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AVOIDING DEAD SPOTS AND OPTIMIZING NEAR-FIELD UHF RFID READING

The size of the tags is not the only difference between far-field and near-field RFID technology. This article aims to look closer at how parameters and antennas can be modified to optimize the reading performance of near-field tags.


Author: Toni Heijari

You can roughly divide UHF RFID into two categories based on the mechanism, in which the energy is transmitted from the reader antenna to the tag antenna. These mechanisms are far-field and near-field UHF RFID. Far-field UHF RFID uses electromagnetic radiation, meanwhile near-field UHF RFID relies solely on a magnetic field. 

THE THEORY BEHIND THE CURTAIN 

In near-field UHF RFID the strength of the magnetic field will decrease very rapidly as the distance between the reader antenna and the tag antenna increases. This makes the read range of a near-field tag quite limited.

A rule of thumb is that near-field UHF RFID is sufficient up to a distance of about one wavelength, which in UHF frequencies equals about 30 cm or 12 inches. If the reading distance is longer than that, the tag is required to have an antenna structure that is capable of capturing electromagnetic radiation. The antenna structure required makes far-field tags quite big. For this reason, the main visible difference between far-field and near-field tags is the size. Whereas typical far-field tags have a length of about 5-10 cm (2-4 inches), near-field tags can be even smaller than a coin.

The extremely small form factor of near-field tags has opened new markets and opportunities to use UHF RFID. It is now possible to tag even the smallest of items. Especially the cosmetics and jewelry sectors in retail are leading the way in near-field RFID technology. Their products were earlier quite impossible to tag on item-level, because of the big size of the tags.

OPTIMIZING THE ANTENNA PARAMETERS

When designing the UHF RFID reader antenna, it is possible to optimize certain parameters, for example gain, SWVR, radiation pattern, polarization etc. to fit the chosen tag type.

Most of these parameters are used for describing and optimizing the performance of far-field transmission. When we want to optimize the antenna for near-field performance, the parameters to be changed are usually the strength of the magnetic field, and especially, how evenly the magnetic field is distributed along the surface of the antenna.

When the magnetic field is evenly distributed, it makes it possible to read near-field tags in all locations along the antenna surface. This will improve the read reliability when reading large near-field tag populations and especially when the tags are located very close to each other.

MODIFYING THE ANTENNA TO AVOID DEAD SPOTS IN NEAR-FIELD READING

Picture 1 below shows the magnetic field distribution of a traditional loop-antenna. As seen in the picture, the field is strong where the loop wires/tracks are located (the green areas), but weakens as we go to the center of the antenna surface (the blue areas). The same kind of distribution is shown in picture 2, which shows the field distribution of a patch-antenna. It is good to remember that patch antennas are the most common antenna types in fixed RFID readers. So, if you have ever wondered why you are unable to read small near-field tags when they are positioned in the middle section of the antenna surface, this is the reason.

Near -field Antenna Magnetic Field 1

Picture 1. Magnetic field distribution of a traditional loop-antenna.

Near -field Antenna Magnetic Field 2

Picture 2. Magnetic field distribution of a patch-antenna.

There are several ways to improve the near-field performance and make the magnetic field more evenly distributed. A widely used way is to modify the traditional loop antenna. In a traditional loop the fading of the magnetic field in the center of the structure is caused by this: The direction of the current flow in the conductor lines changes to the opposed direction every quarter of a wavelength. And, in the same way, the direction of the magnetic field vector in the center is generated by the currents is changing. These opposed field vectors in the middle of the antenna cancel each other, causing the fading in the center. Picture 3 shows the direction of the current flow in a traditional loop-antenna.

Near -field Antenna Magnetic Field 3

Picture 3. Direction of the current flow in a traditional loop-antenna.

What we need to do is to modify the loop structure, so that the direction of the current flow will not change the loop conductors. We can either shorten the loop to be shorter than a quarter wavelength or add series capacitance to the loop conductors, since adding capacitance makes the loop shorter from an electrical point of view.

We can add the capacitance by cutting the conductors evenly. Two metal plates create a capacitor. When capacitance is correctly added, the loop will be electrically shorter than its physical length. Picture 4 shows the current flow in this kind of segmented loop-antenna. Note that the direction is not changing along the way.

Near -field Antenna Magnetic Field 4

Picture 4. Direction of the current flow in a segmented loop-antenna.

Now that the direction is not changing, the magnetic field in the center is not being canceled and for that reason it will be quite evenly distributed along the surface of the antenna structure. This allows near-field tags to be read in every location of the surface. No more dead spots.

The magnetic field distribution of a segmented loop-antenna is shown in picture 5. As you can see, there is a significant difference in the field distribution when compared to the magnetic fields of traditional loop and patch antennas shown in pictures 1 and 2.

Near -field Antenna Magnetic Field 5

Picture 5. Magnetic field distribution of a segmented loop-antenna.

FROM THEORY TO PRAXIS

We have tested the near-field antennas inside our own products; Nordic ID Sampo and Nordic ID Merlin Blade. We noticed that the theory described in this article does work in real-life situations as well.

By using the modified antennas, while reading small near-field tags, inventory could be performed faster and with more accurate results. We also gained verification that there were no longer dead spots, where near-field tags were impossible to read.

For more information about UHF near-field and antennas, contact me at toni.heijari@nordicid.com.

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4 comments on “AVOIDING DEAD SPOTS AND OPTIMIZING NEAR-FIELD UHF RFID READING”

  1. Posted 22 July 2013 at 16:33:07

    need help with labels for bands.&

    develop same for M/device

  2. Posted 25 July 2013 at 09:13:38

    Hello Saul,

    Can you be a bit more specific on what you need?
    You could also send your request to info@nordicid.com

    BR, Mirva

  3. Gravatar of mido medmido med
    Posted 30 March 2017 at 01:59:37

    Hi.
    thank's for those great and rich informations.
    I have a question please.how can I explain the H-field radiation impact on
    S11 parameter .
    thanks for your help
    best regard

  4. Gravatar of Pauliina MäkeläPauliina Mäkelä
    Posted 10 April 2017 at 10:50:29

    Hello,

    S11 represents how much power is reflected from the antenna. That is to say it expresses how well antenna is matched.

    H-field represents magnetic field and it doesn’t have impact to the S11 parameter in UHF RFID technology.

    BR, Pauliina

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