WiFi Link Budget Calculator
How to Work Out Your WIFI Link Budget
Use a wifi link budget calculator when planing to install a point to point Wifi link. It is helpful to understand the equations the calculator uses to work out what equipment components will satisfy your needs. The equation has a number of factors and can be expressed as…
Prx = Ptx + Gtx – Ltx -Lfs -Lm + Grx – Lrx
- Prx = received power (dBm)
- Ptx = transmitter output power (dBm)
- Gtx = transmitter antenna gain (dBi)
- Ltx = transmitter losses (coax, connectors…) (dB)
- Lfs = free space loss or path loss (dB)
- Lm = miscellaneous losses (fading margin, body loss, polarization mismatch, other losses…) (dB)
- Grx = receiver antenna gain (dBi)
- Lrx = receiver losses (coax, connectors…) (dB)
Our aim is to calculate elements of the equation that will provide a good link quality for our point to point connection at our chosen distance (d) between transmitter and receiver.
Free Space Loss Explanation
There are two main effects which lead to free space loss and non of them are influenced by the frequency of the radiation which is a commonly assumed.
First Loss Effect – Radiation Law
Electromagnetic waves propagate energy according to the inverse square law. Which means the energy per unit area away from the transmitting source is inversely proportional to the square of the distance (d) between the source and the observer. So the power per unit area at point d (Pd) away from the source producing power (Ps) is …
Equation 1: Pd = Ps x (1/4πd^2)
Second Loss Effect – Antenna Apature
Now the ability of an antenna to absorb this radiated energy depends on its apature which also depends on the wavelength of the radiation. so that the effective power received(Pr) is proportional to the square of wavelength used and the incident power (Pd) at the antenna thus…
Equation 2: Pr = Pd x (Wavelength^2/4π)
The free space loss FSP equation is the ratio of transmitted power to received power so…
Equation 3:FSP = Ps/Pr
The free space loss FSP = Ps/Pr can be computed by combining Equation 1 with equation 2 and we get…
Equation 4:FSPL = (4πd/wavelength)^2
Equation 5:FSPL = (4πdf/c)^2
- f is frequency in herts
- c is speed of light (3 x 10^8 m/s)
- d is distance meters
Note that the appature effect in this equation introduces the frequency (f) element in the equation when we substitute for wavelength. This gives people the impression that free space path loss is dependent on frequency. This is an error as no such mechanism exists in nature.
The loss due to propagation between the transmitting and receiving antennas is called the path loss and can be written in dimensionless form by normalizing the distance to the wavelength:
Loss (dB) = 20×log(4×π×distance/wavelength) (where distance and wavelength are in the same units)
When substituted into the link budget equation above, the result is the logarithmic form of the Friis transmission equation (this will be covered in a separate page – coming soon).
In some cases it is convenient to consider the loss due to distance and wavelength separately, but in that case it is important to notice which units are being used, as each choice involves a differing constant offset. Here are some examples…
- Lfs (dB) = 32.45 dB + 20×log(f) + 20×log (d) : frequency(f) in MHz, distance(d) in Kms
- Lfs (dB) = -27.55 dB + 20×log(f) + 20×log (d): frequency(f) in MHz, distance(d) in meters
- Lfs (dB) = 36.6 dB + 20×log(f) + 20×log (d) : frequency(f) in MHz, distance(d) in miles
These alternative forms can be derived by substituting wavelength with the ratio of propagation velocity (c the speed of light, approximately 3×10^8 m/s) divided by frequency, and by inserting the proper conversion factors between km or miles and meters, and between MHz and (1/sec).
How to Calculate Effective Distance (d) required in a Point to Point Link
Rearranging the Link Budget Equation above we can find an expression for distance (d) between antennas thus…
Lfs = Ptx – Prx + Gtx + Grx – (Ltx + Lrx + Lm) = 32.45 + 20Log(f) + 20Log(d)
mix = Ptx – Prx + Gtx + Grx – (Ltx + Lrx + Lm) – 32.45 – 20Log(f) = 20Log(d)
x = mix/20
Log(d) = x
d = 10^x in Kms
But to make things easy for you here is a tool to do all the work for you – Enjoy
Wifi channel numbers in the 2.4 Ghz range
Here is a table of the channel numbers allocated to different world regions and there corresponding frequencies.
Channel numbers by World Region
|Channel ID||US / Canada||Europe||France||Spain||Japan|
Wifi Point to Point Link Reliability Tester
We developed a new tool for testing point to point links. This tool does link budget calculations for both ends of the link at the same time. There are lots of set-up options at both ends to give you an idea of your link viability with the equipment you intend to use and also the environment conditions. This is a really handy tool. See it here…Wifi point to point link reliability tester
Wifi Solutions for Point to Point Links
Here are a selection of suggested Wifi kits to look at for different link ranges. Remember the choice of a good directional antenna with good gain makes for high quality link throughput especially when the link speed is operating in IEEE 802.11b/g/n wireless network mode at speeds up to 150 Mbps in the 2.4GHz band
Remember to use a wifi link budget calculator to choose the right equipment for your wifi project.