Where did the decibel come from?

The decibel originated at Bell Telephone Laboratories in the 1920s. Engineers replaced the older 'mile of standard cable' with a logarithmic 'transmission unit' for measuring telephone signal loss, and in 1928 the unit was named the decibel — one tenth of a bel, which honors Alexander Graham Bell. W. H. Martin formally introduced the name in the Bell System Technical Journal in 1929.

Why is radio power measured on a logarithmic scale?

Because radio spans an enormous range of power. A transmitter may put out a tenth of a watt while the same signal arrives at a distant receiver as a fraction of a femtowatt — more than fourteen orders of magnitude weaker. Logarithms compress that range into readable numbers and turn the multiplication of a signal chain into simple addition and subtraction.

Why is the radio noise floor around minus 174 dBm?

Every conductor at a temperature above absolute zero generates thermal noise, a physical phenomenon described by Johnson and Nyquist in 1928. At room temperature (290 K), that noise sets a floor of approximately minus 174 dBm per hertz of bandwidth. It is a hard physical limit: no receiver can hear a signal buried beneath its own thermal noise, which is why deep negative dBm values are normal and expected.

Is dBm used outside radio?

Yes. The decibel is used across acoustics, audio engineering, optics, and telecommunications. dBm specifically — decibels referenced to one milliwatt — is standard wherever absolute electrical or optical power is expressed, from fiber-optic links to Wi-Fi to cellular to LoRa mesh radios.

Why Radio Speaks in Decibels

TLDR: Understanding dBm starts with understanding why radio is logarithmic at all. The decibel is a hundred-year-old idea from telephone engineering: it compresses the enormous range of radio power — from a transmitter’s tenth of a watt down to a received signal a hundred billion times weaker — into numbers you can add and subtract by hand. dBm anchors that scale to one milliwatt. And the reason received signals sit so deep in the negatives is physics: thermal noise sets a hard floor near −174 dBm per hertz that no receiver can beat.

Open any radio datasheet, signal reading, or link calculation and you are reading decibels. Transmit power in dBm. Antenna gain in dBi. Receiver sensitivity in dBm. Signal-to-noise ratio in dB. It is the native language of every wireless system ever built — and like most languages, it is easier to speak once you know where it came from and why it is shaped the way it is.

This is the general explainer: the decibel from first principles, with citations to the work that defined it. The practical, mesh-specific version — what these numbers mean for a network you operate — is in the companion piece, What dBm Actually Measures.

The problem the decibel solves

Radio covers a staggering range of power. A small LoRa transmitter emits around a tenth of a watt. That same transmission, after kilometers of air, terrain, and buildings, may reach a receiver as roughly a picowatt — and the receiver can still decode signals weaker than a femtowatt, a quadrillionth of a watt. The ratio between what leaves the antenna and what the receiver can still hear spans more than fourteen orders of magnitude.

You cannot do useful arithmetic on numbers like that. Writing 0.0000000000001 watts is error-prone, and a signal chain multiplies and divides those values at every stage — transmitter, cable, antenna, free space, antenna, cable, receiver. The decibel fixes both problems at once by working in logarithms. A logarithm turns a vast multiplicative range into a compact additive one: every factor of ten in power becomes a simple step of 10 on the scale, and the whole multiply-and-divide chain of a radio link becomes a sequence of additions and subtractions.

The same four radio power levels, twice. In linear watts the span is unmanageable; in dBm it is a tidy, evenly-readable line. That compression is the entire reason the decibel exists.

A hundred-year-old unit from the telephone

The decibel was not invented for radio. It came out of the telephone network.

In the early 1920s, Bell System engineers needed a way to express how much a telephone signal weakened over a circuit. Their older yardstick — the “mile of standard cable,” the loss in one mile of a particular telephone cable — was frequency-dependent and awkward. So in 1923–1924 they replaced it with a clean logarithmic ratio they called the transmission unit (W. H. Martin, The Transmission Unit and Telephone Transmission Reference Systems, Bell System Technical Journal, 1924). One transmission unit was defined as ten times the base-10 logarithm of a power ratio — the exact definition the decibel still carries today.

The unit was renamed a few years later. The bel — a full factor of ten in power — honors Alexander Graham Bell. The bel proved inconveniently large for everyday work, so engineers used one tenth of it: the decibel. W. H. Martin announced the name formally in Decibel — The Name for the Transmission Unit (Bell System Technical Journal, vol. 8, no. 1, 1929), the paper that put “dB” into the technical vocabulary.

There is a human reason the decibel is sized the way it is: roughly one decibel is about the smallest change in loudness a person can reliably detect under good conditions. The unit was scaled to human perception of a telephone call — and a century later it still measures the signal a mesh node hears from a hilltop repeater.

