Impedance Mismatches & Return Loss

The Reflection Coefficient

When a signal traveling along a 100-ohm cable hits a 110-ohm connector, some energy is reflected. The Reflection Coefficient ($\Gamma$) describes how much energy 'bounces'.

The Physics of Characteristic Impedance ($Z_0$)

A common misconception is that 50-ohm cable has 50 ohms of resistance. If you measure it with a multimeter, it reads near zero. Impedance is dynamic. It relies on the ratio of inductance ($L$) to capacitance ($C$) per unit length:

To maintain constant impedance, the physical geometry (distance between conductor and shield) must be perfectly consistent. A dent in a coaxial cable pushes the shield closer to the core, increasing capacitance ($C$). Since $C$ is in the denominator, $Z_0$ drops, causing a reflection.

VSWR: Voltage Standing Wave Ratio

In RF engineering, the 'echo' interferes with the outgoing signal, creating stationary high and low voltage points on the cable. This is called a Standing Wave. A high VSWR (e.g., 3:1) can physically damage the transmitter's power amplifier by dumping its own energy back into itself as heat.

VSWR vs. Return Loss Lookup

TDR: Time Domain Reflectometry

We find these mismatches using a TDR. By sending a pulse and measuring the time it takes for the 'echo' to return, we can pinpoint the exact distance (in meters) to the damaged part of the cable or the bad connector.

Conclusion

Impedance is about continuity. Any break in the physical symmetry of the cable is a potential point of failure for high-speed data.