3-10However, the picture changes considerably when a long line is used. Since most transmission linesare electrically long (because of the distance from transmitter to antenna), the properties of such linesmust be considered. Frequently, the voltage necessary to drive a current through a long line isconsiderably greater than the amount that can be accounted for by the impedance of the load in series withthe resistance of the line.TRANSMISSION LINE THEORYThe electrical characteristics of a two-wire transmission line depend primarily on the construction ofthe line. The two-wire line acts like a long capacitor. The change of its capacitive reactance is noticeableas the frequency applied to it is changed. Since the long conductors have a magnetic field about themwhen electrical energy is being passed through them, they also exhibit the properties of inductance. Thevalues of inductance and capacitance presented depend on the various physical factors that we discussedearlier. For example, the type of line used, the dielectric in the line, and the length of the line must beconsidered. The effects of the inductive and capacitive reactances of the line depend on the frequencyapplied. Since no dielectric is perfect, electrons manage to move from one conductor to the other throughthe dielectric. Each type of two-wire transmission line also has a conductance value. This conductancevalue represents the value of the current flow that may be expected through the insulation. If the line isuniform (all values equal at each unit length), then one small section of the line may represent severalfeet. This illustration of a two-wire transmission line will be used throughout the discussion oftransmission lines; but, keep in mind that the principles presented apply to all transmission lines. We willexplain the theories using LUMPED CONSTANTS and DISTRIBUTED CONSTANTS to furthersimplify these principles.LUMPED CONSTANTSA transmission line has the properties of inductance, capacitance, and resistance just as the moreconventional circuits have. Usually, however, the constants in conventional circuits are lumped into asingle device or component. For example, a coil of wire has the property of inductance. When a certainamount of inductance is needed in a circuit, a coil of the proper dimensions is inserted. The inductance ofthe circuit is lumped into the one component. Two metal plates separated by a small space, can be used tosupply the required capacitance for a circuit. In such a case, most of the capacitance of the circuit islumped into this one component. Similarly, a fixed resistor can be used to supply a certain value of circuitresistance as a lumped sum. Ideally, a transmission line would also have its constants of inductance,capacitance, and resistance lumped together, as shown in figure 3-9. Unfortunately, this is not the case.Transmission line constants are distributed, as described below.

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