Microwaves are radio waves with wave­lengths rang­ing from as long as one meter to as short as one mil­lime­tre, or equiv­a­lent­ly, with fre­quen­cies between 300 MHz (0.3 GHz) and 300 GHz. This broad def­i­n­i­tion includes both UHF and EHF (mil­lime­ter waves), and var­i­ous sources use dif­fer­ent bound­aries.  In all cas­es, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at min­i­mum, with RF engi­neer­ing often putting the low­er bound­ary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).

Most satel­lite com­mu­ni­ca­tions sys­tems oper­ate in the C, X, Ka, or Ku bands of the microwave spec­trum. These fre­quen­cies allow large band­width while avoid­ing the crowd­ed UHF fre­quen­cies and stay­ing below the atmos­pher­ic absorp­tion of EHF fre­quen­cies. Satel­lite TV either oper­ates in the C band for the tra­di­tion­al large dish fixed satel­lite ser­vice or Ku band for direct-broad­cast satel­lite. Mil­i­tary com­mu­ni­ca­tions run pri­mar­i­ly over X or Ku-band links, with Ka band being used for Mil­star.

Microwaves are wide­ly used for point-to-point com­mu­ni­ca­tions because their small wave­length allows con­ve­nient­ly-sized anten­nas to direct them in nar­row beams, which can be point­ed direct­ly at the receiv­ing anten­na. This allows near­by microwave equip­ment to use the same fre­quen­cies with­out inter­fer­ing with each oth­er, as low­er fre­quen­cy radio waves do. Anoth­er advan­tage is that the high fre­quen­cy of microwaves gives the microwave band a very large infor­ma­tion-car­ry­ing capac­i­ty; the microwave band has a band­width 30 times that of all the rest of the radio spec­trum below it. A dis­ad­van­tage is that microwaves are lim­it­ed to line of sight prop­a­ga­tion; they can­not pass around hills or moun­tains as low­er fre­quen­cy radio waves can.

 

Microwave radio trans­mis­sion is com­mon­ly used in point-to-point com­mu­ni­ca­tion sys­tems on the sur­face of the Earth, in satel­lite com­mu­ni­ca­tions, and in deep space radio com­mu­ni­ca­tions. Oth­er parts of the microwave radio band are used for radars, radio nav­i­ga­tion sys­tems, sen­sor sys­tems, and radio astron­o­my.

Microwave fre­quen­cy bands
Let­ter Des­ig­na­tion Fre­quen­cy range
L band 1 to 2 GHz
S band 2 to 4 GHz
C band 4 to 8 GHz
X band 8 to 12 GHz
Ku band 12 to 18 GHz
K band 18 to 26.5 GHz
Ka band 26.5 to 40 GHz
Q band 33 to 50 GHz
U band 40 to 60 GHz
V band 50 to 75 GHz
E band 60 to 90 GHz
W band 75 to 110 GHz
F band 90 to 140 GHz
D band 110 to 170 GHz

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