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 range from around twenty thousand times per second (20 kHz) to around three hundred billion times per second (300 GHz). This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies; these are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves. Different sources specify different upper and lower bounds for the frequency range.
Electric Current
Electric currents that oscillate at radio frequencies (RF currents) 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 (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 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 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 standing waves. Therefore, RF current must be carried by specialized types of cable called transmission line, such as coaxial cables.
Frequency Bands
The radio spectrum of frequencies is divided into bands with conventional names designated by the International Telecommunications Union (ITU):
| Frequency range |
Wavelength range |
ITU designation | IEEE bands | |
|---|---|---|---|---|
| Full name | Abbreviation | |||
| 3–30 Hz | 105–104 km | Extremely low frequency | ELF | N/A |
| 30–300 Hz | 104–103 km | Super low frequency | SLF | N/A |
| 300–3000 Hz | 103–100 km | Ultra low frequency | ULF | N/A |
| 3–30 kHz | 100–10 km | Very low frequency | VLF | N/A |
| 30–300 kHz | 10–1 km | Low frequency | LF | N/A |
| 300 kHz – 3 MHz | 1 km – 100 m | Medium frequency | MF | N/A |
| 3–30 MHz | 100–10 m | High frequency | HF | HF |
| 30–300 MHz | 10–1 m | Very high frequency | VHF | VHF |
| 300 MHz – 3 GHz | 1 m – 10 cm | Ultra high frequency | UHF | UHF, L, S |
| 3–30 GHz | 10–1 cm | Super high frequency | SHF | S, C, X, Ku, K, Ka |
| 30–300 GHz | 1 cm – 1 mm | Extremely high frequency | EHF | Ka, V, W, mm |
| 300 GHz – 3 THz | 1 mm – 0.1 mm | Tremendously high frequency | THF | N/A |