is a term that refers to alternating current (AC) havingcharacteristics such that, if the current is input to an antenna, an electromagnetic (EM) field is generated suitable for wireless broadcastingand/or communications. These frequencies cover asignificant portion of the electromagnetic radiation spectrum, extending from nine kilohertz (9 kHz),the lowest allocated wireless communications frequency (it’s within the range of human hearing), to thousands of gigahertz(GHz).

When an RF current is supplied to an antenna, itgives rise to an electromagneticfield that propagates through space. This field is sometimescalled an RF field; in less technical jargon it is a “radiowave.” Any RF field has a wavelength that is inverselyproportional to the frequency. In the atmosphere or in outerspace, if f is the frequency in megahertz and sis the wavelength in meters, then

s = 300/f

The frequencyof an RF signal is inversely proportional to the wavelengthof the EM field to which it corresponds. At 9 kHz, the free-spacewavelength is approximately 33 kilometers (km) or 21 miles (mi). At the highestradio frequencies, the EM wavelengths measure approximately one millimeter (1 mm). As the frequency isincreased beyond that of the RF spectrum, EM energy takes the form ofinfrared (IR), visible, ultraviolet (UV), X rays, and gamma rays.

Many types of wireless devices makeuse of RF fields. Cordless and cellular telephone, radio andtelevision broadcast stations, satellite communications systems,and two-way radio services all operate in the RF spectrum. Somewireless devices operate at IR or visible-light frequencies,whose electromagnetic wavelengths are shorter than those of RFfields. Examples include most television-set remote-controlboxes, some cordless computer keyboards and mice, and a fewwireless hi-fi stereo headsets.

The RF spectrum is divided into several ranges, orbands. With theexception of the lowest-frequency segment, each band represents an increaseoffrequencycorresponding to an order of magnitude (power of 10). The tabledepictsthe eightbands in the RF spectrum, showing frequency and bandwidth ranges. TheSHF and EHFbands are often referred to as the microwave spectrum.

Designation
Abbreviation
Frequencies
Free-space Wavelengths

Very Low Frequency
VLF
9 kHz - 30 kHz
33 km - 10 km

Low Frequency
LF
30 kHz - 300 kHz
10 km - 1 km

Medium Frequency
MF
300 kHz - 3 MHz
1 km - 100 m

High Frequency
HF
3 MHz - 30 MHz
100 m - 10 m

Very High Frequency
VHF
30 MHz - 300 MHz
10 m - 1 m

Ultra High Frequency
UHF
300 MHz - 3 GHz
1 m - 100 mm

Super High Frequency
SHF
3 GHz - 30 GHz
100 mm - 10 mm

Extremely High Frequency
EHF
30 GHz - 300 GHz
10 mm - 1 mm

This entry was posted on Monday, January 4th, 2010 at 10:18 pm and is filed under WLAN Switches. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.