ELECTROMAGNETIC COMPATIBILITY AND MEDICAL DEVICES - Design and Development of Medical Electronic Instrumentation - page 156

carrying high current and low voltage, the fi field will be mostly magnetic in nature. If on the

other hand, the fi field is produced by an element placed at high voltage but carrying little or

no current, the fi field will be mostly electric in nature. This is the domain of the near fi field,

while the plane wave is in the domain of the far fi field.

field, as it is known, is thus a circular loop
of area S (m 2 ) carrying an ac current I of wavelength

The ideal generator for a magnetic fi field, or H- fi

λ . It should be noted that although a

static field is generated by a dc current and can be calculated with the method to follow,

static H-

fi

elds do not cause radiated emissions and are thus disregarded for EMI purposes.
If the loop size is smaller than the observation distance D , the magnitudes of the E and H
vectors can be found using the solutions derived from Maxwell's equations. In the near
fi

fi

field, the simpli fi ed values for these magnitudes are

Z
λ

IS
2

0
D

E (V/m) 2

and

IS
D 3

H (A/m) 4

π

where Z 0 equals the impedance of free space, 120 π or 377 Ω . Inspecting these equations,

we and decreases drastically with the

inverse of the cube of the distance. At the same time, the electric

fi find that in the near fi field, H is independent of λ

fi field increases as fre-

quency increases and falls of with the inverse of the square of distance.

The wave impedance may be de

ff

fi ned as the division of E by H in an electromagnetic

version of Ohm's law:

E (
(

V
A/

) /

m

m
)

Z wave ( Ω ) H

Thus,

Z 0 2
λ

D π

Z wave

where D 48/ f (MHz). In the far fi field [i.e., D 48/ f (MHz)], on the other hand, both E- and

H- fi elds decrease as the inverse of the observation distance as described by

Z
λ

2 π D

IS π 2 I

D

S

E (V/m)

0

H (A/m) λ

which maintains a constant impedance equal to Z 0 , allowing direct calculation of radiated
power density in W/m 2 simply by multiplying E and H . Notice that E and H , and thus
power, increase with the square of frequency. This shows, once again, that limiting the
bandwidth of radiated signals by a pulse train is of utmost importance in controlling EMI.
The region dividing the near fi field from the far fi field is called the transition region [i.e.,

at D 48/ f (MHz)]. In it, abrupt transitions occur on the near- fi eld characteristics until a

smooth blending leads to the far- fields can also be

created by passing an alternating current through a straight wire dipole, just as in a radio
antenna. In this case, the near-

fi eld characteristics. Electromagnetic fi

fi eld electric and magnetic vector amplitudes are

Z
π

I l λ 3

0

E (V/m) 8

2

D