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#1 2010-10-31 11:14:10

Registered: 2010-01-08
Posts: 35

Differences between Reader-Antenna and Tag-Antenna (& some general q.)

I've done a bit search on RFID-Antennas: Most - allmost all - material I've found is about reader antennas. There most engineers seem to build antennas with only a few windings - often just 3 . My assumption is, that they want to keep the resistance of the coil low, to get high currents and therefore strong magnetic fields.

on the contrary most tag antennas I've seen (e.g. from inlays or transparent tag-stickers) use antennas with a lot more windings.  Thinking in the "air transformer model" this means, that they get higher voltages on the "secondary" coil (=tag). This sounds reasonable, since it is easier to decode a signal (reader sends ASK in ISO14/15K modes) with clear voltage readings.

But maybe I get this wrong. Can someone explain my observation ?

Basically I can tune any antenna-coil to the 13.56 mhz with a parallel or series capacitor - it's just, that the needed capacitance may get unhandy, when to big or to small - and that I get different bandwidths (= Q-Factor) out of such a (R)LC System. Am I right ?

One last question:

Most descriptions of the backscatter system I've seen use the transformer model: When the tag draws more current from the secondary (=tag) coil (for example by shortening the coil with an transistor), the current on the primary (=reader) coil also increases which also increases the voltage drop on the reader side.

on the contrary there seem to be a second effect: by shortening the coil on the tag, the LC-System is detuned - which leads to less energy transfer from the primary coil. worse tuning means less currents means less voltage drop on the reader side.

how do these both effects match ?


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