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The Decibels (dB) Scale

dB - decibel

Cable (signal) loss is measured in power decibels (dB), and is the power ratio (watts out / watts in) on a base 10 logarithmic (dB) scale.

dB = 10 log10(ratio)
ratio = 10dB/10

Convert dB's and ratio

ratio = Pout / Pin
Pout = signal power in watts out of cable.
Pin = signal power in watts into cable.

dBm - Signal Power

Signal power is commonly measured in watts or dBm - decibels above or below one milliwatt (1 mW). A negative dBm is dB's below a milliwatt, a positive dBm is dB's above a milliwatt, 0 dBm = 1 mW. The dBm unit is often used for dealing with small signal levels, such as in TV receivers.

dBm = 10 log10 (mW)
mW = 10(dBm/10)

dBm

dBm
to watts

watts
to dBm


dBm - decibels above or below 1 mW
µW - microwatts = 0.000 001 W
mW - milliwatts = 0.001 W
W - watts = 1 W
kW - kilowatts = 1000 W

Television broadcast signals are typically between 10 kW (70 dBm) to 1000 kW (90 dBm). Receive signals are between -5 dBm (316 µW) to -65 dBm (0.0003 µW).

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The Decibels (dB) Scale
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