Inductors
| Inductors | |
|---|---|
| What it does | Stores energy in a magnetic field and opposes changes in current |
| Unit | Henry (H); in practice microhenries (µH) and millihenries (mH) |
| Schematic letter | L |
| Reactance | XL = 2 × pi × f × L; rises as frequency rises |
| In practice | |
| Common forms | Air-core coils, toroids, slug-tuned coils, roller inductors, chokes |
| Quality measure | Q = XL / R |
| Series and parallel | Series values add; parallel values combine like resistors in parallel |
| Ham uses | Tuners, filters, antenna traps, loading coils, RF chokes |
Inductors are electronic components that store energy in a magnetic field. Most are simply coils of wire, sometimes wound on a core of iron, ferrite or powdered iron. An inductor lets direct current through easily but opposes alternating current, and it opposes high frequencies more than low ones, the reverse of a capacitor. Coils turn up all over a ham station: in antenna tuners and filters, in the traps and loading coils of antennas, and as chokes that keep RF out of places it doesn't belong.
Inductance
Current flowing through a wire creates a magnetic field around it. When the current changes, the changing field induces a voltage that pushes back against the change. This property is inductance, measured in henries, named after the American scientist Joseph Henry. HF coils are usually a fraction of a microhenry to a few tens of microhenries.
Inductance depends on:
- Turns. For a coil of a given shape, inductance rises roughly with the square of the number of turns, so doubling the turns gives about four times the inductance.
- Size and spacing. A larger diameter raises inductance; spreading the same turns over a longer length lowers it.
- Core. A core of iron, ferrite or powdered iron multiplies the inductance compared with air. Slug-tuned coils are adjusted by screwing a core in or out.
When the current through an inductor is suddenly cut off, it can produce a voltage spike many times the supply voltage, which is why relay coils usually have a diode across them.
Reactance
An inductor's opposition to alternating current is its inductive reactance, measured in ohms:
- XL = 2 × pi × f × L, with f in hertz and L in henries
A 1 µH coil has about 44 ohms of reactance at 7 MHz and 88 ohms at 14 MHz. At DC, only the small resistance of the wire remains.
Inductors combine like resistors: in series their values add, and in parallel 1 / L = 1 / L1 + 1 / L2 + ... This holds when the coils are far enough apart, or at right angles, that their magnetic fields don't interact. When the fields do link, the coils share mutual inductance, the principle behind the transformer.
Air-core coils and toroids
Air-core coils have no core to lose energy or saturate, so they are stable and efficient, and they are the usual choice in high-power tuners and the tank circuits of tube amplifiers. A roller inductor is an air-core coil that turns on a shaft while a small wheel rides along the wire, giving continuously variable inductance. The drawbacks of air-core coils are their size and a magnetic field that spreads outside the coil.
A toroid is a doughnut-shaped core. Nearly all of the magnetic field stays inside the ring, so toroidal inductors are largely self-shielding and can sit close to other parts. The core also gives a lot of inductance with few turns.
- Powdered-iron cores are stable and low-loss, suited to tuned circuits and filters. They are identified by paint color; mix 2 (red) and mix 6 (yellow) are common at HF.
- Ferrite cores give much more inductance per turn but are lossier and less stable, so they suit broadband transformers, baluns and chokes.
Core names describe size and material: a T-50-2 is a powdered-iron toroid 0.50 inch across in mix 2, and an FT-140-43 is a ferrite toroid 1.40 inches across in mix 43. Makers publish an AL value for each core, from which the turns for a given inductance can be calculated. Too much current or power saturates a core: its inductance collapses and it heats up.
Q
A real inductor has resistance as well as reactance, from its wire and from losses in any core. Its Q, or quality factor, is the reactance divided by that loss resistance: Q = XL / R. A higher Q means less energy wasted as heat and sharper tuned circuits. The skin effect pushes RF current to the surface of the wire and raises its resistance, so high-power coils use heavy wire, copper tubing or silver plating.
Every coil also has a little capacitance between its turns. At some frequency, its self-resonant frequency, that capacitance resonates with the inductance, and above it the coil behaves more like a capacitor.
Chokes
A choke is an inductor used to block AC or RF while letting DC or lower frequencies through.
- RF chokes feed DC into a circuit while keeping RF out of the supply, such as the plate choke that carries high voltage to the tube in an amplifier, or the choke that sends DC up a coax to a mast-mounted preamplifier.
- Filter chokes are iron-core inductors of several henries that smooth ripple in older power supplies.
- Common-mode chokes are coax or cables wound through ferrite to stop RF flowing on the outside of a feed line; see Common-mode choke.