What Is Characteristic Impedance?
Characteristic impedance (Z0) is a fundamental property of a transmission line that describes the ratio of voltage to current for a wave propagating along the line. It depends on the physical dimensions of the conductors and the dielectric material between them, but not on the length of the cable.
Impedance matching is critical in RF and high-frequency systems. When the source, transmission line, and load impedances are matched, maximum power transfer occurs and reflections are minimized. Mismatched impedances cause signal reflections that can degrade performance and damage transmitters.
Impedance Formulas
Where D is the outer conductor inner diameter (coax) or wire spacing (twin-lead), d is the inner conductor diameter (coax) or wire diameter (twin-lead), and Er is the relative dielectric constant.
Common Cable Impedances
| Cable Type | Impedance | Common Use |
|---|---|---|
| RG-58 | 50 ohms | RF, radio, laboratory |
| RG-59 | 75 ohms | Video, CCTV |
| RG-6 | 75 ohms | Cable TV, satellite |
| RG-174 | 50 ohms | GPS, WiFi pigtails |
| RG-213 | 50 ohms | Amateur radio, high power |
| 300-ohm Twin-Lead | 300 ohms | FM/TV antenna feedline |
Frequently Asked Questions
Why are 50 and 75 ohms standard?
50 ohms is the geometric mean between 30 ohms (minimum attenuation for air-dielectric coax) and 77 ohms (maximum power handling). It provides a good balance between the two. 75 ohms is close to the impedance for minimum attenuation and is ideal for signal quality in video and broadcast applications.
What happens with impedance mismatch?
Impedance mismatch causes signal reflections quantified by the Voltage Standing Wave Ratio (VSWR). Higher mismatch means more reflected power, leading to signal loss, standing waves, and potential damage to transmitters. A VSWR of 1:1 is perfect (no reflection), while 2:1 reflects about 11% of power.
How does the dielectric affect impedance?
The dielectric constant (Er) appears in the denominator as sqrt(Er), so higher dielectric constants reduce impedance. Dielectric also affects signal velocity: the velocity factor equals 1/sqrt(Er). Foam dielectrics have lower Er values, resulting in higher velocity factors and lower loss.