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WRI Wireless Calculators


  1. Power Budget - total power of your system in
    dBm.
  2. Free Space Loss - power loss over distance.
  3. System Performance - How far will it go, will
    it get there or what is my Operating Margin?
  4. Fresnel Zones - How much more than LOS or how
    high should my antenna be?
  5. Power Conversion - dBm to milliWatts and milliWatts
    to dBm
Power Budget

Use this calculator to walk through all the factors that make up
your Power Budget. The 'Power Budget' is the the total power being
output from the wireless system and is the sum of:

  • PLUS Radio Transmit Power (A above)
  • MINUS cables and connectors losses (B above)
  • PLUS antenna gain (C above).

Enter one parameter in each column (or leave blank) depending on
what you know and the calculator will supply the appropriate conversions
automatically. Up to 2 decimal places may be entered as 0.xx or .xx.

Note: Connector loss is generally small UNLESS you've got
lots of 'em in which case you are probably in trouble anyway, or your
cable is VERY short. Just leave the line blank if you are feeling
lazy.

Calculations and equations used.
Radio and Antenna    
Transmit Power Antenna Gain Power
 mW
OR

 dBm
 dB(i)  dB
Cable Loss    
Cable Properties
(per 100 ft or m)
Cable Length  
 dB (100 ft)
OR
 dB (100 m)
 feet
OR

 meters
 dB
Connector Loss    
Frequency in MHz No. of Connectors  
 MHz.  No.  dB
Budget    
   dB
Free Space Loss

Loss of power over distance (assuming no FRESNEL
Effect
and nothing in the way). This a very idealised calculation
and in practice everything interferes with the signal but it will
give you a reasonable approximation of the actual loss over distance.

Enter the system Frequency in MHz and the distance in either Kilometers
(Km) or Miles
below and then click the 'Calculate' button. 1 GHz
= 1000 MHz e.g. 2400 = 2.4 GHz.

Calculations and equations used.
Frequency Distance   Result
 MHz  Km
OR
 Miles
 dB
Fresnel Zones

Defines how much clearance you need (yes you need more than simple
Line-of-Sight) and for longer links > 3 Km (2 miles) whether you
may have a ground clearance problem from our friendly planet. M.
Fresnel and his theories
.

Enter the Total link distance (in Miles or Kilometers), if
you do not enter an Obstacle distance (in Miles or Kilometers)
the calculator will use the mid-point for all calculations (Note: assumes antennas at same height). Finally enter the system Frequency
in MHz and then click the 'Calculate' button. 1 GHz = 1000 MHz e.g.
2400 = 2.4 GHz.

The calculator will generate the radius of the 1st Fresnel zone only
(at the obstable point or the mid-point), the 60% (no obstacle) radius
and the height of the effective earth curvature at the mid-point of
the Total link distance using the effective earth radius.

Calculations and equations used.
Total Link Obstacle Distance 1st Fresnel Zone Radius
 Km
OR

 Miles
 Km
OR
 Miles
 m
at
 Km
 ft
at
 Miles
Frequency   60% No Obstacle Radius
 MHz    m  ft
  Earth Height (mid-point)
     m  ft
System Performance

This calculator will give you one of three answers:

  1. If you leave distance blank it will apply the defined SAD factor
    (or default to 30%) to the Operating Margin and supply the max.
    distance (in Km and Miles) at which the Margin operates.
  2. If you enter the distance it will calculate the Operating Margin
    and the SAD factor.
  3. If you enter distance but leave RX or TX antenna gain (or both)
    blank it will apply the chosen SAD factor (or default it to 30%
    if none supplied) and generate the required antenna power. If both
    are left blank it will calculate a symmetrical antenna gain.
To RESET any parameter just set it to BLANK before clicking 'Calculate'

Notes: RX Sensitivity is ALWAYS expressed as a negative dBm
(- dBm) and is the lowest power of signal your radio can handle. Its
buried somewhere in your radio spec and will be typically in the range
of -70 to -110 dBm. Don't guess or 'fudge' this number.

Note: If you solve for TX antenna (you leave it blank) and
define a specific SAD factor it is applied to the TX budget and any
TX antenna value will include this factor. If you then click Calculate
again without changing anything, the SAD factor will be calculated
to include the supplied TX antenna value and thus may show
a lower value than the original one.

Calculations and equations used.
Frequency Distance   Result
 MHz  Km
OR

 Miles
(FS Loss)  dB
TX Power TX Cable TX Antenna  
 dBm
OR
 mW
 dBm  dB  dB
RX Sensitivity RX Cable RX Antenna  
 dBm  dB  dB  dB
Margin RX Power SAD Factor Theoretical Margin
 dBm  %  dB
milliWatts to dBm (and vice versa)

Power in milliWatts to dBm (and vice versa) . Enter the 'Transmit
Power' (A or G above) in milliWatts OR the 'Power Ratio'
in dBm and click the appropriate 'Calculate' button. 1 Watt
= 1000 milliWatts.

Calculations and equations used.
Transmit Power   Result
mW  dBm
Power Ratio   Result
 dBm  mW

Power Budget Calculations

  1. mW to dBm = 10Log10(Watts) + 30

  2. feet to meters = .3048

  3. meters to feet = 3.28

  4. Calculator normalises all distances to kilometers and meters

  5. Connectors loss = 0.10 * square root (frequency in GHz)

Free Space Calculations

  1. Free space loss = 36.56 + 20Log10(Frequency) + 20Log10(Distance
    in miles)

  2. Calculator normalises all distances to miles

  3. Miles to Kilometers = 1.609

  4. Kilometers to miles = 0.621

Fresnel Zone Calculations

  1. Calculator normalises all distances to kilometers and meters

  2. Miles to Kilometers = 1.609

  3. Kilometers to miles = 0.621

  4. feet to meters = .3048

  5. meters to feet = 3.28

  6. 1st Fresnel Zone radius (Km) = 17.3 x Sqr root ((Obstacle Distance
    x (Total Link - Obstacle Distance)) / (Frequency in GHz x Total Link))

  7. 1st Fresnel Zone radius (miles) = 72.6 x Sqr root ((Obstacle Distance
    x (Total Link - Obstacle Distance)) / (Frequency in GHz x Total Link))

  8. Obstacle free radius = 0.6 x 1st Fresnel Zone radius

  9. Radius of nth Fresnel zone (meter) = sqr root ( (n x wave length x
    Obstacle distance x (Total Link - Obstacle Distance)) / Total Link )

  10. wave length (meters) = speed of light (299,792,458 m/s) / frequency
    in Hz

  11. Earth curvature calculation = (Total Link) 2 /(8 * effective
    earth radius)

  12. effective earth radius = 4/3 * Earth radius

  13. Earth radius = 3963 miles, 6378 Km

System Performance Calculations

  1. Calculator normalises all distances to miles and feet during calculations.

  2. Free space loss = 36.56 + 20Log10(Frequency) + 20Log10(Dist
    in miles)

  3. mW to dBm = 10Log10(milliWatts) + 30

  4. dBm to mW = 10(dBm/10)

  5. RX Power = Margin - RX sensitivity

  6. Theoretical margin = TX power budget + RX power budget - free space
    loss

  7. SAD factor = Theoretical margin/TX power budget * 100 and shows the
    percentage of spare power on transmission.

dBm to Watts Calculations

  1. mW to dBm = 10Log10(Watts) + 30

  2. dBm to mW = 10(dBm/10)

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