Please enter valid values. Ensure d2 is greater than d1 and all values are greater than zero.
RESULTS
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.
Dielectric Constant (er)
The relative permittivity of the insulating substrate medium separating the active core conductor from the shielding 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: ~1.68 pF/in.
Diagrams & Theory
A coaxial cable consists of a concentric center conductor and an outer circular metal shield separated by a uniform dielectric insulation layer. The electromagnetic fields are fully contained within the dielectric layer, reducing radio frequency interference (RFI) emissions and isolating the signal path.
Formulas & Mathematical Logic
Characteristic Impedance (Zo): Zo = (138 / sqrt(er)) * log10(d2 / d1) Ω
Propagation Delay (Delay): Delay = 0.08472 * sqrt(er) ns/in
Inductance (L): L = 11.7 * log10(d2 / d1) nH/in
Capacitance (C): C = (0.6128 * er) / log10(d2 / d1) pF/in
These formulas define the steady-state distributed parameters of high-frequency wave propagation. Impedance is set by the geometric ratio of conductor dimensions and the substrate’s dielectric constant.
Step-by-Step Practical Calculation Example
Suppose: Conductor ID (d1) = 35 mil, Shield ID (d2) = 116 mil, Dielectric (er) = 2.30
Step 1: Calculate Characteristic Impedance (Zo)
Zo = (138 / sqrt(2.30)) * log10(116 / 35)
Zo = (138 / 1.51657) * log10(3.31428)
Zo = 90.994 * 0.52039 = 47.35 Ω
Step 2: Calculate Propagation Delay
Delay = 0.08472 * sqrt(2.30) = 0.08472 * 1.51657 = 0.128 ns/in
Step 3: Calculate Distributed Inductance
L = 11.7 * log10(116 / 35) = 11.7 * 0.52039 = 6.09 nH/in
Step 4: Calculate Distributed Capacitance
C = (0.6128 * 2.30) / log10(116 / 35) = 1.40944 / 0.52039 = 2.71 pF/in
How to Use This Calculator
Enter the inner conductor diameter in the In Dia (Conductor ID) field and select your multiplier unit.
Enter the inner diameter of the outer shield in the In Dia (Surface Shield ID) field and choose its unit.
Input the relative permittivity of the insulating substrate in the Dielectric (Substrate) input box.
Click the orange Calculate button to evaluate transmission line parameters.
Read the values for characteristic impedance, propagation delay, inductance, and capacitance on the colored results cards.
About This Calculator
Determine high-frequency coaxial cable transmission line characteristics instantly.
The CalcBoy Coax Impedance Calculator evaluates characteristic impedance, propagation delay, inductance, and capacitance per unit length using the mechanical core boundaries and relative permittivity.
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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