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Osnovna jedinica je Bel i ona prestavlja dekadni logaritam odnosa dva nivoa snage signala. Ovaj jedinica se koristi jer je praktičnija za račun. Prvobitno je korišćena u proračunima audio signala, a poznato je da ljudsko uho reaguje logaritamski na promenu jačine signala.

Posto je bel velika jedinica (1 B predstavlja odnos 1:10) , u praksi se koristi decibel (dB) koji je 10 puta manji to jest:

decibel = 10 * log 10 (P1/P2)

Decibel je, dakle, jedinica koja prikazuje relativan odnos snage dva signala, u našem slučaju odnos snage ili jačine polja dva signala. Ona predstavlja neimenovan broj, slično kao i procenat.

U praksi se koristi i dBm, a to je jedinica koja pokazuje apsolutan odnos jednog signala prema referentnom signalu 0.001 W (1 mW). Dakle dB i dBm je ista jedinica, samo što je dBm izražen u odnosu na poznat nivo snage. Matematički receno:

dBm = 10 * log 10 (P1/0.001W)

Napomena: Ovaj tekst je bez dozvole preuzet iz članka UPUTSTVO: ŠTA JE DECIBEL. S obzirom da podržavam Vikipediju, dozvoljavam dalje korišćenje ovog teksta sve dok je u njemu naveden i izvor iz koga je tekst preuzet. Autor.

Više detalja na http://en.wikipedia.org/wiki/DBmhttp://wireless.uzice.net/uputstvo-decibel.htm

dBm levelPowerNotes
80 dBm 100 kW Typical transmission power of FM radio station with 50 km range
60 dBm 1 kW = 1000 W Typical combined radiated RF power of microwave ovenelements
50 dBm 100 W Typical thermal radiation emitted by a human body
40 dBm 10 W
36 dBm 4 W Typical maximum output power for a Citizens' band radiostation (27 MHz) in many countries
33 dBm 2 W Maximum output from a UMTS/3Gmobile phone (Power class 1 mobiles)

Maximum output from a GSM850/900 mobile phone

30 dBm 1 W = 1000 mW Typical RF leakage from a microwave oven - Maximum output power for DCS 1800 MHz mobile phone

Maximum output from a GSM1800/1900 mobile phone

27 dBm 500 mW Typical cellular phonetransmission power

Maximum output from a UMTS/3G mobile phone (Power class 2 mobiles)

26 dBm 400 mW
25 dBm 316 mW
24 dBm 250 mW Maximum output from a UMTS/3G mobile phone (Power class 3 mobiles)
23 dBm 200 mW
22 dBm 160 mW
21 dBm 125 mW Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles)
20 dBm 100 mW Bluetooth Class 1 radio, 100 m range (maximum output power from unlicensed FM transmitter).

Typical wireless router transmission power.

15 dBm 32 mW Typical WiFi transmission power in laptops.
10 dBm 10 mW
6 dBm 4.0 mW
5 dBm 3.2 mW
4 dBm 2.5 mW Bluetooth Class 2 radio, 10 m range
3 dBm 2.0 mW More precisely (to 8 decimal places) 1.9952623 mW
2 dBm 1.6 mW
1 dBm 1.3 mW
0 dBm 1.0 mW = 1000 µW Bluetooth standard (Class 3) radio, 1 m range
−1 dBm 794 µW
−3 dBm 501 µW
−5 dBm 316 µW
−10 dBm 100 µW Typical maximum received signal power (−10 to −30 dBm) ofwireless network
−20 dBm 10 µW
−30 dBm 1.0 µW = 1000 nW
−40 dBm 100 nW
−50 dBm 10 nW
−60 dBm 1.0 nW = 1000 pW The Earth receives one nanowatt per square metre from amagnitude +3.5 star[1]
−70 dBm 100 pW Typical range (−60 to −80 dBm) of wireless (802.11x) received signal power over a network
−80 dBm 10 pW
−100 dBm 0.1 pW
−111 dBm 0.008 pW = 8 fW Thermal noise floor for commercial GPS single channel signal bandwidth (2 MHz)
−127.5 dBm 0.178 fW = 178 aW Typical received signal power from a GPS satellite
−174 dBm 0.004 aW = 4 zW Thermal noise floor for 1 Hz bandwidth at room temperature (20 °C)
−192.5 dBm 0.056 zW = 56 yW Thermal noise floor for 1 Hz bandwidth in outer space (4kelvins)
−∞ dBm 0 W Zero power is not well-expressed in dBm (value is negative infinity)

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