RFID Antennas

- overview of the basics of RFID antennas and RFID antenna design including tags and coils for all frequencies from LF through HF to UHF for RFID tags, readers and writers.

RFID antennas form a particularly important element of any overall RFID system. There are two main types, namely the RFID tag antenna and the RFID reader antenna (this RFID antenna may also be part of an RFID transceiver or RFID printer, etc).

When designing an RFID antenna system, items that are determined by the system need to be considered. For example the frequency of operation, and the coupling mechanism are two obvious points.

As part of the design of the RFID antenna, parameters such as the radiation resistance, bandwidth, efficiency and Q all need to be considered, so that the resulting design for the RFID antenna meets the requirements and allows the required level of performance to be achieved.


Antenna basics

As with any antenna, and RFID antenna follows the basic rules of any antenna system. The antenna is basically a form of tuned circuit. Power is fed into the antenna and much of it is radiated. As all passive antennas are perform in an equivalent manner in reception as they do in transmission, it is often easier to look at them as a radiating element as it is often easier to look at the concepts in terms of radiation.

There are a number of parameters and definitions for antennas that are useful when looking at RFID antennas:

  • Radiation resistance:   The resistance that equates to that which would be required to dissipate any power that is radiated.
  • Resistive losses:   The losses that occur as a result of the resistance of the antenna elements - these losses plus the radiated power equate to the total input power.
  • Bandwidth:   The band over which the antenna will operate satisfactorily. Normally antennas operate as resonant elements and therefore their performance falls either side of the centre frequency. This must be accounted for in the design of the RFID antenna, or any other antenna for that matter.
  • Feed impedance:   The current and voltage will vary along the length of the antenna element. Voltage rises towards the ends and the current falls and is also dependent upon the length of the antenna, etc. As impedance is the ratio of current and voltage this means that the feed impedance varies. TO ensure the maximum power transfer the source and load impedances must match, and therefore the feed impedance of the antenna is particularly important to ensure efficient operation.

These and many other parameters are used when designing antennas and in this case RFID antennas.


RFID antenna bandwidth

With antennas in many RFID systems being very small when compared to wavelength, the bandwidth of the antenna tends to be small. This can create problems when used with modulation systems that may use wider bandwidths.

Some RFID systems send data at very low data rates and in a straightforward manner - others use a subcarrier and this increases the bandwidth required. In view of the small size of the antenna with respect to the wavelength, the bandwidth of the antenna may be sufficiently narrow that the centre frequency may radiate well, but the sidebands arising from the subcarrier and modulation may be outside the antenna bandwidth and may not radiate effectively.

The RFID antenna bandwidth required can be determined from the following formula:

Bandwidth   =   Ftol   +   Fsc   +   Max data rate

Where
  Bandwidth is in Hertz
  Ftol is the frequency tolerance of the antenna
  Fsc is the frequency of the sub carrier in Hertz
  Max data rate is expressed in bits per second


RFID antenna coupling

Many RFID antennas will use near field effects, i.e. inductive coupling. When RFID antennas use these near field effects, the RFID reader and RFID tag will be relatively closely coupled to each other. This has a major effect on the bandwidth and performance of the antenna. Accordingly the RFID antenna coupling must be taken into account when designing the antennas.

The RFID antenna is an essential item within any RFID system. It enables the signals to be radiated or received and as a result, its performance is a crucial to that of the overall system. Accordingly the design of an RFID antenna must be carefully considered and its performance optimised to ensure that the whole system can perform satisfactorily.

By Ian Poole


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