Read range of the UHF tag is from several millimetres to tens of meters, and is limited by the number of the following variables:
- Transmitting power of the reader – maximum power for transmission.
- Sensitivity of the tag – minimum power required for a single activation of the IC tag
- The distance between reader and tag (attenuation path).
- Used carrier frequency.
- Sensitivity of the reader – minimal power of the tag reader to read the tag
Distribution of RFID operating frequencies:
There are four main working frequencies specified in Table 1.
Table 1: Operating RFIF frequency
|Name||Frequency (MHz)||Reading distance (m)|
|Low frequency (LF)||0.125||0.3|
|High frequency (HF)||13.56||1|
|Ultra high frequency (UHF)||850 - 950||10|
|The microwave frequency (MW)||5800 - 9600||several tens|
For our purposes we will consider only the UHF work band.
Transmitting power of the reader:
The amount of energy radiated by the transmitting antenna is limited by three constraints: cost, power consumption, government regulations. The first two constraints are different according to the specific requirements of the task and therefore they will not be further addressed. In contrast, government regulations limitations are fixed. For UHF band it is used the ERP (Effective Radiated Power) expression. The maximum allowed power (ERPmax) is divided in terms of the regions as shown in the following Table 2.
Table 2: Standardization of RFID – Region 1
|Europe (CEPT)||South Africa|
|869.4 – 869.65 MHz / 0.5 W||869.4 – 869.65 MHz / 0.5 W|
|865.6 – 867.6 MHz / 2 W||915.2 – 915.4 MHz / 8 W|
Tab.3: Standardization of RFID – Region 2
|USA, Canada and Mexico||Central and South America|
|902 – 928 MHz / 4 W||902 – 928 MHz / 4 W|
Tab.4: Standardization of RFID – Region 3
|918 – 926 MHz / 1 W||864 – 868 MHz / 4 W|
|950 – 956 MHz / 4 W||910 – 914 MHz / 4 W|
|The rest of Asia - the same as Europe|
Sensitivity of the tag:
This is the minimum received power by the tag, which is still able to activate IC (Integral Circuit). Modern tag ICs require approximately 10-15 uW for its function (reading operation) and a lot more power to write data into the tag memory. However, the tag power supply circuit has an efficiency of 30 %, therefore it is necessary that the received power from the supplied antenna was around 30-50 uW. Sensitivity of the tag for the lower limit 30 uW is -15 dB and -10 dB for the upper limit 100 uW.
The distance between the reader and tag (attenuation paths):
The easiest way to estimate the strength of the received signal by the tag antenna is to imagine that the reader transmits a signal in all directions with the same power density (requires an isotropic antenna). In fact, such an antenna cannot be built and even if so it would not be used since the sought tags are placed in predefine directions and the radiation outside these areas would be only wasting of emitted energy. This antenna is ideal and is only used for calculations.
Figure 1 shows uniformly distributed power of isotropic antenna on an imaginary sphere at a distance r from the transmitter. Part of this power will be reported by receiving antenna. The quantity of the received power PRX depends on the power density incident on the tag and on the effective aperture of the receiving antenna Aef.
Fig. 1: Emission of isotropic antenna (taken from  and modified)
Power density at any distance from the transmitter is easily transmitted power divided by surface of the sphere (4πr2), which has a radius equal to the distance. The power density for isotropic radiator is therefore calculated using the formula:
where PD … power density, R … distance from the antenna, PT … transmitted power
As already mentioned, the RFID readers utilize directional antennas instead of omnidirectional, and therefore it must be taken into account the gain G, which is the ratio of radiated power in a desired direction to the power radiated by isotropic antenna, or:
Many RFID UHF tags use variants of a simple printed dipole as an antenna. The gain of a typical half-wave dipole on the frequency 900 MHz is about 2.15 dBi (G = 1.64). However, we cannot guarantee, that the main beam of the tag antenna aimed directly at the reader so we should count with a minimal gain of antenna tag.
Table 5: Overview of the gain of various types of antennas
The power density at a distant point R from the reader, which has an antenna with a gain GT is equal to the power density of the isotropic antenna multiplied by antenna gain of the reader.
Where ERP is effectively radiated power which is fixed for each of the regions mentioned earlier.
Effective aperture simply represents how much power can be captured from incoming planar wave by the antenna. The principal relation between the effective aperture and the gain of any antenna is:
where λ … wavelength of the electromagnetic wave
where c … speed of light, f… frequency of electromagnetic wave
For half-wave dipole is maximum effective aperture proportional to rectangle with sides ½ λ and ¼ λ and it is independent of the dipole magnitude. For microstrip antenna with an effective aperture, it is approximately equal to its actual surface.
Using equations (2) and (3) we can express the quantity of the received power PRX by the tag at a distance R from the reader. This relation is known as Friis equation:
where GRX … gain of the receiver antenna (tag)
From a previous relation we can derive the calculation of the minimum distance for the forward channel, which is:
where GTX … gain of transmitter antenna, PTX… power of the transmitter, PTag,min… minimum received power needed to turn it on (sensitivity of the tag)
For the return channel calculation we need to consider two questions: How much power is transmitted by the tag and how much of the power is needed by reader for the demodulation and decode the tag data? Passive tag does not generate its own signal but only adjusts the amount of incoming radiation and then radiates back to the reader (backscatter). In principle it is possible that the transmitted power in the return way was the same as the maximum power absorbed by the tag but it is very difficult to meet it in practice. Since all the designs of the tags are not same it is reasonable to consider the reflected power for one-third (-5 dB) versus the absorbed power. The minimum received power needed to successfully read the tag is also comprehensive and depends on the particular reader characteristics. Sensitivity of today readers is around -75 dBm (0,03 nW) to -90 dBm (1 pW). Another thing you need to realize is that the power received from the reader by the return path will decrease by fourth power of the distance. From the Friis equation knowledge (5) and from the previous assumptions we can write a formula for the received power at the reader in the return channel:
where Tb… backscattering losses
From the previous relation we derive the calculation of the minimum distance for the return channel.
However, to determine the theoretical range of the tag we need only to know the minimum distance of forward channel because we think that if the tag receives as much energy to turn it on, it has enough power to transmit back.
List of references
- DOBKIN, Daniel Mark. The RF in RFID: passive UHF RFID in practise [online]. Burlington: Newnes, 2008, ix, 493 s. [cit. 2014-10-24]. ISBN 978-0-7506-8209-1.
- UKKONEN, L., M. SCHAFFRATH, D.W. ENGELS, L. SYDANHEIMO a M. KIVIKOSKI. Operability of Folded Microstrip Patch-Type Tag Antenna in the UHF RFID Bands Within 865-928 MHz. IEEE Antennas and Wireless Propagation Letters. 2006, vol. 5, issue 1, s. 414-417. DOI: 10.1109/LAWP.2006.883085. Dostupné z: http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=1704257
- LEHPAMER, Harvey. RFID design principles: passive UHF RFID in practise [online]. Boston: Artech House, 2008, xii, 293 s. [cit. 2014-10-24]. ISBN 978-1-59693-194-7.
- NIKITIN, P.V. a K.V.S. RAO. Reply to "Comments on 'Antenna Design for UHF RFID Tags: A Review and a Practical Application'" [online]. Boston: Artech House, 2008, xii, 293 s. [cit. 2014-10-24]. ISBN 10.1109/tap.2006.875934.
- MARROCCO, Gaetano a K.V.S. RAO. The art of UHF RFID antenna design: impedance-matching and size-reduction techniques [online]. Boston: Artech House, 2008, xii, 293 s. [cit. 2014-10-24]. ISBN 10.1109/map.2008.4494504.