True Near-field Antennas

By Times 7, Jul 06, 2018

Benefits of Specifically Designed Near-Field Antennas

Antennas designed explicitly for near-Field only provide a close to the surface consistent read zone with no dead spots using the magnetic components of the RF electromagnetic fields. 

Although reading tags at a greater distance is one of the advantages of RAIN RFID, some applications require near only zone tag detection. In these cases, a consistent surface read zone with no dead spots is essential. 

A true near-field antenna will have almost no distance read capability.

Electromagnetic Fields in RAIN RFID

RAIN RFID uses radio frequencies for communication. Radio frequencies are alternating electric currents oscillating in a specific frequency. RAIN RFID works in the range of 860 to 960 MHz, globally. Radio waves are a type of electromagnetic radiation. Other types of electromagnetic waves are X-rays, visible light, gamma rays, infrared and ultraviolet rays.

Electric fields and magnetic fields together constitute the electromagnetic fields produced by an antenna. By their nature, electric fields can propagate longer distances, and magnetic fields are associated with the surface of the antenna.

In simple terms, magnetic fields denote ‘near-field,’ and electric fields represent ‘far-field.’ Following are some examples of near-field and far-field applications:

RF Polarization

The polarization of an antenna refers to the orientation of electric fields concerning a plane. There are three main types of polarizations:

• linear (horizontal or vertical – pertaining to a plane),
• circular
• elliptical

As the name indicates, linearly polarized antennas have their electrical fields oriented only in one plane, and it is the direction of propagation.

In a circularly polarized antenna, electric fields rotate in at a steady rate with equal magnitude in all possible directions.

When the magnitude is varied with respect to the planes, then the fields are elliptically polarized.

The rotation can be clockwise or counter-clockwise in both circular and elliptical polarizations.

Polarization in Near-Field

In a near-field antenna, polarization is not applicable as the magnetic fields are used for communication.  A contactless credit card can be read by the point of sale machine regardless of the orientation because of the magnetic coupling effect between the reader device and the card.     

Importance of Near-Field in RAIN RFID Applications

Certain RAIN RFID applications involve metals and liquids such as jewellery, pharmaceutical vials, food products, bottles, blisters, etc. These pose challenges in reliable detection using far-field antennas and tags. Smaller assets like jewellery cannot bear larger far-field tags. Liquid assets would absorb the electric field components of the far-field radiation, leaving the tags unread. These challenges can be addressed using near-field tags and antennas. Magnetic fields are less harmed by liquids or metals.

 Near-field is also used in applications such as tag commissioning and item sorting where a confined read zone is necessary. A near-field antenna is polarization independent when the tags are in the proximity of the antenna’s surface.

RAIN RFID Antennas and Tags

The RAIN RFID reader antennas and tags can be broadly classified into far-field or near-field antennas and tags. As the name indicates, far-field operates at a greater distance whereas the near-field operates only at short distances. Most of the commercial tags such as the ALN-9640 are far-field tags which can capture the near-field signals as well. They consist of a special loop that captures the near-field signals, and the long-wiggled structure responds to the far-field. There are some ‘near-field only tags’ such as ALN-9713 whose antenna design is as simple as a loop.

Reader Antenna & Tag Compatibility

The following table shows the compatibility between the types of reader antennas and tags:

Surface Dead Spots

The traditional loop and patch antennas’ magnetic field distribution over their surfaces are not even, which results in areas with low magnetic field intensity. These areas contribute to “dead zones” while reading ‘near-field only’ tags, where the tags are not detected.

Times-7’s A1001 and the A1030 antennas specially designed to handle this problem as they do not use traditional loop or patch antennas. Due to this reason, these antennas can also provide very confined power distribution in a given area.