Also known as: dBm
dBm is power expressed in decibels relative to one milliwatt, making it an absolute measure of signal strength rather than a mere ratio.1 The formula is P(dBm) = 10·log₁₀(P/1 mW), so 0 dBm equals 1 mW, +30 dBm is 1 watt, and −30 dBm is one microwatt.
How it works
Because dBm carries a fixed reference (1 mW), a single dBm figure names a real power level, while a plain decibel figure only names a ratio. The two combine naturally: adding a gain in dB to a level in dBm gives a new level in dBm. A +20 dB amplifier turns a −80 dBm input into a −60 dBm output; the units bookkeep themselves.
Received radio signals are tiny fractions of a milliwatt, so they land as negative dBm values, and the one closer to zero is stronger. −70 dBm beats −90 dBm by 20 dB, which is 100× in power. This ordering trips up newcomers who read “−90” as bigger than “−70”; in dBm, less negative always means more power.
A few landmarks build intuition. A handheld transmitter is +30 to +37 dBm (1–5 W). A usable off-air signal at a scanner might be −60 to −90 dBm. The thermal noise floor in a narrow channel sits around −120 dBm, and a good receiver can pull signals out just a few dB above it. That span — roughly +37 dBm to −127 dBm — is more than 16 orders of magnitude in raw power, which is exactly why the logarithmic unit exists.
dBm is a power unit, so it uses the 10·log₁₀ form. It should not be confused with dBFS, which references digital full scale and lives entirely inside the converter, with no fixed relationship to dBm until the analog gain of the front end is known.
In practice
Most receivers report a proxy for dBm called RSSI (received signal strength indicator).2 True dBm requires a calibrated front end; many SDR dongles instead give an uncalibrated number that tracks relative changes but is offset from absolute power by an unknown amount. That is usually fine — what matters for decoding is the gap between signal and noise floor, the SNR, which is a difference of two dBm-like readings and cancels a constant offset.
dBm is the working currency of a link budget: start at the transmitter’s dBm, add and subtract gains and losses in dB, and compare the arriving dBm against the receiver sensitivity threshold, also quoted in dBm.
Relevance to SDR
Receiver meters, waterfall scales, and site-survey tools report signal and noise-floor levels in dBm (or an RSSI proxy). Their difference is the SNR that determines whether a channel decodes. GopherTrunk works primarily with SNR and demod-quality figures rather than absolute dBm, because most supported SDR hardware is not power-calibrated — but the mental model of “signal in dBm, noise in dBm, decode if the gap is wide enough” is exactly right.
Sources
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dBm — Wikipedia, power referenced to one milliwatt and the resulting negative values for received signals. ↩
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Received signal strength indication — Wikipedia, how receivers report signal strength and why RSSI is often uncalibrated relative to true dBm. ↩