Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

762 CHAPTER 14
Figure 14-11. Microwave antenna showing the feeder (tapered horn) and the parabolic reflector.
In a dipole, electromagnetic energy is radiated from solid conductors that are
coupled to a transmitter. The horn is an opening in a waveguide from which electromagnetic
radiation emerges in the direction in which the aperture is facing. The
microwave feed is mounted at the focus of the parabolic reflector and emits radiation
into the reflector. The radiation is then reflected in a (theoretically) parallel
beam. However, the beam does spread, with the amount of spread being inversely
proportional to the size of the reflector. The measure of beam spread is called the
beamwidth. The beamwidth is defined by the width of the beam at the half-power
points and is measured in degrees. The ability of an antenna to concentrate the
radiated electromagnetic energy into a beam is described by a figure of merit called
the antenna gain. The gain of an antenna is defined as the ratio of the intensity of
the radiated signal in the far-field beam to the intensity at the same point if the same
amount of transmitter power were radiated by an isotropic antenna.
Antenna gain = G =
power from antenna P
= . (14.26)
power from isotropic radiator P0
The gain of an antenna is related to its area and to the wavelength of the radiation.
For a parabolic antenna, this relationship is given by
G = 4πA
, (14.27)
λ2 whereA is the area of the antenna aperture and λ is the wavelength of the radiation.