- 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
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 is expressed in Watts or in the Decibel relative units
compared to milliWatts (dBm).
Conversion from Watts (W) to decibels-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:
|RG 174||Thin. Often used for pigtail adapter cables||2||3.7
|RG 58 ||quite common, used for Ethernet, not recommended for WiFi||1||2
|RG 213||Thick black||0.6||1
|CFD-200||Ideal for WiFi||0.54||0.86
|CFD-400||Ideal for WiFi||0.22||0.35
- 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
- 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
- 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
- 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:
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
Free space loss
It is the power loss of a wave travelling in free space
Correspondance between free space gain loss in dB and distance
in kilometer (km) :
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.
|-59 to 0||23.5
Ubiquiti™ Receiver Sensitivities
|Product||2.4 GHz||5 GHz
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:
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 %).
- 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
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.
- 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
- Reference: S. Saunders: Antenna and propagation for
wireless communication systems.
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