RFID线圈设计

In a Radio Frequency Identification (RFID) application,
an antenna coil is needed for two main reasons:
• To transmit the RF carrier signal to power up the
tag
• To receive data signals from the tag
An RF signal can be radiated effectively if the linear
dimension of the antenna is comparable with the
wavelength of the operating frequency. In an RFID
application utilizing the VLF (100 kHz – 500 kHz) band,
the wavelength of the operating frequency is a few
kilometers (λ = 2.4 Km for 125 kHz signal). Because of
its long wavelength, a true antenna can never be
formed in a limited space of the device. Alternatively, a
small loop antenna coil that is resonating at the
frequency of the interest (i.e., 125 kHz) is used. This
type of antenna utilizes near field magnetic induction
coupling between transmitting and receiving antenna
coils.
The field produced by the small dipole loop antenna is
not a propagating wave, but rather an attenuating
wave. The field strength falls off with r
- 3
(where r = dis-
tance from the antenna). This near field behavior ( r
-3
)
is a main limiting factor of the read range in RFID
applications.
When the time-varying magnetic field is passing
through a coil (antenna), it induces a voltage across the
coil terminal. This voltage is utilized to activate the
passive tag device. The antenna coil must be designed
to maximize this induced voltage.
This application note is written as a reference guide for
antenna coil designers and application engineers in the
RFID industry. It reviews basic electromagnetics
theories to understand the antenna coils, a procedure
for coil design, calculation and measurement of
inductance, an antenna-tuning method, and the
relationship between read range vs. size of antenna
coil.
M AN678
RFID Coil Design

Author: Youbok Lee
REVIEW OF A BASIC THEORY FOR
Microchip Technology Inc. ANTENNA COIL DESIGN
Current and Magnetic Fields
INTRODUCTION Ampere’s law states that current flowing on a conductor
produces a magnetic field around the conductor.
In a Radio Frequency Identification (RFID) application, Figure 1 shows the magnetic field produced by a
an a