RF Inductors Information

RF inductors are special inductors designed to be used in radio frequency (RF) and microwave applications.

Inductors are basic passive components which serve to oppose changes in current flow. When current is applied from a battery or power source, the inductor temporarily stores it using a magnetic field. When the current flow changes, the magnetic field induces a voltage which opposes the current change.

RF inductors are inductors designed to handle radio frequency signals, which are much higher in frequency than alternating or direct (AC or DC) current. Along with capacitors and resistors, inductors make up a large majority of the components found in resonant (tunable) circuits essential to radio communication devices.

The image below shows an inductor/capacitor pair in a parallel tuned circuit. If the capacitor and inductor are properly matched, this circuit will pass only signals of a specified frequency and cancel out all others. This capability is especially useful in radio communications for cancelling all signals except a desired frequency.

Parallel tuned circuit graphic from Encyclopedia of Science/David Darling

Construction

Typical inductors used for lower-frequency operations are constructed as wire coils wrapped around a ferromagnetic core. All inductors using magnetic cores, however, are subject to undesirable current loss dissipated as heat or noise. Core losses can be caused by two conditions:

  • Hysteresis, which involves the shrinking of core material when magnetized, causing losses through heat.
  • Eddy currents, which are circulating current loops within a conductive core. The energy from these additional currents is dissipated as heat.

Because core losses increase as a current's frequency increases, ferromagnetic cores are not used in RF inductors. Instead, these inductors use air cores, which have lower inductance but much higher resistance to losses. This term may refer to actual air (to the effect that the coil seems to be wrapped around an "empty" core) as well as nonferromagnetic materials such as glass or polymers.

Air Core Losses

While air cores cut high-frequency signal losses, their higher resistance values lead to unique losses which can reduce a circuit's Q factor. The Q factor describes a tunable circuit's resonance by relating its bandwidth to its center frequency; higher Q values indicate lower losses in relation to higher stored energy. Q factor losses in air core inductors are caused by the factors below.

  • Skin effect loss occurs because high-frequency current does not penetrate into the body of a coil's wire, but instead travels along the surface. Because only a small portion of the wire conducts the current, its resistance—which may already be elevated due to high frequencies—increases.
  • Proximity effect occurs in parallel wire turns within a coil. Eddy currents like those described above pull current to the extreme edge of the wire in relation to the nearest coil turn, causing losses similar to those found in skin effect.
  • Parasitic capacitance, resulting from potential differences within the magnetic field, can occur between coil turns. While this does not specifically cause losses, at high frequencies parasitic capacitance can cause the inductor to become undesirably self-resonant.

Solutions to Air Core Losses

To reduce these conditions, RF inductors are typically designed with coils spaced further apart than low-frequency inductors. They may also feature tubular wire or metal strip to increase surface area. The methods listed below are common means for abating air core losses.

Construction Method

Description

Reduced Effect

Image

Basket-weaving

Uses crossing coil winding to increase separation between turns.

Proximity effect, parasitic capacitance

Basket weave inductors

Spiderweb

Similar to basket-weaving; wire is wound radially through spokes on opposite sides of a support insulator; increases separation.

Proximity effect, parasitic capacitance

Spiderweb coil - RF Inductors

Litz wire

Uses multiple insulated conductors braided into a single wire; distributes current equally.

Skin effect

Litz wire

Design Considerations

In addition to the multilayer designs above, RF inductors may be manufactured as single-layer coils. These have two distinct advantages: they are effective against hysteresis and eddy current losses, and have low self-capacitance and high self-resonance, making them effective for frequencies above 3 MHz.

Single coil inductor from Custom Coils

A single-coil inductor. Note that the coils are well-spaced and are composed of flat metal strip to decrease signal loss.

The inductance of a low-frequency single coil can be calculated using the formula below. Because RF inductors are typically high-frequency devices, 2% of the inductance value must be subtracted for devices designed for frequencies high enough to experience skin effect.

Inductance formula

where:Single coil inductor measurements

L = inductance (in henries)

N = number of turns

r = coil radius (in meters)

l = length of wire (provided it is greater than 80% of the radius)

Optimum inductance is typically achieved when the length of the coil (l) is the same as its diameter (or twice its radius, 2r).

Specifications

Most of the important RF inductor specifications are discussed above and are specified by a manufacturer. Inductance is the most important and is specified in henries, millihenries, or nanohenries (H, mH, or nH). Suppliers may also specify an inductance tolerance value which describes the allowable variation from the specified inductance.

Self-resonant frequency (SRF) is the frequency at which an inductor's distributed capacitance resonates with its inductance. At SRF, inductance reaches its minimum value while impedance spikes and then decreases with increasing frequency, causing the inductor to function as a purely resistive component with a Q value of zero. At frequencies below SRF, the device operates as an inductor, while above this frequency it behaves more like a capacitor. In the context of this discussion, then, inductors intended for high-frequency applications must have relatively high self-resonant frequencies to function as designed.

Self-resonant frequency diagram from TDK Tech Journal

Diagram describing actual inductor capacitance and self-resonant frequency.

Standards

The design and use of RF inductors often adheres to specifications described in published standards, some of which are listed below.

BS EN 129000 -- Generic specification: wound RF inductors

BS EN 129200 -- Specification [for] fixed inductors with ceramic or ferrite core wound with copper wire for RF circuits

BS EN 61248 -- Transformers and inductors for use in electronic and telecommunication equipment (series)

References

Clifton Laboratories - Self-resonant frequency of inductors

R. Clarke/University of Surrey - Air coils

Image Credits:

Encyclopedia of Science -- David Darling | Dave's Homemade Radios | Gollum's Crystal Radio World | Homegrown Audio | University of Surrey | Custom Coils | TDK Tech Journal