RFID antenna – how to choose?

How to choose RFID antenna?

RFID antenna chargers power from RFID reader and transmits it to RFID tag within its reach in form of radio wave. If the readers are brains of RFID system, then the antennas are its arms, as they transmit and receive information between RFID tag and reader. RFID antennas usually look similar. What makes them different is physical specification. When choosing an antenna, several parameters should be taken into account, such as: frequency range, amplification, opening angle/ beamwidth and polarization.

Frequency range

In each country, there are regulations stating acceptable UHF/RFID frequency. The most common three ranges for UHF RFID antennas are:

  • 865-868 MHz (EU / ETSI,
  • 902-928 MHz (US / FCC),
  • 860-960 MHz (global).

When choosing RFID antenna, choose frequency range for the region it will operate in.

Amplification and opening angle

Amplification and opening angle, also called a beamwidth are strongly connected. The bigger amplification (gain), the more narrow radio wave is, which means the covered area is more narrow too, but the beam reaches greater distance. To sum up, amplification influences wave breadth. The bigger amplification, the more focused the wave. It can be easily represented as stronger and weaker flashlight beam. Selection of ideal antenna depends on the solution. If we have many tags in close proximity, the best choice is an antenna with slight amplification and wider beam.

Polarization

Most RFID UHF antennas are circular or linear polarizes. This means that linear polarization antennas transmit RF waves in one level, horizontal or vertical. Circular polarization antennas transmit waves in circular movement, clockwise or counter-clockwise. In this case we differentiate between left circular polarization (LHCP) and right circular polarization (RHCP). If we have a configuration of several or more antennas directed at one another, it is important to know if they are LHCP or RHCP. Circular antennas of the same polarization can overlap. Using opposite circular polarization antennas will allow to create more efficient read zone.

Circular polarization and linear polarization: which antenna is the right one?

Choice between circular polarization antenna and linear polarization antenna can significantly influence RFID solution.

Linear polarization occurs when electromagnetic waves are transmitted on one level (horizontal or vertical). Linear polarization antennas are the best choice when all RFID tags are located in the same orientation and on the same height. Due to concentrated transmission, linear antennas usually have bigger read range than circular antennas with the same amplification.

Omnidirectional antennas are better for solutions where we cannot predict location and orientation of RFID tag. They emit waves that resemble corkscrew movement in two dimensions. As omnidirectional  antennas lose power for two dimensions, their read range is shorter in comparison to linear antennas.

When making a decision about the kind of antenna to be used in RFID system, the key factor is to get to know how the antennas work and how are RFID tags oriented in relation to the antenna.

Types of RFID antennas

Small and large RFID antennas

Antennas are available in different sizes, starting from mobile phone to TV set size. Difference in size usually points for read range. The bigger the antenna, the longer read range and vice versa. Installing location may also influence choice of antenna. Some installation locations will not allow for large antenna due to limitation of space or simply esthetical reasons.

Internal and external RFID antennas

Because RFID can be implemented in almost all environments, elements of the system may be designed to work in conditions where water and dust occur. All electronic elements have certificate of protection before water and dust according to American IEC 60529 norm and British EN 60529 in range between of IP 00 to IP 69. When making decision if the antenna will work outside or inside, work temperature should be taken into consideration. The antennas have specified temperature ranges for optimal work. Outside this range the antenna may work slow or stop working completely.

Integrated and non-integrated RFID antennas

The antennas may be integrated in one case with RFID reader and bought as one device or work separately. Integrated devices help save space and build more mobile system, with no need to worry about cables. Such solution may be optimal for retail and distribution as it is usually small, easy to use and more attractive visually than two inconvenient external devices. On the hand, external antennas provide much more possibilities of configuration and flexibility when it comes to different uses.