Power Sum in dBm
Add two incoherent powers expressed in dBm — the right way.
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The engineering
You cannot add decibels with a plus sign — 10 dBm + 10 dBm is 13 dBm, not 20. Convert to milliwatts, add, convert back; this card is that round trip. The 'increase over larger' row carries the useful intuition: equal powers add 3 dB, a signal 10 dB down adds only 0.41 dB, and anything 20 dB down adds a cosmetic 0.04 dB.
This is the *incoherent* sum — correct for noise sources, separate transmitters, and uncorrelated interferers. Two coherent (same-frequency, phase-locked) signals add as voltages instead and can land anywhere from +6 dB to a perfect null depending on phase; if that's your situation, this card is the wrong tool on purpose.
Where this math comes from
The decibel came out of telephone accounting: Bell System engineers replaced the 'miles of standard cable' loss unit with the Transmission Unit in 1924, renamed the decibel in 1928 in honor of Alexander Graham Bell. It made multiplication of gains into addition — at the cost of making addition of powers into this card.
The logarithms underneath are John Napier's 1614 gift to everyone who computes for a living; RF engineering simply refused to give them up after everyone else moved on, because a 120 dB dynamic range has no comfortable linear notation.
- 1614John NapierLogarithms — arithmetic for multiplicative quantities.
- 1924Bell SystemTransmission Unit replaces miles-of-standard-cable.
- 1928Bell SystemThe TU renamed the decibel.
See the full timeline of the math behind every calculator →
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