Understanding the antenna design challenge
To maintain the 13.56 MHz RFID system in a functional state, it is necessary for the system designer to prevent the coupling factor from getting too high.
In the wireless world, antennas are used to transmit radio frequency energy from location A to location B in the most efficient manner. That is, they radiate the power that is fed to them. When dealing with UHF RFID systems, this hypothesis is true. However, at 13.56 MHz, the picture is quite different.
The wavelength in free space at 13.56 MHz is 22.12 meters. A standard ground plane antenna has a length of one-quarter of the wavelength, which is 5.53 meters. It has a radiation resistance close to 50 Ω. In the world of 13.56 MHz RFID systems, it's unlikely to come close to such dimensions. Even if it does occur, the amount of radiated power will remain quite small.
Consider the following example: a loop antenna with an area that is one square meter. The radiation resistance is given by:
In this case, the equation yields RR = 130 milliOhms, but it would probably require four meters of wire to construct the loop, assuming a square shape. If it doesn't radiate energy, then how can it be transferred to the tag it intends to communicate with?
The answer is magnetic coupling. Some people refer to RFID base stations as “couplers;” a term that is quite appropriate in this case as the RFID system (antenna plus tag) can be considered as a loosely coupled transformer, with the base station antenna acting as the primary of this transformer. This concept is of paramount importance for the system designer. The tag and the base station “antenna” constitute the system, and cannot be studied separately. Another point to remember is that if 5 W are fed to a loop RFID “antenna,” these 5 W, being not radiated, will have to be dissipated somewhere.
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