EIRP and ERP

These two acronyms relate to a scale of measurement for RF transmission. They’re important in the field of UHF RFID as the two scales are used in different regions of the world to measure the power transmitted by the reader. Specifically, those systems that operate under the FCC regulations are limited to 4W EIRP (effective isotropic radiated power), and those within the ETSI region are limited to 2W ERP (effective radiated power) for ETSI Lower Band and 4W ERP for ETSI Upper Band. 

The table below shows a comparison of the two methods of measurement:

*NOTE: the max radiated power as described above is measured from the antenna.  It is up to the installer to ensure that the reader power is set to the correct level so that regulatory limits are not exceeded.

The maximum allowed reader transmit power for the FCC regulations is calculated by the following formula:

Maximum reader transmit power (in dBm) = 36 dBm (FCC limit) – Antenna Gain (in dBi) + Cable loss (in dB).

For the ETSI Lower band, the maximum allowable reader power is calculated by the following formula:

Maximum reader transmit power (in dBm) = 33 dBm (ETSI lower band limit) – Antenna Gain (in dBd) + Cable loss (in dB).

For the ETSI Upper band, substitute 36 dBm for the 33 dBm in the above equation to account for the higher limit.

Please consult the Installation and Operations Guide of the specific reader model you are installing for more information.

It is beyond the scope of this post to discuss the theory behind the differences; however, it is possible to convert EIRP based measurements to ERP; a simple equation governs this conversion:

•  dBm (ERP) = dBm - 2.15 (EIRP)

So, for example, the maximum radiated powers in the ETSI bands and FCC regions can be seen below:

• FCC:                    36 dBm EIRP - 2.15 = 33.75 dBm ERP
• ETSI lower band: 33 dBm ERP + 2.15 = 35.15 dBm EIRP
• ETSI upper band: 36 dBm ERP + 2.15 = 38.15 dBm EIRP

These conversions are also important when selecting antennas and determining legal transmit powers. In order to ensure the maximum transmit power is within regulations it’s important to use the right units for antenna gain.  For regulations that are stated in ERP, the antenna gain must be in dBd (gain compared to a reference dipole) units and if the regulations are stated in EIRP, the antenna gain must be in dBi (gain compared to an isotropic radiator) units. If antenna gain is given in dBi, convert to dBd for standards that use ERP for their radiated power limits (e.g. ETSI). The formula for conversion is dBi = dBd + 2.15.