Characteristic impedance
From Wikipedia, the free encyclopedia
The characteristic impedance or surge impedance Z0 of a uniform transmission line is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm. A transmission line terminated at one end with its characteristic impedance will appear infinitely long to a source at the other end.
Contents |
[edit] Transmission line model
Applying the transmission line model based on the telegrapher equations, the general expression for the characteristic impedance of a transmission line is:
where
- R is the resistance per unit length,
- L is the inductance per unit length,
- G is the conductance per unit length,
- C is the capacitance per unit length,
- j is the imaginary unit, and
- ω is the angular frequency.
The voltage and current phasors on the line are related by the characteristic impedance as:
where the superscripts + and − represent forward- and backward-traveling waves, respectively.
[edit] Lossless line
For a lossless line R and G are zero and the equation for characteristic impedance reduces to
.
[edit] Surge Impedance Loading
The surge impedance loading (SIL) of a power transmission line is the MW loading when reactive power is produced nor absorbed, or
in which VL − L is the line-to-line voltage in kilovolts.
Power system operators use the SIL as a measure to determine whether a line is supplying (actual loading below SIL) or absorbing reactive power from the system (actual loading above SIL).
[edit] See also
[edit] References
- Ulaby, F. T. (2004). Fundamentals Of Applied Electromagnetics, media edition. ISBN 0-13-185089-X.
- Pozar, D. M. (February 2004). Microwave Engineering, 3rd edition. ISBN 0-471-44878-8.
This article contains material from the Federal Standard 1037C, which, as a work of the United States Government, is in the public domain.
