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Art of comparing RFID reader power outputs

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 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

Polarization

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.

14 comments on “Art of comparing RFID reader power outputs”

  1. Posted 20 September 2011 at 16:40:57

    Plse send me white paper about current in antenna

  2. Gravatar of Steve WrightSteve Wright
    Posted 21 September 2011 at 08:57:30

    This article is a bit misleading as the power measurements described are only applicable in the far field (where the antenna beam has formed). In the near field (where a lot of RFID takes place) It is not certain the the power flux density (power on mW or Dbm) is an accurate represenation of actual power level. The correct way to measure in the E and H fields and calculate the power.

    I learned this when measuring RF power levels at aviation navigation aids from LF ( 100 KHz NDB) to S band (3 GHz ATC Radar)

  3. Posted 22 September 2011 at 11:15:30

    Good point Steve, the article is about far field and we had only the "EPC C1G2 supply chain" and passive UHF RFID in mind. With other branches of RFID such as LF, HF and near field UHF a different approach is needed.

  4. Gravatar of ASHOK KUMARASHOK KUMAR
    Posted 19 August 2013 at 13:15:36

    Having understood circular polarization,What is Right Circular polarization and left Circular polarization?

  5. Posted 21 August 2013 at 12:37:39

    Hello,

    R&D provided the following information:
    The right and left side just indicate in which direction the wave is circulating. The geometry of the antenna influences the direction of the circular wave which is generated. Generally in UHF RFID the direction does not matter because tag antennas are linearly polarized. If both transmitting and receiving antennas are circularly polarized then direction of the polarizations must be matched.

  6. Gravatar of jabbar moloodijabbar moloodi
    Posted 28 November 2013 at 18:01:44

    hi. i want know about rfid reader output.
    its analog or digital and how its coded?
    thanks.

  7. Gravatar of Kirsikka DrägerKirsikka Dräger
    Posted 02 December 2013 at 13:20:17

    Hello Jabbar,

    can you specify or elaborate a little bit on your question? I will then be able to check back with R&D.

    BR, Kirsikka

  8. Gravatar of Grishma KhadkaGrishma Khadka
    Posted 19 March 2014 at 12:52:03

    Very nice article, Mirva Saarijärvi can u pls tell me how the interference suppression equation if we can to detect the particular tag from some distance considering other tags as a interference. I saw lot of equation but i was confused with all that equation.
    thanks,

  9. Gravatar of Suraj T SSuraj T S
    Posted 22 April 2014 at 09:23:08

    Nice Article, still in confuse with Linear/Circular ?!

  10. Gravatar of muhammed aymanmuhammed ayman
    Posted 12 July 2014 at 11:49:38

    please. i want to make a RFID Project but with a special specification.
    power:4w, 36dbm
    freq.:850-920 HZ
    i want to calculate the power loses,and gain to not interfere with the gsm band.

  11. Gravatar of Kirsikka DrägerKirsikka Dräger
    Posted 30 July 2014 at 09:59:38

    Hello Muhammed,

    unfortunately with the given specs we cannot help you further but we wish you all the best with your project.

    BR, Kirsikka

  12. Gravatar of Octavio Marques FreitasOctavio Marques Freitas
    Posted 22 December 2014 at 07:23:44

    currently I have been studying in a private university in Indonesia, Telecommunication subject,have semester 2 now. For example, (1). I have a Land Mobile (Base Station) with EIRP 83 Watt (50 W into a 2.15dBi dipole antenna. (2). 41 watt mobile stations (25 Watt into a 2.15dBi 1/4 wavelength monopole antenna. Could you please explain Basic for me, because I do not knew about EIRP.

  13. Posted 23 October 2015 at 22:11:49

    can you recommend me the best UHF Tags reader is suitable for the laundry (bulk reading in a mesh metal trolley. The size 90 cm sqr and the hight is 1.8 meter
    an average of 1000 to 1200 pcs in a Trolley.
    The gate size and the crossing length to get accurate reading
    Waiting for your valuable recommendation
    Best Regards
    Ali Abdul Azeez
    +971505428710
    Dubai - U A E

  14. Posted 22 June 2017 at 16:55:00

    thank you

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