Radio frequency

Electric current {{anchor|Current}}

Electric currents 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 (radio 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 RF burns.
• 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 standing waves. RF current may be carried efficiently over transmission lines such as coaxial cables.

Frequency bands

Main article: Radio spectrum

The radio spectrum of frequencies is divided into bands with conventional names designated by the International Telecommunication Union (ITU): :{| class="wikitable" style="text-align:right" |- ! scope="col" rowspan="2" | Frequency range !! scope="col" rowspan="2" | Wavelength range !! scope="col" colspan="2" | ITU designation !! scope="col" rowspan="2" | IEEE bands |- ! scope="col" | Full name ! scope="col" | Abbreviation |- ! scope="row" | Below 3 Hz | 10⁵ km || || style="text-align:center" | || |- ! scope="row" | 3–30 Hz | 10⁵–10⁴ km|| Extremely low frequency || style="text-align:center" | ELF || |- ! scope="row" | 30–300 Hz | 10⁴–10³ km|| Super low frequency || style="text-align:center" | SLF || |- ! scope="row" | 300–3000 Hz | 10³–100 km|| Ultra low frequency || style="text-align:center" | ULF || |- ! scope="row" | 3–30 kHz | 100–10 km|| Very low frequency || style="text-align:center" | VLF || |- ! scope="row" | 30–300 kHz | 10–1 km|| Low frequency || style="text-align:center" | LF || |- ! scope="row" | 300 kHz – 3 MHz | 1 km – 100 m|| Medium frequency || style="text-align:center" | MF || |- ! scope="row" | 3–30 MHz | 100–10 m|| High frequency || style="text-align:center" | HF || style="text-align:center" | HF |- ! scope="row" | 30–300 MHz | 10–1 m|| Very high frequency || style="text-align:center" | VHF || style="text-align:center" | VHF |- ! scope="row" | 300 MHz – 3 GHz | 1 m – 100 mm|| Ultra high frequency || style="text-align:center" | UHF || style="text-align:center" | UHF, L, S |- ! scope="row" | 3–30 GHz | 100–10 mm|| Super high frequency || style="text-align:center" | SHF || style="text-align:center" | S, C, X, Ku, K, Ka |- ! scope="row" | 30–300 GHz | 10–1 mm|| Extremely high frequency || style="text-align:center" | EHF || style="text-align:center" | Ka, V, W, mm |- ! scope="row" | 300 GHz – 3 THz | 1 mm – 0.1 mm|| Tremendously high frequency || style="text-align:center" | THF || |- |[[File:Spectrum.pdf|Radio Spectrum Allocations in Canada|thumb|top|right]] |[[File:International Telecommunication Union ITU.png|International Telecommunication Union ITU|thumb|top|right]] |}

Frequencies of 1 GHz and above are conventionally called microwave,{{cite book More detailed band designations are given by the standard IEEE letter- band frequency designations and the EU/NATO frequency designations.

Applications

Radio has many practical applications, which include broadcasting, voice communication, data communication, radar, radiolocation, medical treatments, and remote control.

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.

References

References

1. Jessica Scarpati. "What is radio frequency (RF, rf)?".
2. (1976). "Airframe and Powerplant Mechanics: Airframe Handbook". Department of Transportation, Federal Aviation Administration, Flight Standards Service.
3. Jeffrey S. Beasley. (2008). "Modern Electronic Communication".
4. [https://standards.ieee.org/ieee/521/768/ IEEE Std 521-2002 ''Standard Letter Designations for Radar-Frequency Bands''], [[Institute of Electrical and Electronics Engineers]], 2002. ([https://www.nap.edu/read/21774/chapter/10#235 Convenience copy] at National Academies Press.)
5. (2012). "Handbook of RF, Microwave, and Millimeter-Wave Components". Artech House.
6. "RF Radio Frequency Signal Generator » Electronics Notes".
7. (2021). "Measure a DUT's input power using a directional coupler and power sensor". EDN.