A white paper describing how to compare RFID reader power outputs.
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
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
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
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
dBi describes gain compared to isotropic reference antenna
dBd describes gain compared to reference dipole antenna
dBic describes gain compared to reference isotropic circular
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
- dipole antenna radiates more to directions in
90 degree angle to the antenna dipole than ideal isotrphic antenna,
0dBd = 2.14 dBi
- 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.
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.
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
reader with 500mW RF power, antenna with 6dBic gain
- 500mW = 27dBm
- ERP =27 dBm + (6dBic -3dB) -2.15dB = ca. 28dBm
- 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
- EIRP = 26dBm + 4dBi =30 dBm (1W)
For more information, please
*) 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.