Characteristic impedance is the ratio of voltage to current for a signal traveling in one direction along a transmission line. It’s measured in ohms, and for a standard coaxial cable, it’s typically 50 or 75 ohms. What makes it “characteristic” is that it depends entirely on the physical construction of the cable or trace, not on what’s connected at either end or how long the line is.
This concept matters any time a signal travels through a wire, cable, or circuit board trace at high enough frequencies that the signal’s wavelength becomes comparable to the length of the conductor. At that point, you can no longer treat the wire as a simple connection. It behaves as a transmission line, and its characteristic impedance determines how signals move through it and what happens when they reach the other end.
How It Works Physically
Every transmission line has two fundamental electrical properties distributed along its length: inductance and capacitance. The conductors store magnetic energy (inductance per unit length, L) and the gap between them stores electric energy (capacitance per unit length, C). Characteristic impedance is simply the square root of L divided by C. A line with more inductance relative to its capacitance has a higher impedance; more capacitance relative to inductance means lower impedance.
For a lossless line (one where resistance and energy leakage are negligible), this value is purely real, meaning it behaves like a simple resistance even though no energy is actually being dissipated. A single wave traveling down the line sees a constant ratio of voltage to current at every point, and that ratio is the characteristic impedance. This only holds for a wave moving in one direction. If a wave reflects back, the total voltage-to-current ratio at any given point changes, but the characteristic impedance of the line itself does not.
What Determines the Value
For a coaxial cable, three physical factors set the characteristic impedance: the diameter of the center conductor, the diameter of the outer shield, and the dielectric constant of the insulating material between them. A larger center conductor relative to the shield lowers the impedance. A dielectric material with a higher dielectric constant (its ability to store electrical charge) also lowers the impedance because it increases the capacitance per unit length.
On a printed circuit board, the same principles apply but with a different geometry. For a microstrip (a trace running above a ground plane), impedance depends on the trace width, the trace thickness, the height of the dielectric separating the trace from the ground plane, and the dielectric constant of the board material. A wider trace or a thinner dielectric layer lowers the impedance. PCB designers routinely adjust these dimensions to hit a target impedance, most commonly 50 ohms for high-speed digital and RF signals.
The key takeaway is that characteristic impedance is set by geometry and materials alone. Cutting a cable shorter or longer doesn’t change it. A 50-ohm coaxial cable is 50 ohms whether it’s one meter or one hundred meters long, because the cross-sectional structure is the same everywhere along its length.
Why 50 and 75 Ohms Became Standard
The standardization of 50-ohm impedance dates back to the 1930s, when engineers were developing coaxial cables for kilowatt radio transmitters. For air-filled coax, 30 ohms provides the best power handling, while 77 ohms gives the lowest signal loss. Fifty ohms sits right between those two extremes, a practical compromise. When manufacturers switched from air to polyethylene insulation (with a dielectric constant of 2.25), the minimum-loss point for coax shifted to about 51 ohms, reinforcing the choice.
The 75-ohm standard dominates cable television and video applications, where cables don’t carry high power and low signal loss is the priority. There’s also a convenient antenna-matching benefit: a standard half-wave dipole antenna naturally presents an impedance close to 75 ohms, and a simple 2:1 balun transformer converts 300-ohm twin-lead antenna wire down to 75 ohms. Practical considerations like cable flexibility played a role too. Higher impedance means a thinner center conductor, which keeps cables easier to bend, an important factor for cheap commercial installations.
What Happens When Impedances Don’t Match
When a signal traveling down a transmission line hits a point where the impedance changes (a different cable, a connector, or a load at the end), part of the signal reflects back toward the source. The fraction that reflects is called the reflection coefficient, calculated as the difference between the load impedance and the line’s characteristic impedance, divided by their sum.
If a 75-ohm load terminates a 50-ohm line, the reflection coefficient is 0.2, meaning 20% of the signal voltage bounces back. If the load perfectly matches the line at 50 ohms, the reflection coefficient is zero and all the energy transfers into the load. A completely open or shorted end produces a reflection coefficient of 1, meaning the entire signal reflects.
These reflections cause real problems. In a digital system, reflected signals arrive back at the source slightly delayed and can distort subsequent data bits, creating errors. In RF systems, reflected power can damage transmitters and create standing waves that reduce efficiency. In high-speed circuit boards, even small impedance discontinuities at connectors or vias can degrade signal quality. The entire discipline of impedance matching exists to minimize these reflections.
How Characteristic Impedance Is Measured
The most common measurement method is time domain reflectometry (TDR). A TDR instrument sends a fast voltage step into the transmission line and monitors what comes back. Because the reflected signal is separated in time from the original pulse, you can see both the impedance of the line and the location of any discontinuity along it.
The oscilloscope display reveals the characteristic impedance directly. A flat horizontal trace at a given voltage level means the impedance is uniform. Any bump or dip in the trace indicates a change: a small upward bump suggests a series inductance (like a poorly matched connector), while a downward dip suggests added capacitance (like a solder pad or via). The shape of the reflection tells you whether the discontinuity is resistive, inductive, or capacitive.
You can also calculate the distance to any impedance discontinuity from the round-trip time of the reflection. Dividing the propagation velocity of the signal by two (since the pulse travels out and back) and multiplying by the measured delay gives the physical distance to the mismatch. This makes TDR valuable not just for measuring impedance but for locating faults, breaks, or connector problems in installed cables.
Characteristic Impedance vs. Resistance
One common source of confusion is that characteristic impedance is measured in ohms but isn’t resistance in the usual sense. You can’t measure it with a multimeter. If you put an ohmmeter across the ends of a 50-ohm coaxial cable, you’ll read either an open circuit or a short, depending on what’s at the far end. The 50 ohms only manifests when a signal is actually propagating through the line.
Resistance converts electrical energy into heat. Characteristic impedance describes the relationship between voltage and current in a traveling wave, and in an ideal lossless line, no energy is lost at all. The signal propagates forward with a fixed voltage-to-current ratio determined by the line’s inductance and capacitance. That ratio is what your system needs to match for clean, efficient signal transfer.