Radar Technology for Level Gauging - Krohne

Also taking into account the equation for antenna gain G 1 (Section 5.2), and atmospheric
attenuation α and a reflection factor R of the reflecting surface (see Section 7), we obtain
the following interrelationships:
A significant point is that the received power decreases with increasing distance a. Where
a level measuring system is used in a large-area tank, equation (a) should be applied by
approximation: the signal decreases to the square of a. Where extremely tall tanks or interference
reflections from small internals are involved, it is better to use equation (b):
decrease with the 4th power of a.
In both cases, however, there is the same proportional dependence 25 on antenna diameter
D and wavelength λ or transmission frequency f:
Fig. 27 gives a comparison of various radar systems using different antennas and transmission
frequencies.
100
10
1
0.1
Normalized transfer functionn
f=50GHz f=24GHz
f=10GHz
f=5.8GHz
(4") (8")
0.0.1
0 20 60 80 100 120 140 160 180 200
antennea diameter [mm]
The distinct increase in power due to higher frequencies and larger antennas is quite evident.
A 50 GHz system with a 45 mm (1.8”) dia. antenna would have the same power as a
5.8 GHz system with 130 mm (5”) dia. or a 10 GHz system with 100 mm (4”) dia. antenna.
25 Assuming that efficiencies, reflection factor, atmospheric attenuation and effective reflector area are independent
of frequency. This applies only up to a point to the cross-section of the reflected beam σ, since e.g. for a plane face
it is proportional to f 2 (see Section 7.5).
Radar handbook 35
6
Fig. 27:
Transfer function of
radar systems using
different frequencies
and antenna sizes
(normalized to
D = 100 mm (4”)
at 10 GHz)