Radio frequency ( RF) is the oscillation rate of an alternating electric current or voltage or of a Magnetic field, electric or electromagnetic field or mechanical system in the frequency
Electric current
that oscillate at radio frequencies (
RF currents) have special properties not shared by
direct current or lower
audio frequency alternating current, such as the 50 or 60 Hz current used in electrical power distribution.
-
Energy from RF currents in conductors can radiate into space as electromagnetic waves ().
-
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.
[
]
[
] 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 Radiation burn.
-
RF current can 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 . RF current may be carried efficiently over transmission lines such as .
Frequency bands
The
radio spectrum of frequencies is divided into bands with conventional names designated by the International Telecommunication Union (ITU):
-
Frequencies of 1 GHz and above are conventionally called microwave,[ IEEE Std 521-2002 Standard Letter Designations for Radar-Frequency Bands, Institute of Electrical and Electronics Engineers, 2002. ( Convenience copy at National Academies Press.)] and the EU/NATO frequency designations.
Applications
Communications
Radio frequencies are used in communication devices such as transmitters, radio receiver, computers, televisions, and mobile phones, to name a few.
Medicine
Medical applications of radio frequency (RF) energy, in the form of electromagnetic waves () or electrical currents, have existed for over 125 years,
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.
Mechanical oscillations
While RF usually refers to electrical oscillations, mechanical RF systems are not uncommon: see mechanical filter and RF MEMS.
See also
-
Amplitude modulation (AM)
-
Bandwidth (signal processing)
-
Electromagnetic interference
-
Electromagnetic radiation
-
Electromagnetic spectrum
-
EMF measurement
-
Frequency allocation
-
Frequency modulation (FM)
-
Plastic welding
-
Pulsed electromagnetic field therapy
-
Radio astronomy
-
Spectrum management
External links
