Radar Technology for Level Gauging - Krohne

The thinner the pipe, the slower are the waves propagated 21 . The propagation rate in the
cylindrical waveguide (inside diameter D) is 22 :
The diagram in Fig. 24 shows the correction factor as a function of the inside pipe diameter.
The minimum diameter is dependent on the wavelength, or frequency.
6.2 Transmission through dielectric windows
The practical application of radar level gauging systems requires that the tank interior be
separated from the tank exterior in order to provide separation of pressure, temperature and
product. To this end, "windows" of a dielectric material (plastics, glass, ceramics) are used
which for the microwaves should preferably be transparent and low reflecting.
reflected
wave
incident wave transmitted wave
The series of events can be imagined thus: a first (negative) reflection r 1 occurs at the left
interface between air and dielectric and a second (positive) reflection r 2 at the right interface,
which ideally cancel each other out (Fig. 25). It is reasonable to assume that the
difference in the transit time of the waves must be a multiple of the wavelength inside the
dielectric, divided up into incident wave and reflected wave. We therefore obtain the ideal
thickness d of the window with material property ε r as:
21 Relevant is the so-called group velocity, not the phase velocity of the waves
dielectric
window
22 Group velocity; the formula applies to the fundamental mode of the H11 waves; for other modes the factor 1.7 will
change according to the table in section 4.4.5.
Fig. 25:
Reflection of waves
from a dielectric
window
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