Tower Locator Blog


The Decibels (dB) Scale

dB - decibel

Signal loss or gain is commonly measured in power decibels (dB), and is the power ratio (watts out / watts in) on a base 10 logarithmic (dB) scale. Minus three decibels (-3 dB) has a power ratio of about half (0.5012), plus three decibels (3 dB) is almost double (1.995) power. Signal loss is always a negative (-) dB, signal gain is a positive (+) dB, 0 dB is no loss or gain.

dB = 10 log10(Pout / Pin)
Pout / Pin = 10dB/10

Convert dB's and ratio

Pout = output signal power in watts.
Pin = input signal power in watts.
Pout = Pin x 10dB/10
Pin = Pout / 10dB/10
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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 more convenient than watts when dealing with radio, TV, or radar wave signal levels.

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


to 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). Over-the-Air Digital TV (OTA DTv)
The Decibels (dB) Scale