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There are two major propogation methods which have an influence over the range
of detection of the transponders from the readers. In magnetic systems, the
energising field is similar to that shown at school between the poles of a
permanent magnet shown with iron filings. To increase the range the poles of the magnet
need to be seperated (increase the size of the reader) or increase the power
of the field (not very effective). Coupling is by the means of coils for the
antennas, and typically the energising frequency could be 130KHz. Receiving
antenna could have 1000 turns on the antenna.
The electric field coupling uses antennas to propogate the energy from the transmitter to the transponder. The antennas size is dependent on the frequency of operation (typically 400 MHz to 2.5 GHz) and the energising field typically drops off as the inverse of the square of the distance between the transmitter and the transponder. Using this method, ranges of 4 meters are achievable.
Some transponder systems can use a principle called backscatter modulation
to communicate the data from the transponder to the reader, when in the presence of an energising field.
The energising field is emitted from the transmitter in the form of a carrier wave signal at a fixed frequency (in the example this is 915MHz). This energy from the transmitter is collected by the transponder antenna, rectified and used to power the transponder. The transponder generates a data stream comprising a clock signal and the data to be communicated, is a form of a modified manchester code. Typically the clock rate of the transponder might be at 10KHz. The data from the transponder is used to drive a shorting transisitor across the antenna, which has the effect of changing the reflectivity of the transponder antenna and causing some of the received energy from the transmitter to be reflected back towards the receiver. This reflected energy has the form of packets of energy at a frequency that is shifted from the original transmitter carrier by the clockrate of the transponder.
A simple receiver using the transmitter signal as a local oscillator can decode the received energy and extract the modified manchester code.
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