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Understanding RFID part 4: RFID Antennas

Discussion in 'Electronic Design' started by RFIDabc, Aug 2, 2007.

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  1. RFIDabc

    RFIDabc Guest

    The design of the antennas used by RFID tags and RFID readers is one
    of the most critical pieces in RFID infrastructure. This is because
    the antennas facilitate the communication between an RFID reader and a
    tag through free space. Both the reader and the tag utilize an antenna
    to transmit and receive data. In the case of passive RFID systems, the
    antennas have the added burden of being able to efficiently collect
    and radiate energy, respectively. Without well designed antennas there
    can be no efficient communication between the tags and readers, and,
    in the case of passive tags, there may not be enough energy to power
    the tag.

    Characteristics of antennas

    Most antennas are made of a highly conductive material such as copper
    so it is sensitive to electrical and/or magnetic currents found in
    radio waves. When an antenna is receiving, the conductive material
    comes in contact with radio waves and converts the radio wave's
    electrical and/or magnetic currents into signals and energy that can
    be consumed by some type of circuit. The opposite is true for a
    transmitting antenna.

    In most cases, antennas possess a property called reciprocity. This
    means that if an antenna can transmit well at a certain frequency, it
    can receive equally well at the same frequency.

    Most antennas are tuned to operate within a certain bandwidth. This
    means that the properties of the antenna, such as construction
    materials, length, and structure, are all precisely chosen to work
    efficiently in a specified, usually very narrow, range of frequencies.
    The frequency at which the antenna works best is known as its resonant
    frequency. When an antenna works well at multiple frequencies, the
    antenna is said to have harmonic resonance. In most cases, antennas
    that are designed to work at multiple frequencies are not as efficient
    as antennas that are designed to work in a very specific frequency
    spectrum.

    As discussed, the construction material of an antenna enables the
    antenna to convert electric and/or magnetic currents into usable forms
    of energy. The two components of electromagnetic radiation are the
    electric component, E field, and the magnetic component, H field.
    Based on an antenna's design, it will make use of one of these
    components.


    Near field vs. Far field antennas

    RFID tags that operate within one wavelength's distance from the
    reader's antenna make use of the magnetic component. If an antenna is
    designed to operate at 13.56 MHz, its antenna should be designed to
    operate in the "near field" and utilize the magnetic component. This
    is because the wavelength of a 13.56 MHz wave is approximately 22
    meters. This is not to say that the tag will function at a distance of
    22 meters. It won't. The tag's antenna would not be able to derive
    enough energy from the electromagnetic field at that range. It is
    important to recognize that the wavelength of the target frequency
    directly influences how the tag's antenna is constructed.

    Any RFID tag that is designed to operate at 13.56 MHz will function
    best in the near field and will most likely have an antenna that is
    shaped like a coil (see Figure 1). A coil antenna configuration works
    best for making use of the magnetic component of electromagnetic
    radiation.



    Figure 1 - Texas Instruments 13.56 MHz Tag-ItTM Inlay


    It is important to note that the wavelength of a radio wave is
    inversely proportional to its frequency; thus, a radio wave with a
    frequency of 13.56 MHz will have a wavelength (approximately 22
    meters) that is much longer than that of a radio wave with a frequency
    of 915 MHz (approximately 32.8 centimeters). The wavelength of any
    radio wave at a certain frequency can be calculated by the equation ¦Ë
    = c/f, where ¦Ë is the wavelength and c is the speed of light in meters
    per second.





    Figure 2 - Radio Wave

    Far field antennas operate most efficiently outside of one
    wavelength's distance of its target frequency. Far field antennas are
    usually manufactured as straight lines, as opposed to the coils
    because far field antennas utilize the electric component of the
    electromagnetic radiation. The most common type of far field antenna
    is the half wavelength dipole antenna (see Figure 3). The half
    wavelength dipole consists of two antennas, with each antenna being a
    quarter of the target frequency's wavelength. Most RFID tags place the
    tag's processor between both antennas. Based on the previous example,
    a half wavelength dipole antenna that is designed for the 915 MHz
    spectrum would require each dipole to have a length of 8.2 centimeters
    which is one quarter of the 32.8 centimeter wavelength. This antenna
    would be 16.4 centimeters in total length.




