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

where:

• 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 …

#### 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 5:FSPL = (4πdf/c)^2

where…

• 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

#### 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 Link Budget Calculator (Kms)

f (Frequency) MHz Channel 11
Ptx (Power into Tx Antenna) dBm mW dBW W
Srx(Receiver Sensitivity) dBm mW dBW W
M (Gain Margin: signal loss) dB
Gtx (Transmit Antenna Gain) dBi
Cable Tx X loss dB/m dB
Grx ( Rx Antenna Gain) dBi
Cable Rx X loss dB/m dB

Prad (Power from Tx Ant) dBm mW dBW W
Preq (Power needed at Rx Ant) dBm mW dBW W

### 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
1 2412 2412 2412
2 2417 2417 2417
3 2422 2422 2422
4 2427 2427 2427
5 2432 2432 2432
6 2437 2437 2437
7 2442 2442 2442
8 2447 2447 2447
9 2452 2452 2452
10 2457 2457 2457 2457 2457
11 2462 2462 2462 2462 2462
12 2467 2467 2467
13 2472 2472 2472
14 2484 ### 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.