Thermal Noise Floor
kTB noise power for a bandwidth — the −174 dBm/Hz line.
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The engineering
Physics' floor under every receiver: −174 dBm per hertz of bandwidth at room temperature, plus your noise figure. A 1 MHz channel starts at −114 dBm before the first transistor gets a vote.
Halving bandwidth buys 3 dB of sensitivity — the oldest trade in radio.
Where this math comes from
John Johnson measured the noise in 1927–28 at Bell Labs; Harry Nyquist explained it thermodynamically in a paper published back-to-back with Johnson's. Resistors hiss because their electrons are warm — a result so clean it later provided a way to measure Boltzmann's constant.
The 290 K convention (courtesy of Friis's noise-figure work) is why the magic number is −174: room temperature, defined for bookkeeping.
- 1928John B. JohnsonMeasures thermal noise in resistors.
- 1928Harry NyquistDerives kTB from thermodynamics.
- 1944Harald FriisNoise-figure convention fixes T₀ = 290 K.
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