Why is a good ground so important? Why are some circuits grounded and others not grounded? What is the physical and electrical meaning of the ground symbol when it appears in a diagram? Grounding actually means making an electrical connection between a piece of equipment or circuit and the earth, thus bringing the connected point to the earth's neutral potential. There are many ways of accomplishing this. The most common way is by connecting the equipment to a cold water pipe by means of a wire and a metal grounding clamp. Why specifically a cold water pipe? Well, a cold water pipe goes directly to the outside water line which is buried in the earth. A hot water pipe is connected to a furnace or hot water heater and is not a direct ground connection. Another way of making a ground connection is by connecting the equipment to a metal rod which has been driven deep into the earth. The metal rod is called a ground rod and, to be effective, should go at least eight feet into the ground. A properly installed ground rod is shown in Fig. 1.
Fig. 1 - Ground rod system of grounding is good if the rod is driven well into some moist earth. If you check your TV antenna, you may find that a ground rod is part of the installation, as is a lightning arrestor. Should lightning strike the antenna, it would find an easier path through the lightning arrestor to the ground rod, and thus into the earth, than through your house. A Good Ground Whether or not a ground is "good" is determined by the amount of ohmic resistance between the ground rod, or other means of grounding used, and the earth. The less this "earth resistance," the better the ground. The actual resistance measurement is made with an instrument called a megohmmeter which applies a high voltage to a resistance and then measures the current flow. There are many factors that determine the earth resistance. Some of the more important are: 1. Moisture content of the soil surrounding the grounding element. 2. Composition of the soil. For example: clay is a good contact, rock a poor one. 3. Temperature of the soil. 4. Size, shape, and number of the grounding elements buried in the earth. The more element area in contact with the soil, the better the ground. Circuit Grounds The ground in an electrical circuit is the circuit's electrical reference point. Normally when something is "above" ground, it is positive, since the negative side of the circuit is usually grounded. There are times, however, when the positive side is grounded, as in some of the new 12-volt automobile electrical systems. In such cases, the potentials would be considered negative or below ground. Before installing mobile equipment in a car, it is important to determine whether the positive or the negative terminals of the battery are grounded. When a circuit is grounded and the circuit diagram shows various parts to be at ground potential by the ground symbol, it means, in effect, that these parts are electrically connected. This is generally done by using the chassis as a common grounding point and then connecting the chassis to an external ground. Equipment is grounded as a safety measure as well as for proper operation of the circuit. If a ground connection is made to the chassis, possibility of shock through contact with the chassis is eliminated since both you and the chassis are at "ground potential." There are some circuits, in which the chassis is "hot," that should never be grounded. A common example of equipment which should not be grounded is the typical a.c.-d.c. table radio. The instructions which come with these sets generally state, "Caution: Do not connect a ground wire to this set." Since you may have either ground potential or 117 volts a.c. on the chassis (depending on which way you insert the a.c. plug into the power line), these inoffensive-appearing little sets should be handled with due respect, and never be operated outside of their insulated cabinets without taking proper precautions. Grounds and Antennas An antenna is nothing more than a conductor whose specific job is to radiate or receive electromagnetic energy. Very often in ham radio the same antenna serves for both transmitting and receiving through the use of a switching arrangement. In this discussion we shall limit ourselves to the role that the earth, or ground, plays in the functioning of the antenna system.
Fig. 2 - Ground plane system is useful when an antenna must be installed at a distance from an external ground. Although antennas are sometimes discussed without taking the earth into consideration, we cannot ignore the earth. When the antenna radiates electromagnetic energy, the earth acts as a reflector for energy which is directed in a downward direction. These waves are reflected back by the earth and combine with the waves which have been radiated directly from the antenna. If the reflected wave and the direct wave are in phase, that is, if their maximums and minimums coincide, they tend to strengthen each other. If they are out of phase, or do not coincide, the reflected wave weakens the direct wave to the point where, if the two waves are 180° out of phase, cancellation occurs. The way the two waves combine depends to a large extent on the relationship of the antenna to the ground beneath it. Is the ground a good conductor or a poor conductor? Is it rocky? Is it wet or dry? Is the antenna high above the earth? All of these factors are important. Currents are induced in the earth by that portion of the radiated wave which travels along the ground and is known as the ground wave. Valuable energy is dissipated into the earth by the ground wave and every attempt is made to keep ground-wave losses to a minimum. Fewest losses occur when the wave travels over ground which is a good conductor. This is the reason many commercial stations place their antenna systems near water or marsh lands, the water or wet earth being a much better conductor than dry earth. Where this physical placement is not possible, in order to make the ground around the antenna as conductive as possible, metal rods or mesh screens are buried near the surface of the earth. They extend about one-half wavelength to either side or radially around the antenna. The actual height of the antenna then becomes its height above this ground screen. Many times it is practical to mount a vertical antenna on the roof of a building at an inconvenient distance from a good grounding point. A ground system is still for the antenna to operate properly and may be accomplished by simulating a ground condition at the base of the antenna through the use of a ground plane system (see Fig. 2). In the ground plane system, copper wires cut to quarter wavelengths of the frequencies to which the antenna is tuned are attached radially, with wires of the same length opposite each other, to the base of the antenna mounting. They are insulated from the antenna's driven element and the roof but connected to a good earth ground and the transmission line shield. In effect, the ground plane system provides a ground cut to specified wavelengths and then suspended in mid-air at the point where it is needed. In practice, ground plane radials generally act as supports for the vertical antenna mast as well as being part of the electrical installation. Safety and Efficiency A sound knowledge of basic ground theory and procedures is necessary for each person who works or experiments with electronic devices. Good grounding techniques assure the operation of electronic equipment at maximum efficiency and with minimum electrical shock hazard.
Posted March 23, 2020 "After Class" Topics
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