How do dBm and watts relate?

P(dBm) = 10 times log10(P in milliwatts). Conversely, P(mW) = 10 to the power of (P_dBm/10). Key reference points: 0 dBm = 1 mW, plus 10 dBm = 10 mW, plus 20 dBm = 100 mW, plus 30 dBm = 1 W, plus 40 dBm = 10 W, plus 50 dBm = 100 W. Negative dBm means sub-milliwatt: minus 30 dBm = 1 microwatt, minus 60 dBm = 1 nanowatt, minus 90 dBm = 1 picowatt. The advantage of dBm is that multiplication and division in linear power become addition and subtraction in dB. A 20 dB gain amplifier fed with minus 10 dBm produces plus 10 dBm output. This makes cascade calculations trivial: just add gains and subtract losses along the signal chain.

EIRP (Effective Isotropic Radiated Power) is the total power that would need to be radiated by an isotropic antenna to produce the same signal strength in the direction of maximum antenna gain as the actual transmitter and antenna combination. EIRP = P_transmitter plus G_antenna minus L_cable (all in dB), or equivalently EIRP = P_TX times G_ant / L_cable in linear. For a 1 W transmitter (plus 30 dBm) with a 15 dBi antenna and 2 dB cable loss: EIRP = 30 plus 15 minus 2 = 43 dBm (20 W). Regulatory limits are typically specified as maximum EIRP. FCC Part 15 for WiFi at 2.4 GHz: 36 dBm (4 W) EIRP maximum. 5G NR at mmWave (n260): 43 dBm EIRP for mobile devices using beamforming.

How do you calculate a power budget?

A power budget traces the signal power from transmitter to receiver through all gains and losses. Starting from the transmitter output power, add amplifier gains and subtract cable losses, filter insertion losses, switch losses, and connector losses to find the power at the antenna port. Add antenna gain and subtract free-space path loss (FSPL = 20 log(4 pi d f / c)) to find the power at the receive antenna. Then add receive antenna gain and subtract receive chain losses to find the power at the receiver input. Compare this received power to the receiver sensitivity (minimum detectable signal, typically defined at a specific BER or SNR): the difference is the link margin. A positive margin means the link works; a negative margin means it fails. Typical design targets 10 to 20 dB of margin to account for fading, multipath, and component aging.

Fundamental Quantity

RF Power

/pow-er/

Electromagnetic energy per unit time. P(dBm) = 10 log(P_mW). 0 dBm = 1 mW, +30 dBm = 1 W, +43 dBm = 20 W. EIRP = P_TX + G_ant − L_cable (dB). Power budget: add gains, subtract losses along signal chain. Cascade: P_out = P_in + G − L. Link margin = received power − sensitivity. FCC WiFi: 36 dBm EIRP. 5G mmWave UE: 43 dBm EIRP with beamforming.

0 dBm: 1 mW

+30 dBm: 1 W

+60 dBm: 1 kW

Understanding RF Power

Power is the most fundamental quantity in RF engineering. Every link budget starts with transmit power. Every receiver has a minimum required power (sensitivity). Every regulatory standard specifies maximum allowed power. Every PA is characterized by its output power capability. Mastering power calculations, conversions, and budgets is the first skill every RF engineer must develop.

The decibel system (dBm, dBW) transforms multiplicative relationships into additive ones, dramatically simplifying cascade calculations. Instead of multiplying a 0.001 W signal by a gain of 100, a loss of 0.5, another gain of 1000, and a loss of 0.1, you simply add: 0 dBm + 20 dB - 3 dB + 30 dB - 10 dB = +37 dBm. This is why RF engineers think in dB.

Power Equations

dBm conversions:
P(dBm) = 10 log₁₀(P_mW)
P(mW) = 10^P(dBm)/10
dBm = dBW + 30

EIRP:
EIRP = P_TX + G_ant − L_cable (dB)
1 W + 15 dBi − 2 dB = 43 dBm

Free-space path loss:
FSPL = 20 log(4πdf/c) dB
= 32.4 + 20log(f_MHz) + 20log(d_km)
2.4 GHz, 100 m: FSPL = 80 dB

Link margin:
M = P_RX − P_sensitivity (dB)
Target: 10-20 dB for fade margin

Common RF Power Levels

dBm	Watts	Context	Example	Regulation
−90	1 pW	Sensitivity	GPS receiver	N/A
0	1 mW	Reference	BLE transmit	FCC Part 15
+23	200 mW	Handset TX	LTE UE max	3GPP 36.101
+43	20 W	Small cell	5G NR micro	FCC Part 27
+60	1 kW	Macro BTS	LTE eNB PA	Site-specific

Common Questions

Frequently Asked Questions

dBm to watts?

P(dBm) = 10log(P_mW). 0 dBm = 1 mW. +10 = 10 mW. +20 = 100 mW. +30 = 1 W. +40 = 10 W. Negative: −30 = 1 μW, −60 = 1 nW, −90 = 1 pW. dBm = dBW + 30. Cascade: add gains/subtract losses in dB. Much simpler than linear multiplication.

What is EIRP?

EIRP = P_TX + G_ant − L_cable (dB). Power an isotropic antenna would need to match actual peak radiation. 1 W + 15 dBi − 2 dB = 43 dBm (20 W). Regulatory limits in EIRP. FCC WiFi 2.4 GHz: 36 dBm. 5G mmWave UE: 43 dBm with beamforming. ERP uses dipole reference (ERP = EIRP − 2.15 dB).

Power budget?

TX power → add PA gain → subtract cable/filter/switch losses → add TX antenna gain → subtract FSPL (32.4+20logf+20logd) → add RX antenna gain → subtract RX losses = received power. Compare to sensitivity (min power for target BER/SNR). Difference = link margin. Target 10-20 dB for fading/aging margin.

Related Terms

← Power Divider Propagation →

RF Systems

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