Simatic Rf600 - Siemens SIMATIC RF600 System Manual

Definition
When specifying decibels, the ratios between powers or voltages are not defined directly but
as logarithms. The decibel is therefore not a true unit but rather the information that the
specified numerical value is the decimal logarithm of a ratio of two power or energy variables
P1 and P2 of the same type.
This ratio is defined by the following equation:
Example:
If P1 = 200 W and P2 = 100 mW, how large is the ratio a in dB?
Use with other units
As with other units, there are also different versions of the unit for decibel depending on the
reference variable. With this reference, the logarithmic power ratio becomes an absolute
variable. The following table lists the most important combinations in this context with other
units:
Versions of decibel
0 dBm
dBi
Antenna gain
Definition
The antenna gain specifies the degree to which the antenna outputs or receives its power in
the preferred angle segment.
With this theoretical variable, a comparison is always made with an isotropic spherical radiator,
a loss-free antenna which does not exist in reality. It describes how much power has to be
added to the isotropic spherical radiator so that it outputs the same radiated power in the
preferred direction like the antenna to be considered. The unit for the antenna gain is therefore
specified in dBi (dB isotropic).

SIMATIC RF600

System Manual, 06/2010, J31069-D0171-U001-A10-7618
Description
Power level with the reference variable 1 mW.
Power level with the reference variable on the isotropic spherical
radiator (see also Section Antenna gain (Page 187) ).
Draft Version 02.06.2010
6.4 Guidelines for selecting RFID UHF antennas
Antennas
187