Magnetic coupled transponder systems
125kHzThese are the most common transponders available today, manufactured by a wide range of suppliers.
Generally operating at frequencies typically in the order of 125KHz, the tags are characterised by antenna systems that comprise of numerous turns of a fine wire around a coil former to collect energy from a reader's magnetic field. Due to the magnetic method of coupling, range is limited generally to a number of inches, being determined by the fields generated between the effective North and South pole of the reader. Magnetic Tags are manufactured by many suppliers and find application in tagging animals, labelling gas bottles, electronic automobile key identification, and factory automation.
Different forms of communication are used by the different manufacturers. Typical methods are to use the energising signal at a frequency of 125 kHz and to receive data back from the transponder by
Issues around magnetic coupling are that the frequency is low, the energising field is very much stronger than the returned data field strength, that it is difficult to create filters with sufficient tuning to separate the transmit and received signal while both are present, that the tags have very limited energy storage capability, meaning that the energising field needs to be applied in a uniform continuous manner, or data can be received back in that short period of time after the energising field is removed (flyback)
Typical operating distances for 125kHz transponders is 2 cms. The magnetic fields propogate through water and with the correct alighnment, the transponders can be buried below the surface of metal items that are to be identified. The smallest transponder in this form is a glass sleeve 11 millimeterss long and 1 millimeter in diameter. This technology now finds application in identifying pets, car keys for ignition systems, labelling gas bottles and in a few crazy applications like being injected into humans to verify identity. A bolus version which can be swallowed by a bovine animal and which lives in the stomach of the animal is also popular.
Since 1998 a new series of magnetic coupled tags has been available which operates at 13.56Mhz , not needing the high number of turns on the 125kHz transponder antenna and hence are cheaper to manufacture. The tags comprise of a small coil of a few turns, often etched on a flexible printed circuit substrate, and to which a single chip is bonded. These transponders are typically the size of credit cards but might be as small as 1.5cm by 1.5cm in area and couple their energy to the reader via magnetic propogation. By the reader continuously providing an energising field, which can be modulated, the tags can extract energy and data from the reader and communicate back to the reader. Such tags often have read/write capabilities and often anti-collision properties to allow for many tags to be in the reader beam at the same time. The reading and writing distance of such tags is limited by the magnetic means of propogation to typically 50cms, but some manufacturers claim 1 meter operating range. These tags seem to be positioning to replace the more difficult to produce 125KHz tags which required coil winding facilities.
Almost all magnetic based transponder systems are passive, that is they get their energy from the reader's energising field.
Transponder systems operating on magnetic coupling principles operate at frequencies as high as 29MHz.
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