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Micro Coaxial Cable Capacitance: What to Know Per Foot

Q: What’s the typical capacitance per foot for micro coaxial cables?

A: There isn’t one single “typical” value applicable to all micro coaxial cables. However, for common thin micro coax cables like RG178 or similar constructions, you can generally expect a capacitance in the range of 20 picoFarads per foot (pF/ft) to 31 pF/ft (65 pF/m to 100 pF/m).

Understanding Micro Coax Capacitance

Capacitance in a coaxial cable refers to its ability to store electrical energy between the center conductor and the shield. It’s an inherent property caused by the physical separation (dielectric material) and geometry of these two conductors.

Why it Varies: The exact capacitance depends on several factors:
- Dielectric Material: The type of insulation (e.g., PTFE, FEP, Polyethylene) and its dielectric constant (εr).
- Geometry: The inner diameter of the shield and the outer diameter of the center conductor.
- Manufacturing Tolerances: Minor variations during production.
Why it Matters: Capacitance affects signal transmission:
- Signal Distortion: High capacitance loads down driver circuits, slowing down signal edges (rise/fall times) and limiting maximum data rates or usable frequency. This causes distortion, especially over longer runs or at high speeds.
- Impedance: It’s a key factor, along with inductance, in determining the cable’s characteristic impedance (like 50Ω or 75Ω). Capacitance too far out of spec can alter the actual impedance.
- Cable Length Impact: The total capacitance of a cable run is capacitance per foot multiplied by the total length. Even a moderate per-foot capacitance can lead to significant total capacitance over hundreds of feet, degrading signals.

Common Micro Coax Capacitance Values

While precise specs are always product-specific, here’s a rough guide:

PTFE-Based Micro Coax (Common for RG178, RG316):
- Often falls around 29-31 pF/ft (approx. 95-100 pF/m).
- Example: RG178/U is frequently spec’d at 30.8 pF/ft (101 pF/m). RG316 is very similar.
Other Miniature Coax Types:
- Some micro/miniature cables using different dielectrics might have slightly lower capacitance, perhaps down to around 20-25 pF/ft (65-80 pF/m).
Lower Capacitance Options:
- Some specialized “low-capacitance” micro coax cables are designed to minimize this value, sometimes reaching 14-18 pF/ft (45-60 pF/m). These often trade off other characteristics like flexibility or shielding.

The Absolute Rule: Consult the Datasheet!

Don’t rely on “typical” ranges for your specific design! Always obtain the official datasheet for the exact micro coaxial cable part number you are using. Reputable manufacturers will explicitly list the capacitance per unit length (usually pF/ft or pF/m) as a key electrical parameter. Values between different brands or even batches of the “same” cable type can vary within tolerance.

Capacitance Conversion: Feet to Meters

Measurement	Capacitance Range
Per Foot	~20 pF/ft to ~31 pF/ft
Per Meter	~65 pF/m to ~100 pF/m

Key Takeaways:

Common Range: Most standard micro coax cables (e.g., RG178/RG316 equivalents) have capacitance around 20-31 picoFarads per foot (pF/ft), with many close to 30 pF/ft.
No Single Value: Capacitance varies significantly based on construction (dielectric, dimensions). Always check the datasheet.
Impact: High capacitance distorts signals and limits usable length/bandwidth.
Source is King: Your cable’s datasheet provides the only reliable capacitance value for design calculations. Ignoring it risks signal integrity problems. Think of the datasheet as your essential guide to ensuring your micro coax performs as expected in your application.