Most countries regulate the amount of power output allowed for radio frequency identification reader antennas, in order to avoid having an RFID system interfere with other systems operating at nearby frequencies. Output is usually given in watts of effective radiated power (ERP), or effective isotropic radiated power (EIRP).
GS1 maintains a current list of nations that have regulations in place regarding RFID power output. You can download this list in PDF format: http://www.gs1.org/docs/epcglobal/UHF_Regulations.pdf.
It isn’t easy to explain in plain English how ERP and EIRP are calculated, but I will try to do so. Effective radiated power is the power emitted by an antenna (usually via a setting that can be changed in the reader-configuration software), multiplied by the gain of that antenna, compared to some standard antenna. The antenna gain relates to the intensity of an antenna’s output in a given direction, compared to the intensity that would be produced by a hypothetical ideal antenna radiating equally in all directions. A higher gain means the antenna output is more focused.
I reached out to ODIN Technologies, an RFID systems integrator, for the best way to calculate ERP and EIRP, so that you can comply with your government’s regulations. Chetan Karani, one of the company’s technical experts, pointed me to an online calculator that enables a user to input the amount of transmitter power output in watts, as well as the antenna gain (this should be available from the reader manufacturer), thereby enabling the user to calculate ERP or EIRP in watts.
So, for instance, if you purchased a passive ultrahigh-frequency (UHF) reader with a maximum of 4 watts of transmitted power output, and if it came with an antenna with a gain of 2 dBi, you could calculate that the ERP would be 3.8 watts. If you reduced the power output to 2 watts, the ERP would be less than 2 watts.
However, Chetan says, the calculator values should only be used as a reference. “In the practical world,” he states, “there are other factors, such as cable loss, antenna loss and so on, that need to be considered and compared with theoretical values.”
—Mark Roberti, Founder and Editor, RFID Journal
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