Cable Impedance Calculator
Calculate coaxial and twisted pair cable characteristic impedance, capacitance, inductance, propagation delay, and cutoff frequency. Free online transmission line calculator.
About This Calculator
The Cable Impedance Calculator helps engineers, technicians, and electronics enthusiasts determine the characteristic impedance of coaxial and twisted pair transmission lines. Enter the conductor geometry and dielectric properties to instantly compute impedance, capacitance, inductance, propagation delay, and cutoff frequency.
For coaxial cables, the characteristic impedance is calculated using Z₀ = (60/√εr) × ln(D/d), where D is the inner diameter of the outer shield, d is the conductor diameter, and εr is the dielectric constant. For twisted pair cables, the formula is Z₀ = (120/√εr) × ln(2s/d), where s is the spacing between conductors. The calculator also computes capacitance per meter, inductance per meter, propagation delay, and the TE11 cutoff frequency for coaxial cables.
Common cable types include 50 Ω coax (RG-58, RG-213) used in radio communications and test equipment, 75 Ω coax (RG-6, RG-59) used in television and video, and 100 Ω twisted pair (Cat5e, Cat6) used in Ethernet networks. The dielectric constant depends on the insulation material: polyethylene (εr=2.25), PTFE/Teflon (εr=2.1), PVC (εr=3.0-4.0), and air (εr=1.0).
Frequently Asked Questions
What is characteristic impedance of a cable?
Characteristic impedance is the effective resistance a transmission line presents to alternating current. For coaxial cables it is calculated as Z₀ = (60/√εr) × ln(D/d) and for twisted pair cables as Z₀ = (120/√εr) × ln(2s/d), where εr is the dielectric constant, D is the shield inner diameter, d is the conductor diameter, and s is the conductor spacing.
What is the standard impedance of RG-58 coaxial cable?
RG-58 coaxial cable has a standard characteristic impedance of 50 Ω. It uses a 0.9 mm inner conductor, a 2.95 mm inner shield diameter, and a polyethylene dielectric with εr ≈ 2.25.
What is the difference between coaxial and twisted pair impedance?
Coaxial cable impedance depends on the ratio of shield diameter to conductor diameter (D/d), while twisted pair impedance depends on the ratio of conductor spacing to diameter (2s/d). Coaxial cables typically have impedances of 50 Ω or 75 Ω, while twisted pair cables like Ethernet Cat5e/Cat6 have 100 Ω characteristic impedance.
How does dielectric constant affect cable impedance?
The dielectric constant (εr) affects impedance inversely: higher εr reduces impedance and slows propagation velocity. Common dielectric materials include polyethylene (εr=2.25), PTFE/Teflon (εr=2.1), and PVC (εr=3.0-4.0).
What is cable capacitance and why does it matter?
Cable capacitance is the ability of the transmission line to store electrical energy per unit length. Higher capacitance increases signal distortion over long runs. Coaxial cables typically have 50-100 pF/m capacitance while twisted pair cables have 40-70 pF/m.
What is the cutoff frequency of a coaxial cable?
The cutoff frequency is the frequency above which higher-order transmission modes (TE11) can propagate. It is calculated as f_c = 2c/(π√εr(D+d)). For RG-58 coax this is approximately 33 GHz. Above this frequency the cable no longer behaves as a simple transmission line.
Can I use this calculator for Ethernet cable impedance?
Yes, Ethernet cables (Cat5e, Cat6, Cat6a) use twisted pair construction with 100 Ω characteristic impedance. Enter the conductor diameter and spacing for your specific cable to verify its impedance. Typical values are 0.5 mm conductor diameter with 1.0 mm spacing.
Why is impedance matching important?
Impedance matching between a source, cable, and load minimizes signal reflections and maximizes power transfer. Mismatched impedances cause signal degradation, standing waves, and reduced system performance. Common standards include 50 Ω for RF systems and 75 Ω for video.