An anten­na (or aer­i­al) is an elec­tri­cal device which con­verts elec­tric pow­er into radio waves, and vice ver­sa. It is usu­al­ly used with a radio trans­mit­ter or radio receiv­er. In trans­mis­sion, a radio trans­mit­ter sup­plies an oscil­lat­ing radio fre­quen­cy elec­tric cur­rent to the antenna’s ter­mi­nals, and the anten­na radi­ates the ener­gy from the cur­rent as elec­tro­mag­net­ic waves (radio waves). In recep­tion, an anten­na inter­cepts some of the pow­er of an elec­tro­mag­net­ic wave in order to pro­duce a tiny volt­age at its ter­mi­nals, that is applied to a receiv­er to be ampli­fied.

Typ­i­cal­ly an anten­na con­sists of an arrange­ment of metal­lic con­duc­tors (“ele­ments”), elec­tri­cal­ly con­nect­ed (often through a trans­mis­sion line) to the receiv­er or trans­mit­ter. An oscil­lat­ing cur­rent of elec­trons forced through the anten­na by a trans­mit­ter will cre­ate an oscil­lat­ing mag­net­ic field around the anten­na ele­ments, while the charge of the elec­trons also cre­ates an oscil­lat­ing elec­tric field along the ele­ments. These time-vary­ing fields, when cre­at­ed in the prop­er pro­por­tions, radi­ate away from the anten­na into space as a mov­ing trans­verse elec­tro­mag­net­ic field wave. Con­verse­ly, dur­ing recep­tion, the oscil­lat­ing elec­tric and mag­net­ic fields of an incom­ing radio wave exert force on the elec­trons in the anten­na ele­ments, caus­ing them to move back and forth, cre­at­ing oscil­lat­ing cur­rents in the anten­na.

Anten­nas may also con­tain reflec­tive or direc­tive ele­ments or sur­faces not con­nect­ed to the trans­mit­ter or receiv­er, such as par­a­sitic ele­ments, par­a­bol­ic reflec­tors or horns, which serve to direct the radio waves into a beam or oth­er desired radi­a­tion pat­tern. Anten­nas can be designed to trans­mit or receive radio waves in all direc­tions equal­ly (omni­di­rec­tion­al anten­nas), or trans­mit them in a beam in a par­tic­u­lar direc­tion, and receive from that one direc­tion only (direc­tion­al or high gain anten­nas).

 


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