3-14CHARACTERISTIC IMPEDANCE OF A TRANSMISSION LINEYou learned earlier that the maximum (and most efficient) transfer of electrical energy takes placewhen the source impedance is matched to the load impedance. This fact is very important in the study oftransmission lines and antennas. If the characteristic impedance of the transmission line and the loadimpedance are equal, energy from the transmitter will travel down the transmission line to the antennawith no power loss caused by reflection.Definition and SymbolsEvery transmission line possesses a certain CHARACTERISTIC IMPEDANCE, usually designatedas Z_{0}. Z_{0} is the ratio of E to I at every point along the line. If a load equal to the characteristic impedanceis placed at the output end of any length of line, the same impedance will appear at the input terminals ofthe line. The characteristic impedance is the only value of impedance for any given type and size of linethat acts in this way. The characteristic impedance determines the amount of current that can flow when agiven voltage is applied to an infinitely long line. Characteristic impedance is comparable to theresistance that determines the amount of current that flows in a dc circuit.In a previous discussion, lumped and distributed constants were explained. Figure 3-15, view A,shows the properties of resistance, inductance, capacitance, and conductance combined in a short sectionof two-wire transmission line. The illustration shows the evenly distributed capacitance as a singlelumped capacitor and the distributed conductance as a lumped leakage path. Lumped values may be usedfor transmission line calculations if the physical length of the line is very short compared to thewavelength of energy being transmitted. Figure 3-15, view B, shows all four properties lumped togetherand represented by their conventional symbols.Figure 3-15.—Short section of two-wire transmission line and equivalent circuit.Q19.Describe the leakage current in a transmission line and in what unit it is expressed.

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