Results
Impedance (Zo)-
Propagation Delay-
Inductance-
Capacitance-
Input Parameters Specification
Conductor ID (d1)
The baseline external mechanical boundary diameter profile thickness of the interior active signal wire core pin component.
Surface Shield ID (d2)
The internal opening clearance diameter scale measured across the interior boundary face of the outer metal grounding jacket layer.
Practical Operational Examples
Example 1: Standard RG-58 Test Setup
• Conductor ID = 35.0 mil | Surface Shield ID = 116.0 mil
• Dielectric Constant = 2.30 (Solid Polyethylene)
• Output Impedance: ~47.4 Ω | Delay Parameter: ~0.129 ns/in
Example 2: Semirigid Foam Substrate Matrix
• Conductor ID = 20.0 mil | Surface Shield ID = 67.0 mil
• Dielectric Constant = 1.44 (Expanded Teflon)
• Output Inductance: ~6.14 nH/in | Capacitance Profile: ~1.17 pF/in
Coax Cross Section Theory
Formulas
Zo = 60 / sqrt(er) * ln(d2 / d1)
Delay = 84.72 * 10^-3 * sqrt(er)
Inductance = 5.08 * ln(d2 / d1) (expressed in nH/in)
Capacitance = 1.41 / ln(d2 / d1) (expressed in pF/in)
Step-by-Step Example
Example: Conductor ID (d1) = 35 mil, Shield Inner ID (d2) = 116 mil, Dielectric (er) = 2.30.
Step 1: Convert units to standard calculation inches. Conductor ID (d1) = 35 * 0.001 = 0.035 in, and Shield ID (d2) = 116 * 0.001 = 0.116 in.
Step 2: Calculate the natural logarithm of the diameter ratio: ln(d2 / d1) = ln(0.116 / 0.035) = ln(3.314286) = 1.198228.
Step 3: Solve the characteristic impedance (Zo) using the natural logarithm formula: Zo = (60 / sqrt(erv)) * ln(d2 / d1) = (60 / sqrt(2.30)) * 1.198228 = (60 / 1.516575) * 1.198228 = 47.41 Ohms.
Step 4: Solve the propagation delay: Delay = 84.72e-3 * sqrt(2.30) = 0.08472 * 1.516575 = 0.1285 ns/in.
Step 5: Solve the parasitic inductance: Inductance = 5.08 * ln(d2 / d1) = 5.08 * 1.198228 = 6.09 nH/in.
Step 6: Solve the parasitic capacitance: Capacitance = 1.41 / ln(d2 / d1) = 1.41 / 1.198228 = 1.18 pF/in.
Result: The calculated parameters are Impedance = 47.41 Ohms, Delay = 0.1285 ns/in, Inductance = 6.09 nH/in, and Capacitance = 1.18 pF/in.
How to Use This Calculator
Enter the external inner In Dia (Conductor ID) value and select the desired multiplier unit (mil, cm, mm, µm, in).
Enter the inner diameter of the outer In Dia (Surface Shield ID) and select its unit.
Input the relative permittivity of the insulating substrate in the Dielectric (Substrate Constant) field.
Click the orange Calculate button to initiate the coaxial transmission line solver.
Read the computed values for Impedance (Zo), Propagation Delay, Inductance, and Capacitance on the Results cards.
About This Calculator
Determine high-frequency coaxial cable transmission line parameters with precision.
The CalcBoy Coax Impedance Calculator evaluates the characteristic impedance (Zo), propagation delay, parasitic inductance, and capacitance per unit length using outer shield and inner conductor diameters.
Coaxial cables are standard transmission mediums in radio frequency (RF) networks, laboratory instrumentation, and high-speed data links. The geometric spacing between the inner wire and the grounding shield determines how electromagnetic energy propagates along the line. Impedance must be closely controlled to prevent wave reflections and maximize signal delivery.
Using these standard geometric ratios, antenna designers, RF engineers, and lab technicians can predict cable behavior, check matching limits, and select appropriate materials (such as foam or solid dielectric insulators) to meet system specifications.
Best UseRF circuit planning, coaxial line selection, and laboratory impedance matching.
Key OutputCharacteristic Impedance (Zo), Propagation Delay, Inductance, and Capacitance per inch.
Helpful ForHigh-speed layouts, ham radio feedlines, and signal integrity evaluation.
Design ReminderDielectric consistency directly affects transmission speed and shielding.
Tip: Standard RG-58 cables are rated at 50 Ohms, while TV antenna cables (RG-6) are typically rated at 75 Ohms. Match these system values to eliminate power loss.
Frequently Asked Questions
How do the cable dimensions affect impedance?
Impedance increases as the outer shield diameter (d2) becomes larger relative to the inner conductor diameter (d1). Conversely, a higher dielectric constant (er) decreases characteristic impedance.
What are common coaxial dielectric values?
Solid Teflon (PTFE) has a dielectric constant of 2.1, solid Polyethylene (PE) is 2.3, and foamed dielectrics range between 1.3 and 1.6 due to high air content.
Why is propagation delay important?
Propagation delay determines signal transit time along the cable. In phase-matched antenna arrays or high-speed clock routing, matching cable propagation delays prevents skew and phase alignment issues.
Does cable length affect characteristic impedance?
No. Characteristic impedance is a per-unit-length property set entirely by cross-sectional geometry and materials. A 1-inch cable and a 100-foot cable of the same type have the same impedance.
What happens if cable impedance is mismatched?
Mismatches cause standing wave spikes (high SWR) and signal energy reflections. This results in power loss, heating, and signal distortion.
How are capacitance and inductance related to Zo?
Characteristic impedance is mathematically related to these parameters by the equation Zo = sqrt(L / C). Controlling spacing helps achieve the target Zo value.
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