Radio frequency (RF) is theoscillation rate of analternating electric current orvoltage or of amagnetic, electric orelectromagnetic field or mechanical system in thefrequency[1] range from around20 kHz to around300 GHz. This is roughly between the upper limit ofaudio frequencies and the lower limit ofinfrared frequencies, and also encompasses themicrowave range. These are the frequencies at which energy from an oscillating current can radiate off a conductor into space asradio waves, so they are used inradio 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 theskin effect.
RF currents applied to the body often do not cause the painful sensation and muscular contraction ofelectric shock that lower frequency currents produce.[3][4] This is because the current changes direction too quickly to triggerdepolarization of nerve membranes. However, this does not mean RF currents are harmless; they can cause internal injury as well as serious superficial burns calledRF burns.
RF current canionize air, creating a conductive path through it. This property is exploited by "high frequency" units used in electricarc 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 thedielectric insulator of acapacitor. This is becausecapacitive 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 theinductive 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 calledstanding waves. RF current may be carried efficiently overtransmission lines such ascoaxial cables.
Frequencies of 1 GHz and above are conventionally calledmicrowave,[7] while frequencies of 30 GHz and above are designatedmillimeter wave.More detailedband designations are given by the standardIEEE letter- band frequency designations[5] and the EU/NATO frequency designations.[8]
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.[12][citation needed][13]
