Radio frequency ( RF) refers to alternating (AC) electric current or , oscillating in the frequency range used in radio, extending from around twenty thousand times per second () to around three hundred billion times per second (), roughly between the upper limit of audio frequency and the lower limit of infrared frequencies.
[A. A. Ghirardi, Radio Physics Course, 2nd ed. New York: Rinehart Books, 1932, p. 249] The term does not have an official definition, and different sources specify slightly different upper and lower bounds for the frequency range. RF usually refers to electrical rather than mechanical oscillations. However, mechanical RF systems do exist (see mechanical filter and RF MEMS).
Special properties of RF current
that oscillate at radio frequencies have special properties not shared by direct current
or alternating current of lower frequencies.
Energy from RF currents in conductors can radiate into space as electromagnetic waves (). This is the basis of radio technology.
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 easily Ionization 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 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 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 . Therefore, RF current must be carried by specialized types of cable called transmission line.
The radio spectrum
of frequencies is divided into bands with conventional names designated by the International Telecommunications Union (ITU):
Frequencies of 1 GHz and above are conventionally called microwave,
while frequencies of 30 GHz and above are designated millimeter wave.
More detailed radio spectrum are given by the standard IEEE letter- band frequency designations [ IEEE Std 521-2002 Standard Letter Designations for Radar-Frequency Bands , Institute of Electrical and Electronics Engineers, 2002.] and the EU/NATO frequency designations.
Radio frequencies are generated and processed within very many functional units such as transmitters, receivers, computers, and televisions to name a few. Radio frequencies are also applied in carrier current
systems including telephony and control circuits.
Radio frequency (RF) energy, in the form of radiating waves or electrical currents, has been used in medical treatments for over 75 years,
generally for minimally invasive surgeries using radiofrequency ablation including the treatment of sleep apnea
Magnetic resonance imaging (MRI) uses radio frequency waves to generate images of the human body.
Radio frequencies at non-ablation energy levels are commonly used as a part of aesthetic treatments that can tighten skin, reduce fat by lipolysis and also apoptosis,
or promote healing.
diathermy is a medical treatment that uses RF induced heat as a form of physical therapy or occupational therapy and in surgical procedures. It is commonly used for muscle relaxation. It is also a method of heating tissue electromagnetically for therapeutic purposes in medicine. Diathermy is used in physical therapy and occupational therapy to deliver moderate heat directly to pathologic lesions in the deeper tissues of the body. Surgically, the extreme heat that can be produced by diathermy may be used to destroy neoplasms, warts, and infected tissues, and to cauterize blood vessels to prevent excessive bleeding. The technique is particularly valuable in neurosurgery and surgery of the eye. Diathermy equipment typically operates in the short-wave radio frequency (range 1–100 MHz) or microwave energy (range 434–915 MHz).
Pulsed electromagnetic field therapy (PEMF) is a medical treatment that purportedly helps to heal bone tissue reported in a recent NASA study. This method usually employs electromagnetic radiation of different frequencies - ranging from static magnetic fields, through extremely low frequencies (ELF) to higher radio frequencies (RF) administered in pulses.
Effects on the human body
Radio frequency current through tissue will generate heat in the tissue and can cause burns.
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.
Amplitude modulation ("AM")
Frequency modulation ("FM")
Pulsed electromagnetic field therapy