Real-World Problem: "My Multimeter Says 0.2 Ohms, So Why Is It Called a 50-Ohm Cable?"
If you take a digital multimeter set to resistance and touch the leads to both ends of a 50-ohm coaxial cable, you won't measure 50 ohms—you'll measure nearly zero ohms! That's because characteristic impedance is not DC resistance. It is the instantaneous AC resistance an electric wave experiences as it travels down the cable, governed entirely by the physical ratio of wire size to insulation thickness.
1. Why Are Standard Cable Impedances 50, 75, and 100 Ohms?
Industry standards weren't chosen randomly—they are mathematical sweet spots between power handling and signal attenuation:
- 50 Ohms — RF Coax (RG-58, LMR-400, WiFi antennas): The mathematical ideal compromise between maximum power handling (around 30 ohms) and minimum signal attenuation (around 77 ohms).
- 75 Ohms — Video & TV Coax (RG-59, RG-6): Provides near-minimum signal attenuation over long distances where high power handling isn't needed.
- 100 Ohms — Differential Pairs (Ethernet Cat6, USB): Optimized for twisted-pair geometry to reject common-mode noise while maintaining low losses.
2. What Happens When Impedance Changes Along the Cable?
If a manufacturing line lets insulation thickness drift by even ±0.02mm, the characteristic impedance shifts. When a high-speed signal hits an unexpected change in impedance (an impedance mismatch), part of the energy reflects backward toward the transmitter like an echo inside a tunnel. This creates signal corruption, eye-diagram distortion, and dropped data packets.
Key Rule for Hardware Designers
Always match your PCB trace impedance, connector pinout, and raw cable impedance! If your board outputs a 100-ohm differential signal, every link in the chain—PCB, cable, and connector—must be strictly rated for 100 ohms.