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Propagation Constant of Radio-Frequency Transmission LinesThe propagation constant of any transmission line at any frequency is given by equation 37. The attenuation constant a and the phase constant β can be found by equations 43 and 49. At radio frequencies, however, ωL and ωC become so much greater than the resistance R and conductance G that simplifications are possible. Thus, equation 37 becomes5,15
and, when the shunt conductance is negligible,
and
where a is the attenuation constant in nepers per unit length, β is the phase constant in radians per unit length (corresponding to R, L, and C), Z0 is the characteristic impedance (equation 79), and the units ohms, farads, and henrys are used. Since the wave velocity is, from equation 55, V = ω/β, it follows that at radio frequencies the velocity of propagation on a line is approximately
This is the velocity of light, approximately 186,300 miles per second, or 3 X 105 kilometers, 3 X 108 meters, or 3 X 1010 centimeters per second.
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Last Update: 2011-05-18