Radio frequency

Radio frequency (RF) is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals. RF usually refers to electrical rather than mechanical oscillations; however, mechanical RF systems do exist (see mechanical filter and RF MEMS).

Although radio frequency is a rate of oscillation, the term "radio frequency" or its abbreviation "RF" are also used as a synonym for radio – i.e. to describe the use of wireless communication, as opposed to communication via electric wires. Examples include:

• Radio-frequency identification
• ISO/IEC 14443-2 Radio frequency power and signal interface

Special properties of RF current

Electric currents that oscillate at radio frequencies have special properties not shared by direct current or alternating current of lower frequencies.

• The energy in an RF current can radiate off a conductor 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. For this reason, when the human body comes in contact with high power RF currents it can cause superficial but serious burns called RF burns.
• RF currents applied to the body often do not cause the painful sensation of electric shock as do lower frequency currents. This is because the current changes direction too quickly to trigger depolarization of nerve membranes.
• 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.
• 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, so RF current must be carried by specialized types of cable called transmission line.

Radio communication

In order to receive radio signals an antenna must be used. However, since the antenna will pick up thousands of radio signals at a time, a radio tuner is necessary to tune in to a particular frequency (or frequency range). This is typically done via a resonator – in its simplest form, a circuit with a capacitor and an inductor forming a tuned circuit. The resonator amplifies oscillations within a particular frequency band, while reducing oscillations at other frequencies outside the band.

Frequencies

Main article: Radio spectrum

Frequency	Wavelength	Designation	Abbreviation
3 – 30 Hz	104 – 105 km	Extremely low frequency	ELF
30 – 300 Hz	103 – 104 km	Super low frequency	SLF
300 – 3000 Hz	100 – 103 km	Ultra low frequency	ULF
3 – 30 kHz	10 – 100 km	Very low frequency	VLF
30 – 300 kHz	1 – 10 km	Low frequency	LF
300 kHz – 3 MHz	100 m – 1 km	Medium frequency	MF
3 – 30 MHz	10 – 100 m	High frequency	HF
30 – 300 MHz	1 – 10 m	Very high frequency	VHF
300 MHz – 3 GHz	10 cm – 1 m	Ultra high frequency	UHF
3 – 30 GHz	1 – 10 cm	Super high frequency	SHF
30 – 300 GHz	1 mm – 1 cm	Extremely high frequency	EHF
300 GHz - 3000 GHz	0.1 mm - 1 mm	Tremendously high frequency	THF

The inverse relation between frequency and wavelength deduces that the higher the frequency of the RF Signal, the smaller its wavelength and vice versa. Thus, under similar conditions of propagation, the higher frequency signal attenuates faster than the lower frequency signal and becomes too weak to be detected at the end of the receiver, located at larger distances. An RF power amplifier is used to amplify the power level of such a transmitter RF Signal, so that it can travel larger distances with less attenuation.

In medicine

Radio frequency (RF) energy has been used in medical treatments for over 75 years, generally for minimally invasive surgeries, using radiofrequency ablation and cryoablation, including the treatment of sleep apnea. Magnetic resonance imaging (MRI) uses radio frequency waves to generate images of the human body.

See also

• Amplitude modulation
• Bandstacked
• Electromagnetic radiation
• Frequency allocation
• Frequency bandwidth
• Frequency modulation
• Plastic welding
• Radio waves
• RF connector
• RuBee
• Spectrum management

External links

• Definition of frequency bands (VLF, ELF … etc.) IK1QFK Home Page (vlf.it)
• Radio, light, and sound waves, conversion between wavelength and frequency
• RF Terms Glossary