RFID Antennas

One of the key elements of any RFID system is the RF antenna - it enables the link between the RFID tag and the reader, writer transceiver and its performance governs that of the system.

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The RFID antenna is a key element of the RFID system as the performance affects many aspects of the overall system and a number of key parameters need to be considered.

The requirements for the RFID antennas located in the reader / writer and the tags are very different and as a result the antennas are normally different between the two. The tags are normally very small, but the reader / writer antenna limitations are very much less and they can be very much larger.

Some of the key characteristics to be considered when designing an RFID antenna include the frequency of operation, the coupling mechanism as well as the radiation resistance, bandwidth, efficiency and Q.

The RFID antenna design can be quite challenging as the size and overall space requirements can limit the performance in many areas.

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 = F_{tol} + F_{sc} + Max data rate

Where
Bandwidth is in Hertz
F_tol is the frequency tolerance of the antenna
F_sc 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.

RFID antennas vary considerably in their design dependent upon the frequency, coupling type and whether they are used in the tag or the reader / writer. The requirements and parameters vary considerably according to the specific application. The design can be quite interesting because often space is a major limitation and performance is, as with any antenna, key. As a result the RFID antenna often needs careful optimisation to ensure the best performance.