    Figure 3 - Symbol Technologies Single Dipole Inlay



    The principle of Gain

    An antenna's gain is a key characteristic for RF engineers. Gain
    refers to the radiation pattern of an antenna. Low gain antennas
    radiate their energy equally in all directions, while a high gain
    antenna is direction biased.

    High gain antennas have a strong direction and a weak direction. Gain
    is a very important factor in RFID systems because it allows system
    designers to "shape" RFID coverage areas. Antennas can be arranged in
    a portal style configuration (as described in Part 3 of this series)
    where they are directionally biased toward the inside of the portal.
    Passive systems benefit from this type configuration because the
    reader's antennas can concentrate most of their energy on the location
    where the tag is most likely to be; thus providing more energy to
    power a tag as it passes through the portal.

    Active tag systems benefit from gain as well. In a real-time location
    system (RTLS), a directional antenna allows system designers to tweak
    their coverage areas and strictly define the boundaries of the "zone"
    to be covered by an antenna and reader. For example, if an RTLS zone
    is defined by the RF layout designer to only cover a single room, the
    engineers can use the antenna's gain characteristic as one of the
    tools to make sure that the antenna does not read tags beyond the
    borders of the room.


    Enhancing performance through antenna design

    There are two basic rules for designing a passive tag antenna. First,
    the longer the antenna, the more energy it can collect. There is a
    point of diminishing returns with regards to the length of the
    antenna, but, for the most part, longer is better. With respect to
    coil type antennas, increasing the number of rings of the coil is
    equivalent to increasing the length of the antenna. The second antenna
    characteristic that is beneficial to collecting energy is the surface
    area of the antenna. The Alien "M" tag is a great example of a passive
    tag with a large surface area as shown in Figure 4.


    Figure 4 - Alien Technology Corporation "M" Passive Tag


    As the width and height of the antenna increases so does its energy
    collecting efficiency. As discussed earlier, the layout of the antenna
    depends on how the tag is intended to be used and at which frequency
    it will operate. Tags that operate at lower frequencies and work in
    the near field have antennas that are coils. The higher frequency tags
    that work in the far field have straight edge antennas.

    RFID tags are very rarely placed on stationary items (there is usually
    no point in tracking something that does not move unless you want to
    make sure it does not move!). RFID tags tend to move regularly and may
    be placed in many different orientations as the object they are
    attached to is shipped, carted, or carried. The laws of physics
    dictate that an antenna will achieve its best reception when its
    element is oriented orthogonally to the radio wave. This means that
    the antenna works best when it intercepts the wave at a 90 degree
    angle; therefore, orientation is crucial if a tag is to achieve its
    maximum range and transmission data rate capabilities.

    Passive RFID tag antennas sometimes look strange because they may be
    offset at abrupt angles. These angles allow the tag to present some
    part of its antenna to the radio wave at an angle most conducive to
    coupling. The half-dipole antenna mentioned earlier is extremely
    efficient when its orientation is correct, but can be completely
    useless when it is not. Dipole antennas should only be used in
    applications where the antenna's orientation can be ensured, so this
    is why many RFID antennas are "squiggle" type as shown in Figure 5.


    Figure 5 - ALN-9440 Gen2 Squiggle from Alien Technology


    Active tag antennas

    Most of this discussion has focused on passive RFID technology, but it
    is important not to forget about the active RFID world as it has its
    own set of antenna challenges. Active tag antennas do not have the
    added burden of collecting energy from radio waves to power the tag,
    because the active tag's battery provides all the power required. This
    additional power gives active tags some flexibility on how the antenna
    is constructed. The laws of physics have not changed, but the ability
    to blast a signal at a relatively high wattage when compared to
    passive tags can nullify many of the primary design considerations
    associated with passive tag antennas.

    Our next article will discuss the characteristics of radio frequency
    (RF) and why certain frequencies and antennas are chosen for certain
    applications.
     
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