dBm is generally used as a measurement of Power in milli Watts. 0 dBm = 1 milliWatt.Good day, everyone!
I am wondering why we cannot add dBm to dBm? But we can add dB to dB and dB to dBm? Thank you!
Technically, dBm is a measure of power ratio, but not of power level - its units are not Watts. The unit of dBm is a measure of relative ratio of power to 1 mW (the ratio itself is unitless - P/1 mW) - somewhat similarly to how radians can be described as a measure of a unitless ratio of lengths (pi/4 rad = arctan(1/1), for instance). Adding and subtracting dBm is equivalent to scaling the corresponding power. So adding 3 dBm would double (roughly) the power. So adding/subtracting dBm isn't so much "adding/subtracting power" as it is "scaling 1 mW up and down." It doesn't result in W^2 units, as the manipulation is done on dBm, which relies on unitless ratios relative to 1 mW.dBm has the units of Watts (I got this wrong on my first post.)
dB and dBm are on a logarithmic scale. When you add two logarithms, you are doing a multiplication.
So, if you add two dBm values the result would be in square Watts! Not a meaningful value.
dB is pure number. When you add two dB values you are multiplying the numbers, and the result is still a number expressed in dB.
When you add a dB to a dBm, you are multiplying Watts by a number, which is still Watts, so it is valid.
Bob
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by Jake Hertz
by Jake Hertz