System Operating Margin ¦ Diagram #1 ¦
SOM is a measure in dB of the signal loss your system can with stand before it degrades to the error
rate specified at your Receiver Sensitivity threshold. A minimum 20 dB SOM is recommended and many
systems are designed with 30 dB+ due to additional circumstances such as inadequate Fresnel
clearance, desensitization, intermod etc.
Fresnel Clearance Zone ¦ Diagram #1 ¦ Diagram #2 ¦
The concept of Fresnel zone clearance may be used to analyze interference by obstacles near the
path of a radio beam. The first zone must be kept largely free from obstructions to avoid
interfering with the radio reception. However, some obstruction of the Fresnel zones can often
be tolerated, as a rule of thumb the maximum obstruction allowable is 40%, but the recommended
obstruction is 20% or less.
Free-space path loss
Free-space path loss (FSPL) is the loss in signal strength of an electromagnetic wave that would
result from a line-of-sight path through free space, with no obstacles nearby to cause reflection
or diffraction. It does not include factors such as the gain of the antennas used at the
transmitter and receiver, nor any loss associated with hardware imperfections.
Effective Isotropic Radiated Power
Effective isotropic radiated power is the amount of power that a theoretical isotropic antenna
(that evenly distributes power in all directions) would emit to produce the peak power density
observed in the direction of maximum antenna gain. EIRP can take into account the losses in
transmission line and connectors and includes the gain of the antenna. The EIRP is often stated
in terms of decibels over a reference power emitted by an isotropic radiator with an equivalent
signal strength. The EIRP allows comparisons between different emitters regardless of type, size
or form. From the EIRP, and with knowledge of a real antenna's gain, it is possible to calculate
real power and field strength values.
dBm to mW Conversion
dBm (sometimes dBmW) is an abbreviation for the power ratio in decibels (dB) of the measured power
referenced to one milliwatt (mW). It is used in radio, microwave and fiber optic networks as a
convenient measure of absolute power because of its capability to express both very large and very
small values in a short form. Compare dBW, which is referenced to one watt (1000 mW).