AN232

DS00232A-page 2

 2002 Microchip Technology Inc.

To increase sensitivity, one ensures that the transmitter

(TX) tank and receiver (RX) tank resonant frequencies

are the same as the desired magnetic field frequency.

Another aspect to bear in mind is that sensitivity is

dependant on the angle between coil face and the field

lines. Maximum response is obtained when the lines

pass through the coil perpendicularly, as shown in

Figure 1.

FIGURE 1:

ALIGNMENT OF FIELD

LINES WITH COIL FACES

The TX and RX coils can be thought of as a weakly-

coupled transformer, across which data may be trans-

mitted by modulating the source (or transmitter) and

detecting the modulated signal at the receiver.

MAGNETISM BASICS

It is important to note the difference between a mag-

netic field/electric field versus an electromagnetic

wave. A magnetic field is a result of electrical charge

in motion, or a magnetic dipole. One also only gets

magnetic dipoles and not monopoles, as is the case for

electrical particles. A magnetic field can, therefore, be

represented by field lines that form continuous loops

that never cross each other.

Electric fields, on the other hand, are the result of a

distributed electrical charge. What both magnetic and

electric fields share in common is that the field strength

source geometry is assumed to be a point source.

What this means is that the field intensity at a distance

2X away from the source is 1/8th of the field intensity

measured at a distance X from the source.

However, an electromagnetic wave reacts quite differ-

ently than the magnetic or electric field. Assuming the

same point source, the electromagnetic wave propa-

gates with a decay rate of 1/r. Thus, at a distance of 2X

from the point source, the field intensity is only 1/2 com-

pared to that measured at a distance of X from the

source. This means that a magnetic field decays much

more rapidly than an electromagnetic wave.

The magnetic field energy can be thought of as a cloud

of energy packed around the source. On the other

hand, one can imagine an RF wave as a sphere radiat-

ing outward from the source at the speed of light, with

the wave energy spread out across the outer surface of

the sphere.

The question then is; what is the link between mag-

netic/electric fields and electromagnetic waves?

To find the answer, we need to consider some proper-

ties of both magnetic and electric fields. The first is that

a time-varying electric field induces a magnetic field

and, conversely, that a time-varying magnetic field

induces an electric field. These are special cases of

Amperes and Faraday’s laws, respectively. Therefore,

a time-varying field of either kind induces and rein-

forces a field of the other kind.

If

the

signal

wavelength

(magnetic

or

electric)

approaches the dimension of the antenna, the mag-

netic electric reinforcement becomes strong enough to

allow for electromagnetic wave propagation. For an

antenna that is very small compared to the signal

wavelength, one does not have an efficient propagating

wave decaying at 1/r; instead, one has an attenuating

The effect, however, is negligible if the antenna dimen-

sions are small relative to the wavelength of the

exciting signal. The wavelength of a signal can be

calculated using Equation 1, and at 125 kHz is a long

length of 2.4 km!

EQUATION 1:

An antenna approaching this dimension is impractical,

but at 500 MHz the wavelength is only 60 cm.

Higher frequency antenna dimensions are thus much

more practical and a true propagating wave is easily

realizable.

.

Calculating The Magnetic Field Strength

For most LFMC applications when calculating field

strength, a magnetic field is generated by a base sta-

tion by setting up an oscillatory current in a series RLC

network at a typical resonant frequency of 125 kHz.

The current passing through the inductor creates a sur-

rounding magnetic field according to Ampere’s Law.

Using Equation 2, one can calculate the absolute

magnetic field strength B at a point P from the radiating

coil, as shown in Figure 2.

V

Source

Meter

Field Lines

Note:

For LFMC, a small component of the total

energy is in the form of an electromagnetic

wave, but that is negligible compared to

the magnetic energy of a 125 kHz mag-

netic antenna.

ƒ

c

[meters]