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Link Calculator Link Calculator
  • Option 1: Detailed information and worksheet below.
  • Option 2: Ligowave's Link Calculator
  • Option 3: Ubiquiti's Link Calculator (For Ubiquiti products only)

Link Budget Calculator for Wireless LAN (WLAN)

Permission to use and edit this page kindly granted by the original Author: © Mathias Coinchon

This page allows you to calculate if your link will or will not work. It generally assumes a Clear Line of Sight path between the two antennas. This means if the 1st antenna was set on fire, the 2nd antenna could 'see' it.

Power

Power is expressed in Watts or in the Decibel relative units compared to milliWatts (dBm).

Conversion from Watts (W) to decibels-milliWatts (dBm) :

dBm: milliWatts:

(dBm= 10*log10(P/ 0.001))

  • UltraWAP APs have an output power of 60, 90, 130, or 200mW (depending on model purchased)
  • Linksys WRT54GL wireless routers have a maximum undistorted output power of 84mW

Loss in a coaxial cable

Here are some loss value for common coaxial cables:

TypeDescriptionLoss dB/m
2.4 GHz5.8GHz
RG 174Thin. Often used for pigtail adapter cables23.7
RG 58 quite common, used for Ethernet, not recommended for WiFi12
RG 213Thick black0.61
CFD-200Ideal for WiFi0.540.86
CFD-400Ideal for WiFi0.220.35

Choose type of cable:

Frequency Band:

Length (meter): Loss in dB (negative value !):


Antenna

  • Antenna gain is normally given in isotropic decibels [dBi]. It's the power gain in comparison to an isotropic antenna (antenna that spread energy in every directions with the same power....theoretical view it doesn't exist in reality!).
  • Some antennas have their gain expressed in [dBd], it's the gain compared to a dipole antenna. IN this case you have to add 2.14 to obtain the corresponding gain in [dBi].
  • The more gain an antenna has, the more it is directional (narrower beam) it is (energy sent in a preferred direction).
  • Antennas supplied by default with WLAN equipment is generally low gain (2 dBi ).
  • Antenna gain is the same for transmit and receive

Parabolic antennas:

  • The parabolic reflector is independent of the frequency, it only affects antenna gain. So it means you can reuse your TV satellite dish for wifi
  • The higher the gain the higher the directionality and so the more accurately it must be pointed.
  • The big challenge is to illuminate the parabolic reflector correctly. If illumination is too large or to concentrated there will be gain loss.

Here's the maximum theoretical gain of a parabolic antenna:

Frequency Band:

Antenna diameter in meters:
Max theoretical gain in dB:


Radiated power

Radiated power (power sent by the antenna) is expressed in dBm. (0 dBm = 1 milliWatt).

Effective Isotropic Radiated Power (EIRP) is the effective power sent by the antenna at its strongest point in the beam. It is also expressed in dBm:

EIRP [dBm] = Transmitter power [dBm] - cable loss [dB] + antenna gain [dBi]

The legal limit for radiated power (EIRP) for WLAN is country and frequency dependant.
  • 2.4 GHz
    • For Australia the EIRP limit is 4W = 4000mW = 36 dBm
    • For Europe the EIRP limit is 100mW = 20 dBm
  • 5 GHz
    • For Australia the EIRP limit ranges between 50 mW (17 dBm) and 4000 mW (36 dBm). For full details see here.

Free space loss

It is the power loss of a wave travelling in free space (whithout obstacles).

Correspondance between free space gain loss in dB and distance in kilometer (km) :

Frequency Band:

Loss in dB
(negative value !): kilometers:

(Friis formula)


Receiver sensitivity

All receivers have a minimum received power threshold (on the antenna connector) that the signal must have to achieve a certain data rate (speed). If the signal power is lower the maximum achievable, data rate will be decreased or performance will decrease. So we have better use receiver with low threshold value. Here are the actual receiver signal levels vs data rates for the UltraWAP V2 and the Ubiquiti™ range of products.

