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The total energy balance of the route between reader and tag is given by summing up all of the gains and losses on path from the broadcasting part of reader, through the medium (air) to the tag, and back to the receiver of the reader. You can write in simplest form:

received power = transmitted power + gains - losses

To determine all gains and losses we must take into account the transmit antenna (even receiving antenna if we have different antenna for transmitting and receiving), tag antenna and everything in between. Signal attenuation along the route is determined by the individual losses. These losses include polarizing loss, loss of open space, the loss of the cable from the reader to antenna (see. Fig. 1) and mismatch losses.

To have the greatest received power the only thing we can influence is the transmitted power. Setting of transmitted power is limited by region (see Table 1).

 

                                                                   Region 1

Europe

South Africa

869,4 – 869,65 MHz / 0,5W

869,4 – 869,65 MHz / 0,5W

                865,6 – 867,6 MHz / 2 W

                915,2 – 915,4 MHz / 8 W

                                                                   Region 2

Canada and Mexico

Middle and South America

902 – 928 MHz / 4 W

902 – 928 MHz / 4 W

                                                                   Region 3

Australia

New Zealand

918 – 926 MHz / 1 W

864 – 868 MHz / 4 W

Japan

South Korea

950 – 956 MHz / 4 W

910 – 914 MHz / 4 W

 

Table 1Standardization of RFID - The frequency and power

 

Figure 1Power Balance

 

I.       ENERGY BALANCE

A.     Forward channel (Tag recharge)

It describes the flow of energy from the transmission part of reader to the RF tag. Performance Ptag is the amount of RF power of the integrated circuit of bound tag [W].

where Greader the transmitting antenna gain, Gtag the tag antenna gain, λ is the wavelength [m], the total losses (polarization loss, cable loss, loss of mismatch), r is distance between reader - tag [m]

 

B.     Return channel (Route backscatter)

 

After charging the tag there must be a sufficient amount of power backscattered from the tag to the reader in order to transfer information. In our case we consider monostatic antenna configuration [1], that only one antenna is serving as emitter and receiver.

Received power in retrospect scattering (backscattering) is inversely proportional to the fourth power of the distance between the reader and tag. By editing Friisov equation the anticipated received power of reader is given by equation (2). :

where:

PRX,reader is the received power of RFID reader,

PTX,reader is the transmission power of the RFID reader,

Greader is the reader antenna gain,

Gtag is the antenna tag gain,

R are the losses of backscattering.

 

II.      LOSSES ON PATH

 

A.   Polarization losses

 

Polarization losses, Lpol, are given by the angle by which reader antenna and tag are mutually rotated. The linear scale may range from 0 to 1. It is often difficult to determine the exact angle, because the reader and tag can have an unpredictable position with respect to each other. In such a case, we choose the mean value of the polarization mismatch X = 0,5 (linear scale) corresponding Lpol = 3 dB (according to formula (3)).

   

     Lpol = 10log(X) = 10log(cos(α)2)                                                                                             (3)

 

B.   Free space loss (FSL)

 

Electric field intensity decreases inversely with distance during free-space spreading. Received power decreases with the square of the distance.

If the frequency is given in MHz and the distance d in kilometers, we can modify the expression (4) in decibels:

   FSL (d)dB = 32,4 + 20log(r) + 20log(f)                                                                                           (5)

 

C.   Cable losses

This parameter contains all of the losses between the RF reader and antenna connectors. Such losses are signal attenuation through cable and any losses in the connectors on the path.
The java applet consider a meter long cable so the losses are related to one meter. In practice such losses range from 0.3 to 0.8 dB/m.

 

D.   Backscatter losses (backscattering)

 

III.    GAINS ON THE PATH

A.   READER ANTENNA GAIN

The antenna is a passive element and its profit is determined by the transformation of energy radiated in space. Antenna gain is logarithmic expression of directivity factor D . Gain indicates how many times more power is provided by receiving antenna either to the half-wave dipole or to the isotropic radiator.

It is true that half-wave dipole has a gain of 2.16 dB to the isotropic radiator:

     dBi = 2,15 + dBd                                                                                                                             (7)

where:

      dBi expresses gain in comparison to an isotropic antenna,
      dBd expresses gain in comparison to a half-wave dipole.

UHF antenna gain is around 6dBi in practice. The greater the gain the larger antenna.

B.   Tag antenna gain

Tag antenna is predominantly a small dipole with 2.15 dBi gain and there is not much room for gain improvement to all directions. We must also count antenna mismatch.

 

List of references

            [1]     P. V. Nikitin, K. V. S. Rao, Antennas and Propagation in UHF RFID System, in 2008 IEEE International Conference on RFID, Las Vegas, Neveda, USA, pp. 277-288, April 16-17, 2008.

            [2]     SU, Zhuo, Shing-Chi CHEUNG a Koon-Ting CHU. Investigationofradio link budget for UHF RFID systems. 2010 IEEE International Conference on RFID-Technology and Applications.
         IEEE, 2010, s. 164-169. DOI: 10.1109/RFID-TA.2010.5529938. From www: 
http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=552993 

            [3]    IEEE Antennas and PropagationMagazine. 2009, vol. 51, issue 2. ISSN 1045-9243. From: http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=5162013