Radar Technology for Level Gauging - Krohne

3. Radar-Füllstandsmesssysteme
Typical values for antenna efficiency h 1 are approx. 0.5 - 0.8. Due to reciprocity 17 a gain G 2
is imputed to a receiving antenna that is generally equal to the transmission G 1:
received power of antenna in a planar wave field (optimally aligned)
G2 =
received power on an ideal isotropic radiator
The following correlation with the effective receiving area AE is given by:
5.3 Radiation angle
A characteristic quantity for describing the directional effect is the radiation angle or halfvalue
width. This is defined as the cone angle at whose edge the power density is 3 dB
below the maximum power density (i.e. at the edge of this lobe the power density is half
the size it is in the middle).
The smaller the radiation angle, the larger the aperture area, i.e. the higher the antenna gain.
Fig. 21 gives a rough estimation of the half-value width of horn radiators with an aperture
angle of approx. 40°. By approximation the following equation applies (D = antenna diameter):
angle [degrees]
90
80
70
60
50
40
30
20
10
The radiation lobe is slightly asymmetrical (elliptical) due to polarization of the waves
(see subsection below).
For level measuring systems, a small radiation angle, i.e. good focusing, is desirable in order
to avoid interference reflections as much as possible from the tank wall or tank internals.
17 Reciprocity of characteristics for transmission and reception
radiation angle
0
0 1 2 3 4 5 6 7 8 D
λ
9 10
Radar handbook 25
5
Fig. 21:
Radiation angle
of horn antennas
(approximative values
after [Pehl.2])