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Decibels and power — edited Aug 31, 2026 by Unknown editor
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Almost every number in radio is a ratio, and ratios in radio are written in decibels. Learning to
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think in dB saves money, because it tells you which upgrades are worth buying.
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## The three numbers to memorize
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| Change | In dB |
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|---|---|
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| Twice the power | 3 dB |
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| Four times the power | 6 dB |
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| Ten times the power | 10 dB |
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Everything else is built from these. Twenty times is 10 dB + 3 dB = 13 dB. One hundred times is
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20 dB. A tenth of the power is −10 dB.
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The formula behind the table is `dB = 10 × log10(P2 / P1)` for power, and `dB = 20 × log10(V2 / V1)`
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for voltage — the factor of 20 exists because power goes as the square of voltage.
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## dB versus dBm, dBd and dBi
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A plain "dB" is a comparison with no reference and means nothing on its own. The suffixed forms do:
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- **dBm** — power referred to one milliwatt. 0 dBm = 1 mW, 30 dBm = 1 W, 60 dBm = 1 kW. Receiver
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sensitivity is quoted here (a strong signal might be −73 dBm, which is S9).
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- **dBd** — antenna gain compared with a dipole. **dBi** compares with an isotropic radiator and
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reads about 2.15 dB higher for the same antenna. Manufacturers prefer dBi; when comparing two
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antennas, make sure both are in the same units.
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- **dBW**, **dBc**, **dBV** — referred to a watt, a carrier and a volt.
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## What that means for your station
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An S-unit on most receivers is nominally 6 dB, so a full S-unit needs **four times** the power.
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Going from 25 W to 100 W moves the other operator's meter by one unit; going from 100 W to 1500 W
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moves it by about two and a half. Antenna height and pattern usually beat both.
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Losses subtract in the same currency:
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- 100 feet of RG-58 at 28 MHz costs roughly 2.5 dB — nearly half your power, before the antenna sees
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it. The same run of LMR-400 costs well under 1 dB.
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- A 1.5:1 SWR costs about 0.2 dB of mismatch loss. This is why chasing a perfect match matters far
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less than most people believe; see [feedline, SWR and tuners](/wiki/station/feedline-and-swr).
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- Each connector, switch and choke adds a small amount. On HF it is negligible; at 440 MHz and above
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it is not.
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## ERP and EIRP
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Repeater coordination and some rules are written in terms of **effective radiated power** — power at
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the antenna plus gain minus loss, referred to a dipole (ERP) or an isotropic radiator (EIRP). A 50 W
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transmitter, 2 dB of line loss and a 6 dBd antenna gives 48 dBm + 6 − 2 = 52 dBm, about 160 W ERP.
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Part 97 also has a handful of power limits expressed as PEP output at the transmitter and, on 630 m
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and 2200 m, as EIRP — the only bands where you must do the ERP arithmetic to stay legal.
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## Power ratios worth remembering
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- QRP is 5 W. Compared with 100 W that is −13 dB: two S-units. People work the world with it; see
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[QRP operating](/wiki/operating/qrp).
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- FT8 and other weak-signal modes decode about 20–25 dB below what an ear can hear, which is worth
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more than any amplifier you can buy.
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- Doubling antenna height on HF often gains more than doubling power, because it changes the takeoff
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angle rather than the amplitude.