Radar Technology for Level Gauging - Krohne

The attenuation factor is dependent on frequency. It is given in Fig. 26 for air at 20°C and
7.5 g /m 2 moisture content. The absorption maximum at approx. 20 GHz is due to water
vapour, the maxima at 60 GHz and 120 GHz to oxygen. Attenuation performance is
dependent on pressure, temperature and relative humidity.
However, in practical terms microwave attenuation in air is not significant for radar level
measurement systems because at typical tank heights of up to 30 m (100 ft) it is at most
only about 15 dB/km · 2 · 30 m = 1 dB.
The case is different with liquid ammonia (NH 3), which under pressure (approx.10 bar/145
psi at 10°C/50°F) usually forms a dense gas phase above the liquid; in practice, it dampens
microwave signals in the X-band (10 GHz) so much that no reflection is measurable.
6.5 Modified radar equation
Based on the radar equation given in Section 3.7 the relations for antenna gain and propagation
loss can now be included.
According to Section 6.3 the power density of the waves before reaching the reflector at
distance a is given by:
The distinction now needs to be made between 2 cases, depending on the size of reflecting
area A R:
(a) The reflecting area is so large that it intersects the beam cross-section completely,
so that in the ideal case 24 the transmission power is totally reflected. Therefore, the total
path a + a is travelled to the receiving antenna, and the power density is:
24 Reflection factor R = 1, see Section 7.1.
Fig. 26:
Attenuation of
microwaves in air.
Radar handbook 33
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