| a quantity defined for a mode of propagation at a given frequency in a specific uniform transmission line or uniform waveguide by one of the three following relations:|
Z1 = S/ |I|2
Z2 = |U|2 / S
Z3 = U / I
where Z is the complex characteristic impedance, S the complex power and U and I are the values, usually complex, respectively of a voltage and a current conventionally defined for each type of mode by analogy with transmission line equations
1. — For a parallel-wire transmission line, U and I can be uniquely defined and the three equations are consistent. If the transmission line is lossless, the characteristic impedance is real.
2. — For a waveguide, the conventional definitions for U and I depend on the type of mode and generally lead to three different values of the characteristic impedance.
3. — For a circular waveguide in the dominant mode ТЕ11, U = r.m.s. voltage along the diameter where the magnitude of the electric field strength vector is a maximum, I = the r.m.s. longitudinal current.
4. — For a rectangular waveguide in the dominant mode TE10, U = the r.m.s. voltage between midpoints of the two conductor faces normal to the electric field strength vector, I = the r.m.s. longitudinal current following on one surface normal to the electric field strength vector.