Balluff BIS U Basishandbuch Seite 42

BIS U Basic Manual
4
Reference Antennas and Antenna Parameters
4.4 Dispersion Angle
4.5 Front-to-back
Ratio
12
By specifying the dispersion angle, another parameter is created for the directional
characteristics of an antenna. The identified dispersion angle is the angle at which only half the
power is radiated, which represents a decrease in power of 3 dB. The reference variable is the
maximum value in the main dispersion direction. Since antennas are always passive
components, the antenna gain correlates directly with the dispersion angle: The higher the
antenna gain, the smaller the dispersion angle.
In some countries, such as European Community countries, the maximum permitted radiated
power depends on the dispersion angle of the antenna in use.
In the applicable product standard, EN 302 308 (issue V1.1.2 2006-07), the permitted radiated
power of an antenna correlates with the antenna dispersion angle as follows:
– Dispersion angle ≤ 70 degrees
– Dispersion angle > 70 degrees
60
90
120
Figure 5: Radiation diagram of a real antenna - horizontal section
Two dispersion angles are specified to provide a full description: The vertical dispersion angle
(elevation) and the horizontal dispersion angle (azimuth).
The electromagnetic waves are also radiated by directional antennas, not only in the main
dispersion direction, but also in other spatial directions, in particular a backwards spatial
direction. These minor lobes should be suppressed as efficiently as possible to allow the radio
fields to be aligned correctly towards the selected data carrier.
Attenuation in a backwards dispersion direction in relation to power radiated in the main
dispersion direction is described as the front-to-back ratio (see figure 4).
A typical value > 18 dB is specified for the BIS U 300 antenna.
Radiated power
Radiated power
3 dB dispersion angle
0
0
30 330
-3
-6
-9
-12
-15
-18
-21
-24
150 210
180
Front-to-back ratio
up to 2 watts
ERP
up to 0.5 watts
ERP
300
270
240