UltraWAP V2
Sig Level
(dBm)
Data Rate
(Mbps)
-59 to 023.5
-6023.4
-6123.1
-6222.8
-6322.7
-6421.8
-6520.3
-6619
-6717.3
-6817.1
-6915.5
-7015.4
-7114.7
-7213.3
-7312.6
-7411.7
-7510.7
-7610.2
-778.79
-787.38
-833.34
-84dropout
Ubiquiti™ Receiver Sensitivities
Product2.4 GHz5 GHz
802.11b802.11g802.11n802.11a802.11n

Link budget

Link budget is the computation of the whole transmission chain. Here's a budget for free space loss transmission:

  • Transmit [dBm]: transmitter power [dBm] -cable loss [dB]+ antenna gain [dBi]
  • Propagation [dB]: Free space loss [dB].
  • Receive [dBm]: antenna gain[dBi]- cable loss [dB]- receiver sensitivity [dBm]

Link working condition is that the total : Total Transmit + Total Propagation + Total Receive must be greater than 0 . The remainder gives the fade margin of the system.

Warning: These rules are theoretical. It represents the maximum achievable for a system. In reality we will have interferences (other WLAN networks, bluetooth), industrial noise (microwave ovens), atmospheric losses (air moisture, scattering, refraction), badly pointed antenna, reflections,... that will affect performances. It is so necessary to take a sufficient security margin (5-6 dB or more on large distances).

The table below has been pre-populated with some 'typical' values assuming the following equipment at both ends of the link:

General Frequency Band :
Transmit Transmitter output power :
dBm mW
Cable loss
(negative value!) :

dB

m
Antenna gain : dBi
Propagation Free space loss
(negative value!) :
dB km
Reception Antenna gain : dBi
Cable loss
(negative value!) :

dB

m
Receiver sensitivity
(always a negative value) :
dBm
Consult Receiver Sensitivity tables above
to decide what to enter here.
Total

Remaining margin = dB

Max. range = km (assuming 5dB margin)

EIRP = (dBm)

Comments  

 


Propagation: Fresnel ellipsoid

A simple and quick explanation of Fresnel ellispsoid role in radio propagation is to see the thing like a virtual "pipe" where most of the energy travels between a transmitting and receiving site. So in order to avoid losses there should be NO obstacles inside this zone (forbidden region) because an obstacle will disturb "the energy flow". (the explanation is really simplified !).

For example, if half of the forbidden zone is masked (antenna at the limit of line of sight), there will be a signal power loss of 6 dB (power loss of 75 %).

Distance "D" between transmitter and receiver [meters] :

Distance "d" between transmitter and obstacle [meters] :

Radius "R" of forbidden zone at this distance [meters] :

  • These values are only valid for a frequency of 2.45 GHz ! (would you like them for another frequency ?)

(The radius of forbidden region here is 0.6 x Radius of first Fresnel ellipsoid)


Propagation: Diffraction

When an obstacle is located between the transmitter and the receiver some energy still pass through thanks to the diffraction phenomenon on the top edge of the obstacle. The higher the frequency of the transmission the higher the loss will be.

Height "h" between antenna top and obstacle top [meters] :

Distance "D1" between transmitter and obstacle [meters] :

Distance "D2" between receiver and obstacle [meters] :

Power loss at 2.45 Ghz [dB] :

  • These calculation are valid in the case of D1 and D2 far greater than h.
  • This loss is to add to the free space propagation loss.
  • The loss is the same in a transmission in the opposite direction (transmitter replaced by receiver and vice versa).
  • Reference: S. Saunders: Antenna and propagation for wireless communication systems.

Propagation: Polarisation

Wave polarisation is given by the type of your antenna and its orientation (radiating element) respectively to the ground . For a example a whip antenna will give a vertical polarised wave when set vertically ( | ) and horizontal polarisation when lying horizontal (--). The same hold for Yagi antennas ( |-|-|-| ). Helical antennas produce neither vertical nor horizontal polarisation but circular polarisation. Circular polarisation can turn either right or left...like normal cork openers and joke cork openers ;-)

Practically in a transmission system transmitter and receiver antennas should have the same polarisation for best performance. (As polarisation change with diffractions and reflection this rule does not always hold). Vertical polarisation is prefered for long range transmission because the ground effect attenuate the signal power in horizontal polarisation case in long range.

A transmission system with circular polarisation antennas is a good way to attenuate the effect of reflections (principle used for GPS).


References, documentation


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