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 (Lower Band) and 4W ERP (Upper Band).
So, what does this mean? The table below shows a comparison of the two methods of measurement:
|What does it stand for?||Region Used||Max Radiated Power||Antenna Gain Measurement|
|ERP||Effective Radiated Power||ETSI (Lower Band)||33 dBm or 2W||dBd|
|ERP||Effective Radiated Power||ETSI (Upper Band)||36 dBm or 4W||dBd|
|EIRP||Effective Isotropic Radiated Power||FCC||36 dBm or 4W||dBi|
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 lower band 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. The gain of an antenna is often measured in dBi regardless of where it is intended to be used. In order to ensure the maximum transmit power is within regulations it’s important to ensure the system is set up using like for like measurements (i.e. dBd = dBi - 2.15).