Direct Energy, 2018a

72 4.2 Electromagnetic Radiation
are enough to cook with, so for this reason, it would be dangerous to touch
or even, depending on the frequency, be close to the transmitting antenna.
The antenna in this example needs to be mounted on an antenna tower
not only for mechanical reasons but also for safety reasons. The amount
of power through this surface depends on distance from the transmitter as
1
r 2 , so the danger level is strongly dependent on distance from the antenna.
4.2.3 Near Field and Far Field
The region within about a wavelength of an antenna is called the near eld
region. The region beyond multiple wavelengths from an antenna is called
the far eld or Fraunhofer region. For aperture antennas, instead of wire
2(aperture size)2
antennas, distances larger than are considered in the far
λ
eld [15, p. 498]. The radiation pattern in the near eld region and in
the far eld region are quite dierent. Near eld electromagnetic radiation
is used for some specialized applications including tomographic imaging of
very small objects [51]. However, receiving antennas used for communication
signals almost always operate in the far eld region from transmitting
antennas. As an example of the dierence between near eld and far eld
behavior of an antenna, consider the innitesimal dipole antenna. The
electric eld intensity is given in Eq. 4.2. The near eld electric eld from
this innitesimal antenna is found by taking the limit as r → 0.
−→ E =
I 0 · dl · cos θ
4πɛω
· sin ( ωt − 2π λ r)
r 3 (2â r +â θ ) (4.6)
The far eld electric eld is found by taking the limit as r →∞.
−→ E =
−I 0 ω · dl · sin θ
4πɛ
· sin ( ωt − 2πr)
λ
â θ (4.7)
r
4.2.4 Environmental Eects on Antennas
The electromagnetic radiation from an antenna is aected by the environment
surrounding the antenna, specically nearby large conductors.
Sometimes conductors are purposely placed nearby to make an antenna
directional. Other times, the conductors, like metal roofs or bridges, just
happen to be nearby. If an antenna is placed near a salty lake, the lake
surface will reect the electromagnetic radiation. In other cases, the electrical
properties of soil underneath an antenna will aect the electromagnetic
radiation [50, ch. 8] [15, p. 635].