UHF

Ultra-high fre­quen­cy (UHF) des­ig­nates the ITU radio fre­quen­cy range of elec­tro­mag­net­ic waves between 300 MHz and 3 GHz (3,000 MHz), also known as the decime­tre band or decime­tre wave as the wave­lengths range from one to ten decime­tres; that is 10 cen­time­tres to 1 metre. Radio waves with fre­quen­cies above the UHF band fall into the SHF (super-high fre­quen­cy) or microwave fre­quen­cy range. Low­er fre­quen­cy sig­nals fall into the VHF (very high fre­quen­cy) or low­er bands. UHF radio waves prop­a­gate main­ly by line of sight; they are blocked by hills and large build­ings although the trans­mis­sion through build­ing walls is high enough for indoor recep­tion. They are used for tele­vi­sion broad­cast­ing, cord­less phones, walkie-talkies, satel­lite com­mu­ni­ca­tion, and numer­ous oth­er appli­ca­tions.

The point to point trans­mis­sion and recep­tion of TV and radio sig­nals is affect­ed by many vari­ables. Atmos­pher­ic mois­ture; solar wind; phys­i­cal obstruc­tions, such as moun­tains and build­ings; and time of day all affect the sig­nal trans­mis­sion and the degra­da­tion of sig­nal recep­tion. All radio waves are part­ly absorbed by atmos­pher­ic mois­ture. Atmos­pher­ic absorp­tion reduces, or atten­u­ates, the strength of radio sig­nals over long dis­tances. The effects of atten­u­a­tion degra­da­tion increas­es with fre­quen­cy. UHF TV sig­nals are gen­er­al­ly more degrad­ed by mois­ture than low­er bands, such as VHF TV sig­nals. The ionos­phere, a lay­er of the Earth­’s atmos­phere, is filled with charged par­ti­cles that can reflect some radio waves. Ama­teur radio enthu­si­asts pri­mar­i­ly use this qual­i­ty of the ionos­phere to help prop­a­gate low­er fre­quen­cy HF sig­nals around the world: the waves are trapped, bounc­ing around in the upper lay­ers of the ionos­phere until they are refract­ed down at anoth­er point on the Earth. This is called sky­wave trans­mis­sion. UHF TV sig­nals are not car­ried along the ionos­phere but can be reflect­ed off of the charged par­ti­cles down at anoth­er point on Earth in order to reach far­ther than the typ­i­cal line-of-sight trans­mis­sion dis­tances; this is the skip dis­tance. UHF trans­mis­sion and recep­tion are enhanced or degrad­ed by tro­pos­pher­ic duct­ing as the atmos­phere warms and cools through­out the day.

The main advan­tage of UHF trans­mis­sion is the phys­i­cal­ly short wave that is pro­duced by the high fre­quen­cy. The size of trans­mis­sion and recep­tion anten­nas is relat­ed to the size of the radio wave. The UHF anten­na is stub­by and short. Small­er and less con­spic­u­ous anten­nas can be used with high­er fre­quen­cy bands.

The major dis­ad­van­tage of UHF is its lim­it­ed broad­cast range and recep­tion, often called line-of-sight between the TV sta­tion’s trans­mis­sion anten­na and cus­tomer’s recep­tion anten­na, as opposed to VHF’s very long broad­cast range and recep­tion, which is less restrict­ed by line of sight.

UHF is wide­ly used in two-way radio sys­tems and cord­less tele­phones, whose trans­mis­sion and recep­tion anten­nas are close­ly spaced. UHF sig­nals trav­el over line-of-sight dis­tances. Trans­mis­sions gen­er­at­ed by two-way radios and cord­less tele­phones do not trav­el far enough to inter­fere with local trans­mis­sions. Sev­er­al pub­lic-safe­ty and busi­ness com­mu­ni­ca­tions are han­dled on UHF. Civil­ian appli­ca­tions, such as GMRS, PMR446, UHF CB, 802.11b (“WiFi”) and the wide­ly adapt­ed GSM and UMTS cel­lu­lar net­works, also use UHF cel­lu­lar fre­quen­cies. A repeater prop­a­gates UHF sig­nals when a dis­tance greater than the line of sight is required


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