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 log_{10}(P_{out} / P_{in}) 
P_{out} / P_{in} 
= 
10^{dB/10} 
P_{out} 
= 
output signal power in watts. 
P_{in} 
= 
input signal power in watts. 
P_{out} 
= 
P_{in} x 10^{dB/10} 
P_{in} 
= 
P_{out} / 10^{dB/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 log_{10} (mW)
mW = 10^{(dBm/10)}

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).
OvertheAir Digital TV (OTA DTv)
The Decibels (dB) Scale
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