Radio frequency (RF) is the
oscillation rate of an
alternating electric current or
voltage or of a
magnetic, electric or
electromagnetic field or mechanical system in the
frequency[1] range from around 20
kHz to around 300
GHz. This is roughly between the upper limit of
audio frequencies and the lower limit of
infrared frequencies, and also encompasses the
microwave range, though other definitions treat microwaves as a separate band from RF.[2][3] These are the frequencies at which energy from an oscillating current can radiate off a conductor into space as
radio waves, so they are used in
radio technology, among other uses. Different sources specify different upper and lower bounds for the frequency range.
RF current does not penetrate deeply into electrical conductors but tends to flow along their surfaces; this is known as the
skin effect.
RF currents applied to the body often do not cause the painful sensation and muscular contraction of
electric shock that lower frequency currents produce.[4][5] This is because the current changes direction too quickly to trigger
depolarization of nerve membranes. However, this does not mean RF currents are harmless; they can cause internal injury as well as serious superficial burns called RF burns.
RF current can easily
ionize air, creating a conductive path through it. This property is exploited by "high frequency" units used in electric
arc welding, which use currents at higher frequencies than power distribution uses.
Another property is the ability to appear to flow through paths that contain insulating material, like the
dielectric insulator of a
capacitor. This is because
capacitive reactance in a circuit decreases with increasing frequency.
In contrast, RF current can be blocked by a coil of wire, or even a single turn or bend in a wire. This is because the
inductive reactance of a circuit increases with increasing frequency.
When conducted by an ordinary electric cable, RF current has a tendency to reflect from discontinuities in the cable, such as connectors, and travel back down the cable toward the source, causing a condition called
standing waves. RF current may be carried efficiently over
transmission lines such as
coaxial cables.
Frequencies of 1 GHz and above are conventionally called
microwave,[8] while frequencies of 30 GHz and above are designated
millimeter wave.
More detailed
band designations are given by the standard
IEEE letter- band frequency designations[6] and the EU/NATO frequency designations.[9]
Radio Frequency or RF energy is also being used in devices that are being advertised for weight loss and fat removal. The possible effects RF might have on the body and whether RF can lead to fat reduction needs further study. Currently, there are devices such as
trusculpt ID, Venus Bliss and many others utilizing this type of energy alongside heat to target fat pockets in certain areas of the body. That being said, there is limited studies on how effective these devices are.
Measurement
Test apparatus for radio frequencies can include standard instruments at the lower end of the range, but at higher frequencies, the test equipment becomes more specialized.[13][citation needed][14]
Mechanical oscillations
While RF usually refers to electrical oscillations, mechanical RF systems are not uncommon: see
mechanical filter and
RF MEMS.