Inductors
An inductor is a coil of wire that stores energy in a magnetic field. If a capacitor is like a tiny rechargeable battery for electric charge, then an inductor is like a flywheel for electrical current — it resists changes in current flow, and it stores energy that it releases when the driving source is removed. Inductors are everywhere in radio equipment: they are the heart of filters, tuned circuits, RF chokes, transformers, and antenna matching networks.
- What is an inductor and how does it work?
- Inductance: the henry
- Types of inductors
- Inductors in series and parallel
- The RL time constant
- Inductive reactance
- Inductors in ham radio
What Is an Inductor and How Does It Work?
Think of current flowing through a wire as water flowing through a pipe. When you try to start the flow suddenly, the water has inertia — it resists being set in motion. An inductor behaves the same way with electrical current. When you apply voltage across an inductor, current does not jump instantly to its final value. Instead it rises gradually as the magnetic field builds up around the coil.
This happens because of a principle called electromagnetic induction. When current flows through a coil, it creates a magnetic field that threads through every turn. If you try to increase that current, the growing magnetic field induces a voltage in the coil that opposes the increase — a phenomenon called Lenz's law. This self-opposing voltage is called the back-EMF. The result is that current changes in an inductor are always gradual, never instantaneous.
When the current source is removed, the magnetic field collapses. As it collapses, it induces a voltage in the coil that tries to maintain the original current flow — the inductor releases its stored energy back into the circuit. This is why a collapsing inductor field can produce voltage spikes much higher than the supply voltage; a property that is both useful and potentially damaging if not managed properly.
A current-carrying coil creates a magnetic field that threads through every turn. The more turns and the higher the current, the stronger the field and the more energy stored.
View LargerInductance: The Henry
The property of a coil that determines how strongly it opposes changes in current is called its inductance, measured in henrys (H). One henry is a large amount of inductance — most practical inductors for radio work are measured in millihenrys (mH) or microhenrys (µH). A small toroidal RF choke might be 100 µH; a large power supply filter choke might be several henrys.
The inductance of a coil depends on:
- Number of turns (N): Inductance increases with the square of the number of turns. Double the turns, quadruple the inductance.
- Core material: Ferromagnetic materials (iron, ferrite) concentrate the magnetic field, multiplying inductance dramatically compared to an air core.
- Core cross-sectional area: A larger core area means more flux for the same current, increasing inductance.
- Coil length: Spreading turns over a longer winding reduces inductance.
- Symbol: L Unit: henry (H), millihenry (mH), microhenry (µH)
- 1 H = 1,000 mH = 1,000,000 µH
- Inductance opposes changes in current, not current itself
- Energy stored: E = ½ × L × I²
Types of Inductors
Different inductor constructions suit different applications. Choosing the right type for a radio circuit involves balancing inductance value, frequency range, current rating, Q factor, and physical size.
Common inductor types used in electronics and radio. Each core material and winding geometry suits a different frequency range and application.
View Larger| Type | Core | Typical range | Ham radio use |
|---|---|---|---|
| Air-core solenoid | Air | 1 µH – 10 mH | RF tank circuits, VHF/UHF tuned circuits |
| Toroidal (ferrite) | Ferrite | 100 nH – 1 mH | RF chokes, broadband transformers, EMI filters |
| Ferrite rod (loopstick) | Ferrite rod | 100 µH – 10 mH | LF/MF receive antennas, AM broadcast receivers |
| SMD chip inductor | Ferrite | 1 nH – 100 µH | VHF/UHF circuits, switching power supplies |
| Powdered iron toroid | Powdered iron | 1 µH – 100 µH | HF band-pass filters, antenna tuners |
Toroidal inductors are especially popular in ham radio because the closed magnetic path means virtually no external field — neighboring components are not affected and the toroid does not pick up external interference. This makes toroids ideal for RF chokes and filter inductors in crowded circuit boards.
Inductors in Series and Parallel
Inductors combine in the same way as resistors. When connected in series, their inductances simply add. When connected in parallel, the total inductance falls below the smallest individual value — like taking lanes off a motorway.
- Series: Ltotal = L1 + L2 + L3 + …
- Parallel (two): Ltotal = (L1 × L2) / (L1 + L2)
- Parallel (many): 1/Ltotal = 1/L1 + 1/L2 + 1/L3 + …
Inductors in Series
Enter up to four inductor values in microhenrys (µH) to find the total series inductance.
Inductors in Parallel
Enter up to four inductor values in microhenrys (µH) to find the total parallel inductance.
The RL Time Constant
Just as an RC circuit has a time constant that governs how fast a capacitor charges, an RL circuit has a time constant that governs how fast current rises through an inductor. When you apply a DC voltage across a resistor and inductor in series, current does not jump to its final value instantly — it rises exponentially, reaching 63.2% of its final value after one time constant, and 99.3% after five time constants.
