ELECTROMAGNETIC COMPATIBILITY AND MEDICAL DEVICES - Design and Development of Medical Electronic Instrumentation - page 157

and

I l

H (A/m) 4

π D

2

where l is the dipole length in meters. In contrast with the near- fi eld H of a loop which falls

with the inverse of D 3 , the near- fi eld H of a dipole falls of ff as 1/ D 2 . Similarly, the near- fi eld

E of a dipole falls of eld E of a loop that falls as 1/ D 2 . The

wave impedance of emissions radiated by a dipole is also a

ff as 1/ D 3 , in contrast to the near- fi

ff ected di ff erently by frequency:

Z
π λ

0

Z wave 2

Compare this equation with the equation describing Z wave in the near field. The change in

wave impedance as a function of frequency in the case of a dipole is inverse to that of a loop.
In the far

fi

elds is again similar to that of electro-
magnetic radiation from a loop; that is, they decrease as the observation distance increases
as described by

fi field, the behavior of the E- and H- fi

Z
λ

Il Il

D

0

E (V/m) 2 H (A/m) 2

D λ

Beyond the transitional point, the wave impedance again remains constant at the value of
Z 0 . The result of a constant impedance in the far fi field means that the ratio of E to H com-

ponents remains constant regardless of how the field was generated.

Of course, real-life circuits are neither ideal open wires nor perfect loops, but rather,
hybrids of these two. In a simpli

fi

ed form, as shown in Figure 4.6, a more realistic model
of a circuit which radiates electromagnetic emissions can assume that an ac voltage source

fi

Z SOURCE

V o @ f

Probe

Figure 4.6 A simplified but realistic model of a circuit that radiates electromagnetic emissions. In it, an ac voltage source causes the flow
of a current I in a rectangular loop enclosing an area S . The voltage seen by the load depends on the source and load impedances.