3-49. You learned earlier that the maximum (and most efficient) transfer of

electrical energy takes place when the source impedance is matched to the

load impedance. This fact is very important in the study of transmission lines

and antennas. If the characteristic impedance of the transmission line and

the load impedance are equal, energy from the transmitter will travel down

the transmission line to the antenna with no power loss caused by reflection.

3-50. Every transmission line possesses certain characteristic impedance,

usually designated as Z0. Z0 is the ratio of E to I at every point along the line.

If a load equal to the characteristic impedance is placed at the output end of

any length of line, the same impedance will appear at the input terminals of

the line. The characteristic impedance is the only value of impedance for any

given type and size of line that acts in this way. The characteristic impedance

determines the amount of current that can flow when a given voltage is

applied to an infinitely long line. Characteristic impedance is comparable to

the resistance that determines the amount of current that flows in a DC

circuit.

3-51. Lumped and distributed constants were explained earlier in this

chapter. Figure 3-15, view A, shows the properties of resistance (R),

inductance (L), capacitance (C), and conductance (G) combined in a short

section of two-wire transmission line. The illustration shows the evenly

distributed capacitance as a single lumped capacitor and the distributed

conductance as a lumped leakage path. Lumped values may be used for

transmission line calculations if the physical length of the line is very short

compared to the wavelength of energy being transmitted. Figure 3-15, view

B, shows all four properties lumped together and represented by their

conventional symbols.

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