TopTechnical DictionaryE.I.R.P. (Effective Isotropic Radiated Power) - an equivalent effective isotropic radiated power

E.I.R.P. (Effective Isotropic Radiated Power) - an equivalent effective isotropic radiated power

E.I.R.P. (Effective Isotropic Radiated Power) - is an equivalent effective isotropic radiated power defined as a power that would have to be radiated by a hypothetical isotropic antenna to achieve identical signal level in the direction of maximum radiation of a specific antenna.”


In accordance to Polish and EU regulations, the maximum power that can be used to transmit in the specific WLAN frequency range (exceeding the power means that you are breaking the law):


  • 2400,0 – 2483,5 MHz (2.4 GHz band) - power up to 100 mW E.I.R.P. (20 dBm),
  • 5150 – 5350 MHz (5 GHz band) - power up to 200 mW E.I.R.P. (23 dBm) - for indoor use only,
  • 5725 – 5875 MHz (5 GHz band) - power up to 1000 mW E.I.R.P. (30 dBm).


  • Consider the following to prevent E.I.R.P. thresholds from being exceeded:


  • transmitter (e.g. network adapter, access point) output power,
  • cable type, length and attenuation at operating frequency and connector attenuation,
  • antenna power gain. 

  • Remember, that the Access Points manufacturers often specify the transmitter power in E.I.R.P. which means that the device conforms to the regulations when using an external or built-in antenna only. For custom WLAN applications, a simple calculation will verify if the signal power meets the requirements.


    For a system including a transmitter (e.g. a wireless router), cable and antenna, E.I.R.P. is calculated from the following formula:


    E.I.R.P. = P – l x Tk + Gi


    P – transmitter power in dBm
    l – cable length in metres
    Tk – attenuation for 1 metre of cable at operating frequency of the transmitter
    Gi - power gain of the isotropic antenna in dB


    To simplify:


    E.I.R.P. = transmitter power (dBm) + antenna gain (dBi) – cable attenuation (dB) – connector attenuation (dB)


    To simplify, attenuation of a single connector is = 0.5 dB


    Example. WLAN operating at 2,4 GHz band:


  • 16 dBm access point,
  • 8 dBi omnidirectional antenna,
  • 8 metre cable TRI-LAN-240 (attenuation for 2.4 GHz is 0.4 dB / metre), i.e. 8 x 0.4 dB = 3.2 dB,
  • two connectors – attenuation + 2 x 0.5 dB = 1 dB.


  • Calculation:


    E.I.R.P. = 16 dBm + 8 dBi – 3,2 dB – 1 dB = 19,8 dBm (i.e. the power level meets the requirements - less than 20 dBm).


    Example: for a 13 dBi gain antenna:


    E.I.R.P. = 16 dBm + 13 dBi – 3,2 dB – 1 dB = 24,8 dBm (i.e. exceeded by 4,8 dBm!)


    Remember, not all access points reduce the output power. Thus, it is better to use a high gain antenna and a low power transmitter than a low gain antenna and a high power transmitter. The devices operate not only in transmission, but also reception mode, and thus the sensitivity of the receiver is also important.