System Operating Margin
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.
- Calculator normalises all distances to miles and feet during calculations.
- Free space loss = 36.56 + 20Log10(Frequency) + 20Log10(Dist in miles)
- mW to dBm = 10Log10(milliWatts) + 30
- dBm to mW = 10(dBm/10)
- RX Power = Margin - RX sensitivity
- Theoretical margin = TX power budget + RX power budget - free space loss
- SAD factor = Theoretical margin/TX power budget * 100 and shows the percentage of spare power on transmission.
Fresnel Clearance Zone
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.
- Calculator normalises all distances to kilometers and meters
- Miles to Kilometers = 1.609
- Kilometers to miles = 0.621
- feet to meters = .3048
- meters to feet = 3.28
- 1st Fresnel Zone radius (Km) = 17.3 x Sqr root ((Obstacle Distance x (Total Link - Obstacle Distance)) / (Frequency in GHz x Total Link))
- 1st Fresnel Zone radius (miles) = 72.1 x Sqr root ((Obstacle Distance x (Total Link - Obstacle Distance)) / (Frequency in GHz x Total Link))
- Obstacle free radius = 0.6 x 1st Fresnel Zone radius
- Radius of nth Fresnel zone (meter) = sqr root ( (n x wave length x Obstacle distance x (Total Link - Obstacle Distance)) / Total Link )
- wave length (meters) = speed of light (299,792,458 m/s) / frequency in Hz
- Earth curvature calculation = (Total Link) 2 /(8 * effective earth radius)
- effective earth radius = 4/3 * Earth radius
- Earth radius = 3963 miles, 6378 Km
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.
- Free space loss = 36.56 + 20Log10(Frequency) + 20Log10(Distance in miles)
- Calculator normalises all distances to miles
- Miles to Kilometers = 1.609
- Kilometers to miles = 0.621
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 Convertion
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).
- mW to dBm = 10Log10(Watts) + 30
- dBm to mW = 10(dBm/10)
Calculators Links