Relative and absolute: dB versus dBm

The decibel by itself is a ratio — it describes a change, not an amount. A cable with 3 dB of loss halves whatever power passes through it, no matter the starting level. That is dB: relative, unanchored.

To describe an absolute power you need a reference, and that is what the suffix does. dBm is decibels referenced to one milliwatt: 0 dBm is exactly 1 mW, +30 dBm is 1 watt, −90 dBm is a thousandth of a billionth of a watt. The “m” is the whole difference between a ratio and a real, measurable quantity.

The arithmetic that follows is the backbone of every link calculation: add a gain or loss (dB) to an absolute level (dBm) and you get a new absolute level (dBm). Transmit power minus cable loss plus antenna gain minus path loss equals the power at the far receiver — all addition and subtraction, exactly as the Bell engineers intended. The full set of rules, with the worked numbers, is in What dBm Actually Measures.

The floor every receiver fights

Here is the part that surprises people new to radio: received signal levels are almost always deeply negative in dBm, and there is a hard physical reason they cannot simply be made stronger at the receiver.

Every electrical conductor at any temperature above absolute zero generates a faint random voltage, because its charge carriers are in constant thermal motion. This is thermal noise, and it was pinned down in 1928 in a pair of back-to-back papers in the same volume of Physical Review: John B. Johnson measured it experimentally (Thermal Agitation of Electricity in Conductors, Phys. Rev. 32, 97, 1928), and Harry Nyquist derived it from thermodynamics (Thermal Agitation of Electric Charge in Conductors, Phys. Rev. 32, 110, 1928). Their result is elegant and unforgiving: the available noise power depends only on temperature and bandwidth — not on the circuit, not on the components — and equals kTB, where k is Boltzmann’s constant, T is absolute temperature, and B is bandwidth.

Put room temperature (290 K) into that formula and you get approximately −174 dBm per hertz of bandwidth. That is the noise floor: the quietest a receiver’s own physics will ever let it be. A signal weaker than the noise in its band cannot be recovered, no matter how good the radio. This is why a receiver’s job is not to be “loud” but to be quiet — to add as little noise as possible above that thermal floor — and why deep-space and radio-astronomy receivers are cryogenically cooled to push T, and the floor, lower.

Two more results complete the picture and explain why the numbers land where they do. Harald Friis’s transmission formula (A Note on a Simple Transmission Formula, Proc. IRE, 1946) describes how power falls with distance and frequency in free space — the reason a +20 dBm transmission arrives at −90 dBm or lower. And Claude Shannon’s founding paper of information theory (A Mathematical Theory of Communication, Bell System Technical Journal, 1948) ties it all together: the rate at which you can move information through a channel is set by its bandwidth and its signal-to-noise ratio. The gap between your signal in dBm and the noise floor in dBm — measured in plain dB — is, quite literally, the budget you have to work with.

From the physics to your network

This is why dBm is worth understanding from the ground up: it is not a radio quirk, it is the shared language of a century of communication engineering, anchored in human perception at one end and thermodynamics at the other. The same −174 dBm floor that limits a deep-space probe limits the LoRa node on your roof.

For people who run MeshCore mesh networks specifically, these abstractions become daily, practical numbers. Receiver sensitivity in dBm tells you how weak a neighbor’s signal can get before the link drops. The noise floor tells you what you are fighting in your local RF environment. And the gap between them — signal-to-noise ratio, in plain dB — is the single most useful health number a mesh operator can read. Waev surfaces that SNR on every packet in Live Packets and tracks it across your network over time, because the difference between a mesh that “mostly works” and one you can rely on is usually written in a few dB.

Learn dBm once, from the physics up, and every wireless system you ever touch — including your own mesh — speaks a language you already know.

Sources

W. H. Martin, “Decibel — The Name for the Transmission Unit,” Bell System Technical Journal, vol. 8, no. 1, pp. 1–2, 1929.
W. H. Martin, “The Transmission Unit and Telephone Transmission Reference Systems,” Bell System Technical Journal, vol. 3, no. 3, pp. 400–408, 1924.
J. B. Johnson, “Thermal Agitation of Electricity in Conductors,” Physical Review, vol. 32, no. 1, pp. 97–109, 1928.
H. Nyquist, “Thermal Agitation of Electric Charge in Conductors,” Physical Review, vol. 32, no. 1, pp. 110–113, 1928.
H. T. Friis, “A Note on a Simple Transmission Formula,” Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
C. E. Shannon, “A Mathematical Theory of Communication,” Bell System Technical Journal, vol. 27, pp. 379–423, 623–656, 1948.