Authors: Teemu Ainasoja and Toni Heijari

I have recently tumbled into few RFQ's and RFP's where RFID readers radiating power figures are asked. And at the same time in some customer discussions it became obvious that it is not always clear how to compare figures of different formats.  We collected this paper together with one of Nordic ID's RF gurus - Mr. Toni Heijari. Our goal is to explain in an easy way what the different RFID power figures mean, and how they can be used.

RFID radiating power is an essential figure describing the performance, although it must be noted that it is not the only one. To put it simply - more power out from antenna mean that tags further away hear the reader - it is a good thing if long reading distance is the goal. But power output does not describe all aspects of the reader performance. It does not describe receiver sensitivity - from how far can a reader hear the tag, it does not describe readers speed or ability to read tags without orientation sensitivity etc.

There are two common ways to show radiating power: milliwatt (mW) and some form of decibels (dB).  Complexity arises when different types of decibel figures are compared - fine if it is done correctly, but mistakes are easily done. An example: Other things equal - which reader has longer reading distance, a reader with 30dBm (1.0W) ERP or a reader with 32dBm (1.6W) EIRP? We will give the answer later in this article.

Decibels (dB) describe relations of two figures in logarithmic scale.
0dB:                    x=1*y
3dB:                    x=2*y
6dB:                    x=4*y
10dB:                  x=10*y
20dB:                  x=100*y
and
-3dB:                  x=y/2
-10dB:                x=y/10

When RFID readers power is discussed dB is not just a plain dB, but dBm, dBi etc. the part after dB describes into which the figure is compared to.  For example an antenna with gain 3dBi emits to the main direction 2 times the power of an isotropic reference antenna. The "i" in dBi stands for the isotropic reference antenna.

RFID readers power output depends on 2 components. Power output going into the antenna and antenna gain. Power going into the antenna (RF power) is usually given as milliwatt (mW) or in dBm. In this case dBm describes the power compared to 1mW.  In table one below the dBm to mW relation is described.

Other component - antenna gain - is given as compared to some reference antenna.

dBi describes gain compared to isotropic reference antenna
dBd describes gain compared to reference dipole antenna
dBic describes gain compared to reference isotropic circular polarized antenna
We have seen also dBiL being used, it refers to reference linear polarized antenna, but the relation to dBi is not clear*), and we don't recommend use of dBiL.

There is a clear relation between the established reference antennas.
-    dipole antenna radiates more to directions in 90 degree angle to the antenna dipole than ideal isotrphic antenna, 0dBd = 2.14 dBi

(source: www.ti.com)

-    Isotropic circular polarized antenna "looses" some of the radiation power in the circulating polarization due to polarization mismatch when tag antenna is linear. As a result 0dBic = -3 dBi

Picture: Illustration of linear and circular polarization. (www.ti.com)

In European legislation radiating power limits are described in relation to dipole antenna and ERP (Efficient Radiating Power)  is used as a measure. An example: RF power is 27dBm (500mW) and antenna has gain of 5dBi. ERP = 27dBm + 5dBi -2.15 = ca. 30dBm (1.0W)

In US legislation EIRP (Equivalent Isotropical Radiated Power) is used. Power is compared to isotropic reference antenna. With same reader and antenna as above (RF power is 27dBm (500mW) and antenna has gain of 5dBi) EIRP = 27dBm + 5dBi = 32 dBm (1.6W)

Answer to the question posted above reader with radiating power of 1W ERP same as reader with radiating power of 1.6W EIRP.

Formulas:
ERP in dBm = (RF power in dBm) + (antenna gain in dBi) -2.15dB, this can be converted to mW using table above
EIRP in dBm = (RF power in dBm) + (antenna gain in dBi), this can be converted to mW using table above
dBi = dBic - 3dB (in RFID use)
dBi = dBd +2.15dB

More examples:
reader with 500mW RF power, antenna with 6dBic gain
-    500mW = 27dBm
-    ERP =27 dBm + (6dBic -3dB) -2.15dB = ca. 28dBm (630mW)
-    EIRP =27 dBm + (6dBic-3dB) = 30 dBm (1W)
reader with 400mW RF power, antenna with 5 dBi gain
-    400mW = ca 26dBm
-    ERP = 26 dBm + 4dBi -2.15dB = ca. 28 dBm (630mW)
-    EIRP =  26dBm + 4dBi =30 dBm (1W)

For more information, please contact:  support@nordicid.com

*) http://www.nitehawk.com/rasmit/ve3ont.html . In the reference calculation of dBiL is done by substracting 3dB from dBic, which in fact means that dBiL would be same as dBi. From antenna performance point of view linear antenna is usually a dipole antenna.