7
Lumped and Distributed Transmission Lines
Ideal and Lossy Transmission Lines
Transmission lines that are lossless, that is R=G=0, are termed ideal transmission lines. This is valid if
attenuation and skin effect are either negligible or not of concern for the signal frequencies being
analyzed.
For real lines, the series resistance is not quite zero, and the phase velocity is slightly dependent on the
applied frequency. These non-idealities result in attenuation and dispersion.
Attenuation
Attenuation results in a reduction in signal amplitude, which may be a function of frequency.
Dispersion
Dispersion results from the propagation velocity being different for the various frequencies.
These effects can cause the frequency components of a signal to be quite different at the far end of the
line, compared to the source. The fast rise and fall times of the input signals can be reduced and become
"rounded". It should be noted that there is a theoretical condition where there is attenuation without
dispersion, when R/L = G/C. This is normally not of practical significance.
An attenuation vs frequency curve is often provided by cable manufacturers to show susceptibility to
these effects:
Figure 5 – Attenuation vs. Frequency for a 100 meter lossy coax cable
Quantitatively, attenuation is the real part of the complex propagation constant,
