Power Reference Units
dBm = power in dB relative to 1 milliwatt P(dBm) = 10·log₁₀(P_mW) 0 dBm = 1 mW, +30 dBm = 1 W, +43 dBm = 20 W
EIRP (Effective Isotropic Radiated Power)
EIRP (dBm) = P_tx (dBm) + G_antenna (dBi) − cable_loss (dB) Example: LTE eNB PA output: +43 dBm (20W) Cable + jumper loss: −0.5 dB Antenna gain: +17 dBi (3-sector panel) EIRP = 43 − 0.5 + 17 = +59.5 dBm (89.1 kW EIRP)
Free-Space Path Loss
FSPL (dB) = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c) FSPL (dB) ≈ 20·log₁₀(f_MHz) + 20·log₁₀(d_km) + 40.0 At 1.8 GHz, 1 km: FSPL = 65.1 + 0 + 40 = 105.1 dB At 3.5 GHz, 1 km: FSPL = 70.9 + 0 + 40 = 110.9 dB (+5.8 dB at 5G NR vs LTE)
Simple Link Budget
Received power = EIRP_tx − FSPL + G_rx_antenna − losses Example: LTE downlink, 1.8 GHz, 1 km EIRP_tx = +59.5 dBm FSPL = −105.1 dB Rx antenna (phone): +0 dBi Additional losses (body, buildings): −10 dB P_rx = 59.5 − 105.1 + 0 − 10 = −55.6 dBm Rx sensitivity = −100 dBm → margin = 44.4 dB ✓
RF View dBm Calculator: Convert between dBm, watts, mW, and dBuV in RF View's Utilities tab. Offline on Android. Free.