EIRP

EIRP (Effective Isotropic Radiated Power)

EIRP (Effective Isotropic Radiated Power) is the total RF power that would need to be radiated by a theoretical isotropic antenna (radiating equally in all directions) to produce the same signal strength in the direction of maximum antenna gain. EIRP is the primary metric used by regulatory bodies to limit RFID transmitter output and is a key variable in link budget calculations.

Calculating EIRP

EIRP combines the reader's conducted transmit power with the antenna's directional gain:

EIRP (dBm) = Transmit Power (dBm) + Antenna Gain (dBi) - Cable Loss (dB)

For example, a reader transmitting 30 dBm (1 W) through a cable with 1 dB loss to an antenna with 9 dBi gain produces:

EIRP = 30 + 9 - 1 = 38 dBm (6.3 W)

This exceeds the US FCC Part 15 limit of 36 dBm EIRP for UHF RFID, so the reader power must be reduced to 28 dBm to comply.

Regional Regulatory Limits

Different regions impose different EIRP limits for UHF RFID:

The distinction between EIRP and ERP (Effective Radiated Power, referenced to a dipole) is important: EIRP = ERP + 2.15 dB. European regulations are typically specified in ERP, while US and Asian regulations use EIRP.

Impact on System Design

EIRP limits constrain the tradeoff between reader power and antenna gain. A high-gain antenna (12 dBi) enables a narrower, longer-range beam but requires reducing transmit power to stay within EIRP limits. Conversely, a lower-gain antenna (6 dBi) allows higher transmit power and wider coverage but shorter maximum read range.

For multi-antenna installations, each antenna port must independently comply with EIRP limits. Multiplexers that switch between antennas ensure only one antenna transmits at a time, simplifying compliance. Understanding EIRP is essential for global deployments where systems must operate within different regulatory frameworks.