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Trolleyponder®/EcoTag® Development User Group

Transponder Sensitivity

Trolley Scan through their EcoTag® developments have been responsible for rewritting the books as far as operating range versus energising power for UHF passive transponders.

With the continual advance of technology, electronic circuitry for operating on the tag uses less and less operating voltage and current with time. However as long as the operating power of the transponder is lower than the RF power available for the tag, the lower operating currents have no impact on improving the sensitivity of the tags as the key performance parameter is the RF operating sensitivity.

Powerdensity changes with range The physics of communicating energy from a reader to the transponders is shown in the attached diagram of a sphere. The reader radiates an amount of power at an instant. This travels away from the source at the speed of light in all directions. The power density at a range is the amount of power radiated divided by the surface area of the sphere at that range. As the surface area of the sphere is
4*pi*range2,
doubling the range as shown in the diagram means that the area increases by a factor of four for each doubling of the range. meaning that the power density quarters for each doubling of the range.
The power of the reader energising field in calculating the true power density in a specific direction, is the gain of the transmitter antenna times the transmitted power. Hence a 1 watt transmitter operating through a 10 gain antenna, uses 10 watts for the calculation

The energy passing close to the transponder is collected by the transponder in its antenna aperture, an area around the antenna. For a 915MHz dipole antenna system on a tag, the effective collecting aperture of the antenna will be 134sq cms and the impedance of the antenna is 72 ohms.

The transponder converts this power received (power density * antenna aperture) to operating energy to operate its circuits. The minimum amount of power needed to operate is called the RF sensitivity of the transponder.

Operating range is determined by how far the reader can deliver enough power that can be collected over the aperture of the antenna of the tag/transponder to make the transponder operate.

Voltage doublerIn the 1994 era,state of the art was 5 volt electronic circuitry which when using two schottky diodes in a voltage doubler format attached to a dipole (72 ohm) required 55 MILLIWATTS (RF sensitivity) of RF power falling in its aperture to operate.

Since 1994, operating voltages have dropped to 3 volts meaning that a modern transponder would still need 23 MILLIWATTS to operate when attached to a dipole.

Power = ((Operating voltage+0.6)/2.8)2/Antenna impedance

Trolley Scan have been leading the way in increasing the sensitiviy of transponders. In March 2001 they have announced the availability of a Trolleyponder RFIDmodule transponder made from commercial parts which requires only 1 MILLIWATT to operate and still uses a dipole. New Isocard sized transponderIn December 2003, Trolley Scan announced the availability of their single chip EcochipTagTM version, which was only 80mm by 33mm in size, and yet operates on just 250 microwatts of energy in its aperture. This uses their new EcoTag patented technology and is available in the small system for users to test.

This power is for fully working transponders and is so low on power requirements, that a 100 watt light bulb uses the equivalent power of 400 000 of the new Trolleyponder modules transponder.

The major impact of low power is two fold, namely -

  • increased ranges on low power
  • smaller and simpler reader systems as much less energy is needed to be radiated by the reader, impacting portability of the reader.
  • Reader power (watts)
    for different transponder sensitivities
    . Transponder sensitivity
    range
    (meters)
    55mW 23mW 250uW EcochipTag
    6 185 78 0.9
    4 82 35 0.37
    2 20 9 0.1

    The above table is based on using a 10 times gain transmit antenna.

    Below is a graph showing how sensitivity impacts range and energising power. WERP(watts) is the product of the power from the transmitter in the reader multiplied by the gain of the transmit antenna. A matched dipole has a gain of 1.6 and a 6 element Yagi antenna has a gain of 15.

    Transponder sensitivity


    Order a complete system using 250 microwatt EcochipTag transponders.

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