How it works
Power in RF work is almost always expressed in dBm — decibels relative to one milliwatt. It’s a logarithmic scale, which makes arithmetic on large ranges manageable and link budget calculations additive.
P(mW) = 10 ^ (dBm / 10)
P(dBm) = 10 × log₁₀(P_mW)The relationship between dBm and dBW is fixed:
dBW = dBm − 30Because 1 W = 1000 mW and 10 × log₁₀(1000) = 30.
Why logarithmic?
A LoRa link budget spans roughly 150 dB from transmitter to sensitivity floor. In linear terms that’s 10^15 — a factor of a quadrillion. Working in dBm, path loss is just subtracted, antenna gain is added, and the end result is directly comparable to the receiver’s sensitivity figure. Try that with milliwatts.
Link budget arithmetic
Received power (dBm) = TX power (dBm)
+ TX antenna gain (dBi)
− cable loss (dB)
− free-space path loss (dB)
+ RX antenna gain (dBi)
− fade margin (dB)Everything stays in dB until the final comparison against sensitivity.
Common TX power settings
| Standard | Typical | Max | Regulatory limit |
|---|---|---|---|
| BLE 5.x | 0 dBm | +8 dBm (nRF52840) | +20 dBm (region dependent) |
| LoRa 868 MHz | +14 dBm | +20 dBm | +27 dBm EIRP (EU) |
| WiFi 2.4 GHz | +20 dBm | +30 dBm | +36 dBm EIRP (FCC) |
| LTE UE | +23 dBm | +33 dBm (power class 3) | — |
Note the difference between conducted power (at the antenna port) and EIRP (effective isotropic radiated power, which folds in antenna gain). Regulatory limits are typically EIRP.
RSSI and noise floor
Received Signal Strength Indicator (RSSI) is reported in dBm — a negative number. A WiFi RSSI of −70 dBm means the received signal is 70 dB below 1 mW, which is 100 picowatts. The thermal noise floor at room temperature is −174 dBm/Hz. At a 200 kHz BLE channel bandwidth that’s −174 + 10×log₁₀(200,000) = −121 dBm. The receiver’s noise figure (typically 5–10 dB for BLE) adds on top.
Common mistakes
Confusing dBm with dB. dBm is an absolute power level. dB is a dimensionless ratio. A 3 dB antenna gain means the signal is doubled; a received power of 3 dBm means 2 mW. They are not interchangeable.
Forgetting that dBm is referenced to milliwatts, not watts. 0 dBm = 1 mW = 0.001 W = −30 dBW. 30 dBm = 1 W = 0 dBW. The 30 offset trips people when comparing datasheets that use different references.
Assuming TX power equals EIRP. An nRF52840 at +8 dBm conducted into a dipole antenna with +2 dBi gain radiates +10 dBm EIRP. Regulatory compliance is checked against EIRP, not conducted power.
Ignoring cable and connector loss. A 1 m coax run at 2.4 GHz can easily drop 1–2 dB. At the milliwatt level that looks negligible, but in a link budget where you have 3 dB of margin, it’s the difference between reliable and flaky.