Appendix
A.2 Designing and calculating wireless systems (for example RCoax)
SCALANCE W786-xPRO
236
Operating Instructions, Release 08/2007, C79000-G8976-C221-02
Specifying power in dBi
If power is specified in dBi, the reference variable is the transmit power of an isotropic
antenna or unipole. Such a (hypothetical) antenna radiates energy from a central point
uniformly in all directions.
From the directional radiation of a real antenna, a dBi value is obtained known as the
antenna gain. This term is misleading in as far as no energy is gained by an antenna in the
physical sense. The higher radiation of a passive antenna results solely from the
concentration of radiation in a certain direction. In other spatial segments, there is
accordingly less power.
Losses due to longitudinal attenuation
The longitudinal attenuation of the leaky feeder cable depends on its length and is calculated
according to the following formula:
a
rc
=
α
rc
* l
a
rc
Longitudinal attenuation of the cable in dB
α
rc
Attenuation coefficient in dB/m as specified in the technical specifications of the cable:
RCoax Cable 0.17 dB/m at 2.4 GHz
connecting cable: 0.55 dB/m at 2.4 GHz
l Total length of the cable in m
Losses due to coupling loss
Coupling loss cd includes the losses at the transition from the cable to the surrounding
space. The coupling loss depends on the construction of the cable and its physical
properties. Values for coupling loss are therefore specified for the particular cable in the
technical specifications.
Losses due to spatial attenuation
Spatial attenuation afr specifies the attenuation between the RCoax cable and the
communications partner. The decisive factor here is therefore the distance between the
RCoax cable and communication partner. The following formula is used:
a
fr
= 20 * log(4
π
d /
λ
)
a
fr
Spatial attenuation in dB.
d Distance between cable and antenna in m.
λ
Wavelength of the electromagnetic oscillation in m; at a frequency of 2.4 GHz, the
wavelength is 0.125 m.
Note
The formula is valid only for the 2.4 GHz RCoax cable.