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Tests on the accuracy of RFID-radartm and developing long-range transponders
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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.
AccuracyThe 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.
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.
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 transpondersDifferent 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:- 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 :- 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:- 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 |
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