TC 9-64 _________________________________________________________________________
CHARACTERISTIC IMPEDANCE OF A TRANSMISSION LINE
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.
Definition and Symbols
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.
3-14
Previous Page
