Decibel, dBm, dBi, dBd



LORA / LORAWAN TUTORIAL 5

Decibel, dBm, dBi, dBd

v1.0.0

INTRO



? In this tutorial I will explain what the decibel is and what the purposes are of the units dBm, dBi and dBd.

DECIBEL: dB



? A deciBel (dB) is a unit of measurement used to express the ratio of powers, sound pressures or others things on a logarithmic scale.The deciBel was invented by Bell Labs and named after Alexander Graham Bell. In this tutorial the deciBel is used to express the ratio of electrical powers (meaning Watts).

A = 10 x log10(Po / Pi) dB Where Pi is the input power and Po the output power.

2W

6W

Amplifier

A(gain)= 10 x log10(6/2) = 4.7 dB

10mW

8mW

Cable

A(loss)= 10 x log10(8/10) = -0.96 dB

? Convert deciBel to power ratio: Po/Pi = 10(A/10)

DECIBEL: dB



? The logarithmic scale is used to avoid very big or very small numbers, meaning the ratios. Using decibels is easier to work with.

? For example: A power ratio of 1,000,000 (1 million) can be expressed by 60 dB. A power ratio of 0,000001 can be expressed by -60 dB.

? A power ratio of 1,000,000,000,000,000 (1 quadrillion) can be expressed by 150 dB. A power ratio of 0,000000000000001 can be expressed by -150 dB.

? A power ratio of 1,000,000,000,000,000,000,000,000 (1 septillion) can be expressed by 240 dB. A power ratio of 0,000000000000000000000001 can be expressed by -240 dB.

DECIBEL: dB



? Remember:The unit dB is a ratio, it does not represent an absolute value.

Pi device Po The only thing you know about this

3dB

device is that it has a gain of 3dB.

? Question: If Pi = 1 mW. Do you know what Po is?

? Answer: Po/Pi = 10(A/10) Po/1 = 10(3/10) Po = 10(3/10) = 1.995 mW

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