τ = L / R
- τ (tau) = time constant in seconds
- L = inductance in henrys
- R = resistance in ohms
- After 1τ: current = 63.2% of final value
- After 5τ: current = 99.3% of final value (effectively steady state)
τ = L / R = 0.010 H / 100 Ω = 0.0001 s = 100 µs
After 100 µs the current has reached 63.2% of its final value (V/R).
RL Time Constant Calculator
Enter inductance and resistance to find τ = L / R.
⚖ Experiment: Observe Inductor Kick
This experiment demonstrates the voltage spike produced when an inductor's magnetic field collapses — the same effect that destroys relay driver transistors if a flyback diode is omitted.
- 9 V battery
- Small 12 V relay (or any coil-type component with a DC resistance of 50–500 Ω)
- 1N4001 or any general-purpose silicon diode
- One LED and a 470 Ω resistor
- Breadboard and jumper wires
- Connect the relay coil across the 9 V battery through a momentary push-button switch. Do NOT include the diode yet.
- Connect an LED in series with the 470 Ω resistor across the relay coil, oriented so it does NOT light with the battery connected (reverse-biased).
- Press and quickly release the push-button. Observe the LED briefly flash as you release the button.
- Now add the 1N4001 diode in parallel with the relay coil, oriented to conduct the fly-back current (anode to the negative side of the coil). Repeat the test.
Without the diode, the LED flashes brightly when you release the button — the collapsing magnetic field of the relay coil generates a voltage spike (sometimes exceeding 100 V from a 9 V supply) that forward-biases the LED through the circuit. With the flyback diode installed, the spike is clamped to about 0.7 V and the LED no longer flashes. This proves that inductors really do produce large voltage spikes on switch-off, and shows exactly why relay driver circuits always need a flyback diode.
Inductive Reactance
An inductor offers no DC resistance beyond the wire resistance of its windings — but when AC flows through it, the inductor resists that AC current. This AC opposition is called inductive reactance, symbol XL, and it is measured in ohms.
Unlike resistance, inductive reactance increases with frequency. The higher the frequency, the more the inductor's back-EMF opposes current change, and the higher its reactance. This is the foundation of how inductors are used in filters: they pass low frequencies and block high frequencies.
XL = 2πfL
- XL = inductive reactance in ohms (Ω)
- f = frequency in hertz (Hz)
- L = inductance in henrys (H)
- 2π ≈ 6.2832
Inductive Reactance Calculator
Calculate XL = 2πfL. Enter frequency in MHz and inductance in µH.
Inductors in Ham Radio
Inductors appear in virtually every part of a ham radio station. Here are the most common roles:
- RF chokes (RFC): A high-reactance inductor placed in a signal line to block RF while passing DC. Used to feed DC bias to RF transistors and to keep RF out of power supply lines.
- Resonant tank circuits: Paired with a capacitor to form the tuned circuits that select a single frequency in oscillators, filters, and amplifiers. The resonant frequency is f = 1 / (2π√LC).
- Band-pass filters: Multiple LC sections form the bandpass filters that separate amateur bands and reject adjacent signals in transceivers.
- Antenna tuners: Variable inductors (roller inductors) allow the antenna system's impedance to be matched to 50 Ω across a wide frequency range.
- Ferrite beads and chokes: Small ferrite core inductors wound with a few turns suppress common-mode RF on coax shields and power leads, reducing TVI and RF feedback into audio equipment.
- Baluns: Wound on ferrite toroids to convert between balanced and unbalanced transmission lines, such as at a dipole feedpoint.
Frequently Asked Questions
Why does inductive reactance increase with frequency?
At higher frequencies, current changes direction more rapidly. Each change in current direction produces a change in the magnetic field, which induces a back-EMF opposing that change. The faster the changes (higher frequency), the larger the back-EMF for the same current amplitude, so the inductor appears to have more opposition — higher reactance. This is why inductors are used as low-pass filter elements: they pass DC and low frequencies easily but increasingly block higher frequencies.
What is Q factor for an inductor?
Q (quality factor) measures how close a real inductor is to ideal. A perfect inductor has only inductance; a real one also has winding resistance. Q = XL / Rwinding. A higher Q means the inductor is more efficient — it stores energy well and loses little to heat. In resonant circuits, a higher Q produces a sharper, more selective response. Toroidal cores with low-loss ferrite generally give higher Q than open winding styles.
Can I use an inductor as a resistor?
Not usefully. A small inductor has very low DC resistance — often just a few ohms or less. It can limit DC current if that resistance is significant, but you would never choose an inductor for that purpose. However, at radio frequencies, the inductive reactance can be very high, making the inductor effectively a large AC impedance. This is exactly how RF chokes work: low DC resistance (so the transistor biasing works correctly) but high RF impedance (so the RF signal sees an open circuit at that point).
Why do toroids have almost no external magnetic field?
In a toroidal (doughnut-shaped) core, the magnetic field follows the circular path inside the core. Because the path closes on itself, the field is almost entirely contained within the core material and does not radiate outward. This is ideal for ham radio circuits because toroidal inductors do not couple magnetically to neighboring components, and they do not pick up hum or RF from nearby transformers or wiring.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.