13.56 MHz RFID systems
and antennas design guide
390119012107
Rev 001 Page 1 of 8 March 2004
1 Abstract:
This document is aimed at providing 13.56 MHz RFID systems designers with a practical cookbook on
how to optimize RFID systems and antennas. A thorough analysis of the most important RFID system
parameters is presented. The emphasis is placed on physical concepts, rather than on lengthy theoretical
calculations.
2 Antenna ? You said Antenna ?
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 do radiate the power that is fed to them. If we were dealing with
UHF RFID systems, this hypothesis would be true. However, at 13.56MHz, the picture is quite different.
The wavelength in free space at 13.56 MHz is
λ
= =
300
13
56
2212
.
.
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 ohms. In the world of 13.56 MHz RFID systems, we are unlikely to
come close to such dimensions. And even if we do, the amount of radiated power will remain quite small.
Let us consider an example. Say that we have a loop antenna and that its area is one square meter.
The radiation resistance is given by:
RR
A
≈ ×
31200
2
2
λ
. In our case, this yields RR=130 milliohms
and we probably have used 4 meters of wire to construct the loop, assuming a square shape… So, if we
do not radiate energy, how do we transfer it to the tag we intend to communicate with ?
The answer is magnetic coupling. Some people refer to RFID base stations as “couplers”. This
terminology is certainly quite appropriate in our case. We have indeed to consider the RFID system,
antenna plus tag, 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. One must always
remember it. The tag AND the base station “antenna” constitute THE system, and cannot be studied
separately.
The other point to remember is that if we feed five Watts to a loop RFID “antenna”, these five Watts, being
NOT radiated, will have to be dissipated somewhere…
3 Coupled circuits: a journey to Hell
In the preceding paragraph, we surreptitiously introduced another important, to say the least, concept. We
said that we had a loosely coupled transformer. The complete theory of coupled circuits is beyond the
scope of this document. It involves quite lengthy calculations, but we can somewhat alleviate this burden
thanks to the use of a circuit simulator like SPICE. Now, we must define the characters, and assign a role
to each of them.
The first character is our base station antenna. In order to maximize the communication range with the
tag, we must create the strongest possible magnetic field, so that the tag will be able to pick up enough
power in order to energize itself. Since the magnetic field from the loop is proportional to the current
flowing through the conductor that actually constitutes the loop, we have to maximize this current.
The second character is the tag. The tag wants to be able to collect in as much energy as possible from
the ambient magnetic field generated by the base station loop antenna. We must maximize this energy
gathering capacity.
These goals can be achieved in various ways. However, the next paragraphs will show that the art of
RFID system design requires a careful understanding of the pitfalls and conflicts that will inevitably arise.
Our characters are not team players. More often than not they are unfair to each other.
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