4.
Inductive Coupling Inductive coupling means that the transponder and the antenna are coupled by the magnetic flux through both coils, much like a transformer. All the energy used in the tag is drawn from the primary coil of the antenna.
5.
Inductance
Electromagnetic induction is the production of voltage across a conductor situated in a changing magnetic flux.
Faraday found that the voltage produced around a closed path conductor is proportional to the rate of change of the magnetic flux through any surface bounded by that path.
Faraday's law:
6.
Oscillator
A capacitor and an inductor connected together form an oscillator. Both capacitors and inductors store energy.
A capacitor stores energy in the form of an electrostatic field an inductor stores energy in a magnetic field
Once the capacitor is charged:
The capacitor will start to discharge through the inductor. As it does,
the inductor will create a magnetic field.
Once the capacitor discharges, the inductor will try to keep the current
in the circuit moving, so it will charge up the other plate of the capacitor.
Once the inductor's field collapses, the capacitor has been recharged
(but with the opposite polarity), so it discharges again through the inductor.
7.
Resonators
The antenna coil of the transponder and the capacitor form a resonant circuit tuned to the transmission frequency of the reader. The voltage U at the transponder coil reaches a maximum due to resonance in the circuit.
This is way a radio receiver works
The efficiency of power transfer between the antenna coil of the reader and the transponder is proportional to:
the operating frequency f
the number of windings n
the area A enclosed by the transponder coil
the angle of the two coils relative to each other
the distance between the two coils.
8.
The rest of the picture
The energy in the coil can is harvested using diodes and capacitors (C 2 ) to rectify the current.
LC circuit oscillator
9.
Method of Coupling
All RFID systems have two basic ways of exchanging information:
Electromagnetic backscatter is quite similar to radars.
Depending on its characteristics, an antenna reflects part of an incoming electromagnetic wave back to the sender.
Electromagnetic wave are reflected by most objects that are larger than half the wavelength.
The efficiency of reflection is particularly large for antennas that are in resonance with the incoming waves.
The short wavelengths of UHF facilitate the construction of antennas with smaller dimensions and greater efficiency.
12.
Far field not near field
Opposite to inductive coupling, electromagnetic backscatter works beyond the near field.
The energy available at the transponder is found by calculating the free space path loss a F between the reader and the transponder.
a F = −147 . 6 + 20 log (r) + 20 log (f ) − 10 log (G T ) − 10 log (G R )
13.
How it works
The power reflected from the transponder is radiated into free space. A small proportion of this (free space attenuation) is picked up by the reader’s antenna.
The reflected signal travels into the antenna connection of the reader in the backwards direction and can be decoupled using a directional coupler.