DescriptionTransmit Power 101
Here's all you need to know about transmit power:
Three concepts/terms
- TxP ‐ internal Tx Power ‐ this is the level of power cranked out by the radio hardware inside the AP.
- EIRP ‐ (effective isotropic radiated power) ‐ basically the transmit power at the tip of the antenna as it goes out over the air.
- AntGain ‐ Antenna Gain‐ The amount by which the antenna can strengthen the signal.
The relationship between the 3 terms:
EIRP = TxP + AntGain.
or
TxP = EIRP ‐ AntGain
In other words: EIRP is the transmit power including antenna gain, TxP is the transmit power excluding antenna gain.
Steps to configure the desired output power in Meru Systems Director:
1. Select the country that you're in, so that the system can enforce local regulatory limits.
2. Configure the antenna gain to the actual values of the actual antennas (No Cheating!!). If you use the default antennas or APs with internal antennas, you can skip this step.
3. Configure the local‐power setting of the radio to the desired level. Note that this is configured as EIRP!!!
Examples
- You have an AP320 with default antennas. With default EIRP set to 20 dBm and default AntGain of 3dBi, the system will program the radio to crank out a TxP of 17dBm.
- You plan to use a 13 dBi gain antenna:
‐ configure the antenna gain (AntGain) to 13
‐ configure local power (EIRP) to 30
‐ the system will program the radio to crank out a TxP of 17dBm.
Caveats
It is possible to configure combinations of EIRP and antenna gain that lead to a TxP that exceeds the capabilities of the radio. For example, if you configure EIRP = 30 and AntGain = 0, the resulting TxP would be 30 and that is more than what the radio can handle. In such cases, the radio would simply drop down the TxP to the maximum capacity of the radio. There is no indication to the user when that happens. The maximum TxP of a radio depends on many factors (AP type, channel, data‐rate, etc.) Usually it is somewhere between 16 and 19 dBm per chain.
