- Effective transmitting power (EIRP): [ transmitter power (dBm) ] PLUS [ connector losses (dB) ] PLUS [ tx antenna gain (dBi) ]
- Propagation loss: [ Free space loss (dB) ]
- Effective receiving sensibility: [ rx antenna gain (dBi) ] PLUS [ cable loss (dB) ] PLUS [ receiver sensitivity (dBm) ]
The beauty of working in dB is that we can just add everything up like a budget calculation! For proper wireless networking performance we want, in theory:
EIRP + Propagation loss + Effective receive sensitivity must be GREATER THAN 0
In reality we want a good margin, around 6 to 10 dB is a good margin to have for a reliable wireless networking link. A wireless network link is only useful if the connection works BOTH ways, therefore the link budget calculation must be performed in both directions to determine if the total link will work. The calculator below will perform the calculation both ways.
This page can help you predict:
- If a proposed link will work
- If a proposed link contains transmitters over the legal limit of EIRP 36 dBm for 2.4 GHz or EIRP 30 dBm for 5.8 GHz.
- The maximum symmetrical link* distance, within legal EIRP limits, for a given wireless card and bit rate.
- How much transmitter power you might need to trade for more antenna gain to achieve a given bit rate over a longer distance.
- Choose 802.11b/g or 802.11a
- Enter distance in kilometers and hit km -> dB.
- Choose transmitting card and bit rate (or type in transmitter output power and receive sensitivity of transmitter manually)
- Choose receiving card and bit rate (or type in receiver sensitivity and the output power of receiver manually)
- Type in antenna gains on both ends
- Select cable loss options (or type in a negative value on both ends manually)
- Hit the 'Calculate' button to find out the margins both ways. Both margins should be 10 or more for a reliable link
- Read information about possible extra propagation losses due to Fresnel ellipsoid and diffraction
- Choose 802.11b/g or 802.11a
- Choose only a transmitting card and bit rate.
- Select some cable loss options on both ends, this will give a more accurate idea of maximum antenna gain
- Hit the 'Max Legal' button and the following information will be calculated
- The receiver card will be filled in automatically - ie we assume a symmetrical link
- Maximum antenna gain allowed to stay legal, to help decide what antenna to buy
- Lowest free space loss achievable with a 9 dB margin in reserve
- The distance this loss equates to in kilometers, which is the maximum distance achievable using legal EIRP for the selected card and bit rate
- Follow the instructions of 'Max Legal' above
- Click the 'Trade Gain' button to increase the antenna gain but at the same time reduce the transmitter power. The new max legal distance will be calculated and shown
- Keep clicking 'Trade Gain' until you are satisfied with the distance
- Keep note of the antenna gains, to help decide what antenna to buy
- Keep note of the transmitter power - this is the power level that you must reduce the wireless card to, to stay legal
dBm - milliWatts converter
Free Space Loss calculator
Select Frequency (frequency selection affects all calculations below)
The Link Budget Calculation
Notes- The max legal distance calculator is only intended for 2.4 GHz and 5.8 GHz frequencies, as selected above. If custom frequency is selected, it will default to 2.4 GHz.
- If max legal distance is less than you expect, don't despair - you can always use a bigger antenna and reduce the output power of the wireless cards manually. YES this is possible - click the "Trade Gain" button repeatedly to see!
- To achieve a very reliable link, a margin of at least 10 dB is needed. This accommodates for local fading (ie variations of signal strength caused by refelections). A 4 to 6 dB margin is needed if the link reliability is moderate.
- Check if Fresnel and/or diffraction limitations apply. Add extra losses to the required margin as needed.
- *A symmetric link means the wireless card and antenna on both ends are identical.
Propagation Losses: Fresnel Ellipsoid
A simple explanation of the role of the Fresnel ellispsoid in RF propagation is to visualise it as an elliptical "pipe" where most of the energy travels between the antennas. In order to avoid losses there should be NO obstacles inside this zone (forbidden region.)
For example, if half of the forbidden region is masked (aka antenna at the "limit of line of sight"), there will be a signal power loss of 6 dB. Thus 6 dB needs to be added to our base margin of 6-10 dB to account for this.
The bottom line is, the longer distance the link, the wider this forbidden region can become, therefore the taller the antenna towers need to be to prevent this being masked somewhere in the middle.
(The radius of forbidden region here is 0.6 x Radius of first Fresnel ellipsoid)
Propagatin Losses: Diffraction
When an obstacle is located between the transmitter and the receiver some energy still passes 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.
- Add this loss to the free space propagation loss.
- The loss is the same in the opposite direction (station B transmitting back to station.)
- Reference: S. Saunders, Antenna and propagation for wireless communication. systems.
