In Episode 28 of No Dumb Questions, we explain electrical inductance in wire and cable components and demonstrate how to measure it in a laboratory environment.
Inductance is the property of an electrical conductor or transmission line that quantifies its ability to store energy in a magnetic field when electric current flows through it. Designated by the letter $L$ and measured in henries (typically microhenries, $\mu\text{H}$, or nanohenries, $\text{nH}$, in cable specifications), inductance opposes rapid changes in electrical current.
Three primary physical parameters determine a cable’s inductance:
- Conductor Geometry: The cross-sectional area and shape of the conductors.
- Conductor Spacing: The distance separating the forward and return current paths.
- Magnetic Permeability: The magnetic constant of the insulation and dielectric materials between the conductors.
To measure inductance, New England Wire utilizes an LCR meter (measuring inductance, capacitance, and resistance). Unlike capacitance testing—which evaluates an open circuit between conductors—inductance testing on a coaxial cable or twisted pair requires the opposite (far) end of the cable to be terminated and shorted together. This closed loop allows current to circulate and sustain the magnetic field; leaving the far end open causes the stored magnetic field to dissipate, producing an invalid reading.
In the demonstration, testing a 10-foot sample of coaxial cable with its far end terminated yields a reading of 1 microhenry ($1\,\mu\text{H}$ per 10 feet, or $0.1\,\mu\text{H}$ per foot).
Key Technical Takeaways
- Definition: Inductance ($L$) is the ability of an electrical component or conductor pair to store energy in the form of a magnetic field.
- Units of Measurement: Expressed in henries (H), most commonly measured as microhenries ($\mu\text{H}$) or nanohenries ($\text{nH}$) per unit length.
- Design Drivers: Governed by conductor cross-sectional area, physical spacing between conductors, and the magnetic permeability of the insulating material.
- Opposing Current Changes: Inductance opposes instantaneous shifts in current, directly influencing transmission line impedance and high-frequency signal rise times.
- Measurement Requirement: Accurate measurement with an LCR meter requires terminating the far end of the cable to complete the circuit loop and contain the magnetic field.
Coming up next…Episode 29: What is the difference between AC and DC?
Episode 28 Transcript – What is Inductance?
Welcome to No Dumb Questions
Hello! Looks like we’re ready for another episode of No Dumb Questions. Follow me.
Welcome to another episode of No Dumb Questions. Today we’re going to be talking about inductance.
What is Inductance?
So what is inductance and how is it measured? Inductance is a measurement used to describe an object or component’s ability to store energy in the form of a magnetic field. This is measured between two conductors, where the area of the conductors, insulation spacing, and the magnetic constant all contribute to the amount of magnetic field that the object can hold.
Measuring Coaxial Inductance with an LCR Meter
We measure inductance using an LCR meter, with the L standing for inductance.
An example here today is going to be a coax, where we measure the coax for inductance from the center conductor to the outer shield. In order to perform this measurement, we have to have the opposite end terminated. That way we get a full loop measurement throughout the coax. If we disconnect the opposite end, it will dissipate the stored field and will provide a false reading.
We have a ten-foot length here. And we can provide a one-trigger test to get a result of one microhenry per ten-foot length.
All right. Well, thank you for joining us here at New England Wire for another episode of No Dumb Questions. We’ll see you next time.