Effective Isotropic Radiated Power
Understanding EIRP
EIRP combines all transmitter-side parameters into a single number that describes the effective radiated signal. It allows direct comparison between different antenna/transmitter combinations regardless of their individual specifications. Two systems with the same EIRP produce the same signal strength at a distant receiver.
EIRP Calculation
- EIRP (dBm) = P_TX (dBm) + G_ant (dBi) - L_cable (dB)
- Example: 30 dBm TX + 20 dBi antenna - 2 dB cable = 48 dBm EIRP (63 watts equivalent)
EIRP vs ERP
- EIRP: Referenced to isotropic antenna. Used for satellite and microwave.
- ERP: Referenced to half-wave dipole. ERP = EIRP - 2.15 dB. Used for broadcast and land mobile.
Regulatory Limits
- Wi-Fi (2.4 GHz EIRP limit): 36 dBm in US, 20 dBm in EU.
- 5G mmWave EIRP: up to 75 dBm from base station.
Frequently Asked Questions
What is EIRP?
EIRP is the total power a transmitter and antenna system would radiate if using an isotropic antenna. EIRP = TX power + antenna gain - cable loss. It is the key parameter for link budget calculations and regulatory compliance.
Why is EIRP important?
EIRP determines the signal strength at a distant receiver and is the single most important transmitter parameter for link range. Doubling EIRP (+3 dB) extends range by ~41% in free space. Regulators limit EIRP to prevent interference.
What is the difference between EIRP and ERP?
EIRP uses an isotropic antenna as reference. ERP uses a half-wave dipole as reference. ERP = EIRP - 2.15 dB. EIRP is standard for satellite and microwave systems. ERP is used for broadcasting and land mobile radio.