What determines characteristic impedance?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Characteristic impedance Z₀ is the ratio of voltage to current of a single traveling wave on a line, set by geometry and dielectric permittivity, not by length. 50 Ω is the dominant RF standard; 75 Ω is used in video and cable TV. In air-dielectric coax, attenuation is minimized near 77 Ω and peak power handling (breakdown-limited) is maximized near 30 Ω; 50 Ω is a compromise.^[1]

How is Z0 calculated?

For a lossless line Z₀ = √(L/C). Closed-form expressions exist for coax ((60/√ε_r)·ln(D/d)), microstrip (Hammerstad–Jensen), stripline and CPW; field solvers handle arbitrary cross-sections and are used by PCB stackup tools.^[1]^[2]

Why 75 ohms for video?

For receive-only systems such as video and cable TV, low loss matters more than power handling. The 77 Ω loss minimum applies to air dielectric; with solid polyethylene (ε_r ≈ 2.25) the minimum-loss impedance drops to about 77/√2.25 ≈ 51 Ω. 75 Ω remained the video convention for low-loss practical cables, and it is close to the roughly 73 Ω feed resistance of a half-wave dipole.^[3]

How is Z0 measured?

With a TDR, which displays impedance versus distance directly, or from S-parameters of a line section. PCB fabricators verify controlled impedance on test coupons per IPC methods, commonly to ±10% tolerance.^[4]

Frequently asked questions

Can I mix 50 and 75 ohm parts?

A 75 Ω load on 50 Ω has |Γ| = 0.2 (VSWR 1.5, return loss 14 dB), acceptable for some uses but not precision work; connectors of different impedance may also be mechanically incompatible.

Is Z0 complex?

For lossy lines slightly; at RF it is usually treated as real.

Does Z0 change with frequency?

Slightly, through dispersion and conductor internal inductance.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
“Accurate Models for Microstrip Computer-Aided Design”. E. Hammerstad and Ø. Jensen, IEEE MTT-S International Microwave Symposium Digest, pp. 407-409. 1980.
“Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
“IPC-2141A: Design Guide for High-Speed Controlled Impedance Circuit Boards”. IPC. 2004.

Categories: Transmission lines

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