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Trolleyponder


Transponder News

A news service reporting on developments regarding the use of radio based tagging transponder systems for commerce and scientific applications. Covering RFID and EAS technologies as well as magnetic/electric field coupled techniques.


Tests on the accuracy of RFID-radartm and developing long-range transponders

2 February 2006

In August 2005, Trolley Scan announced that they had developed a technique for accurately measuring the distance a signal travelled from a lowcost transponder to a reader over long distances, and hence were able to measure the identity and location of many transponders in a zone at a time.

The method of making these measurements is complex and the question arises as to how accurate and repeatable can such a new technique be?

A further impact of such a discovery is that it provides an incentive to develop a new range of very low power transponders to allow the radar to operate over much longer ranges than were needed with the existing RFID reader technology.

Accuracy

The two graphs show measurements made on transponders at far ranges on 23rdJanuary 2006. The first shows 180 repeat measurements made on three targets, one set per second over three minutes. The second shows the scatter from 14000 (fourteen thousand) independent successive measurements made on two transponders.

In the graphs the axis are in meters.

Radar accuracy 1
In the first there are three transponders, namely at (10,0); (13,4) and (34,-11) meters. One hundred and eighty successive range measurements were made on each transponder. The reader accurately determines the range, and calculates the angle of arrival based on the difference in distance between two adjacent readers.

These measurements were made using a 915MHz energising signal to UHF tag- talks- first protocol Trolleyponder/Ecotag transponders. The system only uses 10 kilohertz of bandwidth and measures with good precision despite the speed of travel of the radio signals being at 300 000 kilometers per second.

Radar accuracy 1
The second graph shows the result of fourteen thousand measurements on two targets, one set per second over four hours. Targets were at (10,0) and (41,12) meters. These measurements were made over open fields and have the occasional bird flying through the beams and people moving in the vicinity which causes some excursions on the angular measurements.

Both these graphs show the remarkable ability this new technique exhibits for measuring range of the transponder from the reader.

At present the maximum distance over which we can measure is limited by the energising requirements of the transponders. Trolley Scan expect the range measuring algorithm to operate with similar performance at ranges up to 100 meters.

Increasing the maximum operating range of backscatter transponders

Different operating frequencies and the associated propagation of energy results in different operating ranges for transponders. The first transponders in the 1970's operated at 125/135kHz and have an operating range of a few centimeters. Moving the operating frequency to 13.56Mhz resulted in an increase in range to 50 cms with developments in the 1990s. UHF transponders were developed in the 1990s and dramatically increased the operating range up to 15 meters. Microwave transponders at 2.45Ghz suffer from reducing aperture of the antenna and offer 1 meter ranges. Superimposed on the choice of frequency is the size of the antenna structure which is large at UHF and shrinks as the frequency increases.

UHF frequencies offer the greatest operating range in terms of the laws of physics. Globally countries have allocated frequencies in the 860 to 960MHz bands which allows design of systems with frequency agility which can allow for goods labelled in any one country to be read by readers in other countries. This is achieved by making the transponders responsive to signals over a 100MHz bandwidth.

Hence the challenge for developing long range transponders becomes making UHF backscatter tags that have 100MHz of bandwidth and operate on very low powers.

RFID readers have a practical limit to their needs for long range transponders. As the energy from the reader passes through walls and floors, one does not want too much range or else you are reading goods in adjacent rooms when doing an asset scan and do not know the physical location of the goods you are seeking. This feature puts a typically limit to the range to 6 to 10 meters for these type of applications.

The arrival of RFID-radar - where identity and exact location can be reported, results in a need for much longer operating ranges for the reader which means there is a need for transponders that can operate at distances beyond the cap of 10 meters needed for conventional readers.

The operating range of a reader comes from two elements, namely:-

  • The energy radiated from the reader to energise the transponder and provide an RF signal that can be returned to the reader for backscatter transponders.
  • The ability of the reader to detect very weak signals in the presence of the strong energising signal operating on the same frequency.
  • If you build a transponder with a 5 volt logic circuit and use a dipole antenna structure, it will need 54 milliwatts of RF energy to operate - this 54 milliwatts must fall in the antenna aperture of 149sq cms.

    The energy leaves the reader from the amplifier at some power level, is focussed by the reader antenna as it launches into space and from then on dissipates as the inverse square of the distance travelled. This means that a power density at 1 meter from the reader will be four times the density at the 2 meter point, or 100 times the density at 10 meters, or 10 000 times the density at 100 meters. Hence to increase range from the energising field perspective you can :-

  • Increase the power of the transmitter - this is limited by the radio regulations of the different countries so that it does not interfere with other radio users.
  • Increase the focus of the transmit antenna - causes more energy to be focussed down the beam at the expense of wider coverage.
  • Decrease the power requirements of the transponder
  • The detection of signals coming back from the transponder depends on its ability to detect the weak signals in the presence of noise. To do this the reader needs to optimise:-
  • Use narrow bandwidth data signals to reject as much of the noise as possible as the quality of the signal being received is a function of its signal to noise ratio. Noise is proportional to bandwidth. Low clock rates of 10 kHz are better than 100kHz in this parameter.
  • The sensitivity of the receiver to detect weak signals buried in the noise.
  • The antenna system on the receiver that can focus energy only from the desired location to the reader, rather than from all interfering sources in the vicinity.
  • The modulation index of the transponder - how big a percentage of the energy the transponder receives can be sent back to the reader- this returned energy is a function of how much energy the transponder receives at its distance from the reader and this returned energy decreases as the inverse square of the distance travelled. Hence a transponder 1 meter from the reader has the ability to generate a return signal that is 100 000 times 100 000 (1010) that of the signal coming from a transponder 100 meters away.
  • Trolley Scan have long been supplying the 200uW Ecochiptags, Ecowoodtags and laundry tags that have operating ranges as far as 13 meters. In the past the receive path was never a problem due to the high sensitivity of the receiver. Trolley Scan are now supplying 5uW long range tags, with an operating distance of 30 to 35 meters, where receive paths start becoming an issue. These 5uW tags now need 10000 times less RF energy than needed by that standard 5 volts circuit on a dipole.

    These are the next step into supplying transponders with 100 meter ranges to meet fully the needs of RFID-radar users.

    More info on RFID-radar products can be obtained from Trolley Scan at http://www.rfid-radar.com

    RFID-radar(tm) is the trademark of Trolley Scan (Pty) Ltd


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