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(Sebelumnya) RackspaceRadio-Electronics (Berikutnya)

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 acronym "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:

Contents

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.[2][3] 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).[4] 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
FrequencyWavelengthDesignationAbbreviation[5]
3 – 30 Hz104 – 105 kmExtremely low frequencyELF
30 – 300 Hz103 – 104 kmSuper low frequencySLF
300 – 3000 Hz100 – 103 kmUltra low frequencyULF
3 – 30 kHz10 – 100 kmVery low frequencyVLF
30 – 300 kHz1 – 10 kmLow frequencyLF
300 kHz – 3 MHz100 m – 1 kmMedium frequencyMF
3 – 30 MHz10 – 100 mHigh frequencyHF
30 – 300 MHz1 – 10 mVery high frequencyVHF
300 MHz – 3 GHz10 cm – 1 mUltra high frequencyUHF
3 – 30 GHz1 – 10 cmSuper high frequencySHF
30 – 300 GHz1 mm – 1 cmExtremely high frequencyEHF
300 GHz - 3000 GHz0.1 mm - 1 mmTremendously high frequencyTHF

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 up to larger distances with lesser attenuation.

In medicine

Radio frequency (RF) energy has been used in medical treatments for over 75 years,[6] generally for minimally invasive surgeries, using radiofrequency ablation and cryoablation, including the treatment of sleep apnea.[7] 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

References

  1. ^ "ISO/IEC 14443-2:2001 Identification cards — Contactless integrated circuit(s) cards — Proximity cards — Part 2: Radio frequency power and signal interface". Iso.org. 2010-08-19. http://www.iso.org/iso/iso_catalogue/ catalogue_tc/catalogue_detail.htm?csn umber=28729. Retrieved 2011-11-08.
  2. ^ Curtis, Thomas Stanley (1916). High Frequency Apparatus: Its Construction and Practical Application. USA: Everyday Mechanics Company. pp. 6. http://books.google.com/books?id=GKA3 AAAAMAAJ&pg=PA6&lpg=PA6&d q=%22electric+shock%22+pain+%22high+f requency#v=onepage&q=%22electric% 20shock%22%20pain.
  3. ^ Mieny, C. J. (2003). Principles of Surgical Patient Care, 2nd Ed.. New Africa Books. pp. 136. ISBN 186928005. http://books.google.com/books?id=TSxQ 6ZzovgkC&pg=PA136&lpg=PA136&a mp;dq=%22electric+shock%22+pain+%22hi gh+frequency#v=onepage&q=%22elect ric%20shock%22%20pain%20%22high%20fre quency.
  4. ^ Brain, Marshall (2000-12-07). "How Radio Works". HowStuffWorks.com. http://electronics.howstuffworks.com/ radio8.htm. Retrieved 2009-09-11.
  5. ^ Jeffrey S. Beasley; Gary M. Miller (2008). Modern Electronic Communication (9th ed.). pp. 4–5. ISBN 978-0132251136.
  6. ^ Ruey J. Sung and Michael R. Lauer (2000). Fundamental approaches to the management of cardiac arrhythmias. Springer. p. 153. ISBN 978-0-7923-6559-4. http://books.google.com/books?id=S1fW hl2c5zIC&pg=PA153&dq=rf+coagu lation+75-years&lr=&as_brr=3& amp;ei=IIhuSpfBJ6aqlQT-n7nmDg.
  7. ^ Melvin A. Shiffman, Sid J. Mirrafati, Samuel M. Lam and Chelso G. Cueteaux (2007). Simplified Facial Rejuvenation. Springer. p. 157. ISBN 978-3-540-71096-7. http://books.google.com/books?id=w1fQ K21WK28C&pg=RA1-PA157&dq=rf+c oagulation+sleep-apnea&lr=&as _brr=3&ei=ZYduSumzKJCckgT_-bHdDg.

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(Sebelumnya) RackspaceRadio-Electronics (Berikutnya)