![]() ![]() It’s a lot easier, and more useful in some calculations, to describe a weak signal as -100 dBm as opposed to 0.0000000001 mW. Likewise, a negative dBm means that you’re applying a negative exponent in your power calculations 0 dBm equals 1 mW of power, so -10 dBm equates to 0.1 mW, -20 dBm equates to 0.01 mW, and so forth. In logarithms, the value indicated represents an exponent… for example, under a log 10 scale, a value of -2 represents 10 to the -2 power, which equals 0.01. ![]() The reason you see negative values is that you’re representing small but positive numbers, on a logarithmic scale. It’s all due to the confusing magic of decibels. Even the volume numbers on your home theatre receiver are usually negatives. People may hear distinct sounds at varying frequencies ranging from 20 to 20,000 cycles per second. ![]() On the decibel scale, near absolute silence is designated as 0 dB while a sound 15 times louder is labeled as 15 dB 11. Satellite signals, over-the-air antennas, cellular signals always negative numbers. A decibel is defined by a ratio that represents a logarithmic scale in respect to a reference pressure level. So, we specify dBm, indicating that our scale is relative to 1 milliWatt of power. Spend enough time measuring signals and you’ll realize they’re almost always negative numbers. While sound intensity (in W/m 2) is the SI unit, the sound intensity level in decibels (dB) is more relevant for how humans perceive sounds. So, we specify dBm, indicating that our scale is relative to 1 milliWatt of power. You may have noticed that when people talk about the loudness of a sound, they describe it in units of decibels rather than watts per meter squared. The bel or decibel system can only describe relative power- a gain of 3 dB means your signal is 2 times as strong as it was before, but the dB scale doesn’t define where you’re starting from or what your ‘zero’ is. The bel or decibel system can only describe relative power- a gain of 3 dB means your signal is 2 times as strong as it was before, but the dB scale doesn’t define where you’re starting from or what your ‘zero’ is. On the surface that would appear confusing – how can decibels be negative?!įortunately an anonymous poster explains at Toms Hardware:Ībsolute power of a signal is measured in wattage. If you use “iwconfig” on Linux to get statistics on your wireless LAN you’ll note that the “Signal Level” is expressed in negative decibels. ![]()
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