Theory; Principle Of Operation - JRC Handy Search NJJ-95B Instruction Manual

5.1 Principle of Operation

The principle of this radar is basically the same as for general radars widely used at present.
Electromagnetic waves are transmitted from the antenna toward the concrete as shown in Figure
5-1. The electromagnetic waves are reflected by an interface with the reflecting objects (e.g.,
reinforcing steel bars or cavities) whose electrical property is different from that of concrete. The waves
are reflected back onto the surface of concrete and received by the receiving antenna placed near the
concrete surface. The distance to the reflecting objects can be calculated from the time the reflected
waves need to reach the receiving antenna. The horizontal locations of the objects can be detected by
moving the main unit on the surface of concrete.
Since this radar is designed to probe objects with high resolution that are near from the surface of
concrete, it transmits pulse waves having a width of only about one nanosecond (one-billionth of a
second) or less.
Trans antenna
Figure 5-2 shows a wave reflected by a reinforcing steel bar or the like in a concrete structure, which
is obtained by this radar.
Reflection
5
Trans.
Rebar
Figure 5-1 Diagram of Principle
Trans. wave
T
Figure 5-2 Sample of Reflected Waveform
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Theory

Rec.
Rec. antenna
Reflected wave from rebar
Concrete surface
Inside concrete
Time