Geoscience Reference
In-Depth Information
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(a) Linear dipole antenna
(a) Linear horn antenna
(a) Helix antenna
fIGURe 7.6
Basic antenna designs: (a) linear bowtie dipole, (b) horn, and (c) helix.
antenna is to detect the electromagnetic wave after it has traveled through the material in the subsurface.
The details of antenna design and implementation are very complex, and the performance of a GPR
antenna may be somewhat different from the theoretical design envisioned by the design engineer.
Antennas can be considered to be transducers that convert electric currents on the metallic
antenna elements to transmit electromagnetic waves that propagate into a material. Antennas radi-
ate electromagnetic energy when there is a change in the acceleration of the current on the antenna.
The acceleration that causes radiation may be either linear (e.g., a time-varying electromagnetic
wave traveling on the antenna) or angular acceleration. Radiation occurs along a curved path, and
radiation occurs anytime the current changes direction (e.g., at the end of the antenna element).
The primary design objective for a GPR antenna is to launch an electromagnetic wave into the
ground that has a known shape and amplitude. There are many types of antennas, and they are all
designed to optimize the characteristics of the wave that is launched into the ground. An antenna is
designed to propagate a wavefront with a simple pattern, with particular input pulse characteristics
(in the case of a time-domain system), at a high level of power of the pulse. A few of the more com-
mon designs include the linear dipole, the horn, and the helix, as shown in Figure 7.6. The linear
dipole and horn antennas launch a linearly polarized wave, and the helix launches a circularly
polarized wave.
Ideally, all of the energy from the input pulse will be expended during one trip of the electric
currents on the antenna from the input feed to the tip of the antenna, with the wavelength of the
propagating wave equal to twice the length of one half of the total length of the antenna. However,
some of the energy in the form of electric current remains on the antenna after the pulse has traveled
the full length of the antenna. The electric currents can reverberate between the tip of the antenna
and the input feed, transmitting another pulse of propagating energy each time the currents travel
from the tip to the input feed. This phenomenon is called antenna ringing, and part of the task of
an antenna design engineer is to minimize this ringing effect. There are a lot of design “tricks”
to reducing the antenna ringing, including placing resistors on the tips of the antenna elements to
absorb some of the energy (this is called “loading,” and the energy in the electric current is con-
verted to heat by the resistors on loaded dipole antennas), and changing the shape of the dipoles so
that most of the energy on the antenna element reflects at an angle and the energy of reflections from
different parts of the antenna cancels by destructive interference. This is where the art of antenna
design comes in, and a pair of tin-snips is the antenna designer's secret tool.
Impedance mismatch is another cause of antenna ringing. GPR antennas are designed to operate
on the surface of the ground, and the ground becomes part of the antenna. Antennas are designed
to minimize the contrast in electrical properties between the antenna and the ground by “matching”
the electrical impedance between the antenna and the ground surface. A perfect impedance match
maximizes the amount of energy that goes into the ground, and an imperfect match means that a lot
of the energy is reflected back (backscattered) off of the surface, which also causes antenna ring-
ing. In practice, a perfect impedance match that covers all conditions is not possible, and antenna
designers must compromise on the issue of obtaining a perfect impedance match by designing
the antennas with an intermediate impedance value that will match many types of rock, soil, and
