Maximum Transmission Power in WLAN Radio?
“What is the maximum Wi-Fi transmission power of your access points or wireless router?” This is one of the common questions I always come across during the first meeting with clients in the RF designing stage. Very often we will hear the answer like 100mW or 20 dBm from the engineers, it is the maximum transmit power we can generally configure in most of the WLAN radio transmission power settings.
However, as a WLAN professional, we would like to dig in the IEEE 802.11 specification and the local regulatory domain authority who is responsible for the maximum transmit power regulations. WLAN emissions are subject to local regulatory limits. Countries apply their own regulations to allow devices and maximum power levels within the WLAN frequency ranges. Besides, different radio bands also require different regulatory power limits.
In Hong Kong SAR, we would refer to specification from OFCA (The Office of the Communications Authority) who is the responsible body for telecommunication regulatory and also allocation of the radio frequency portion of the electromagnetic spectrum.
According to HKCA 1039 from OFCA – Performance Specification For Radio communications Apparatus Operating In The 2.4GHz Or 5GHz Band. The peak output power of the apparatus shall not exceed the levels indicated below:
Short overview representing E.I.R.P in dBm:
2.400 - 2.4835 GHz (ch1-11) 4W (36dBm)
5.15 - 5.35 GHz (ch36-64) 200mW (23dBm)
5.470 - 5.725 GHz (ch100-140) 1W (30dBm)
5.725 - 5.850 GHz (ch149-165) 4W (36dBm)
The 5GHz Hong Kong regulatory is similar to ETSI Standards. The maximum allowed output power actually depends on which frequency band that we select according to the channels that we assign to the access points.
However, what is EIRP?
EIRP (Effective Isotropic Radiated Power) is the measured radiated power of an antenna in a specific direction. It is also called Equivalent Isotropic Radiated Power. It is the output power when a signal is concentrated into a smaller area by the Antenna. The EIRP can take into account the losses in transmission line, connectors and includes the gain of the antenna. It is represented in dB. Enter the transmitted power, cable loss and antenna gain to calculate the EIRP (Effective Isotropic Radiated Power)
RF Components:
EIRP = P(T) – L + G
Where P(T) = Output power of transmitter (dBm)
L = Antenna cable & Connectors loss (dB)
G = Gain (dBi)
AP122#show int wifi1
EIRP=Effective Isotropic Radiated Power (Transmit Power + Max Antenna Gain + Max TX Chains Gain);
Freq(Chan)=5200Mhz(40*); EIRP power=23.01*dBm(14dBm + 6.00dBi + 3.01dBi); Diversity=enabled;
From above access point setup, the 5GHz is being assigned with channel 40 using frequency 5200 MHz. The emitted maximum EIRP power is 23 dBm. This is the summary of Transmit Power, Maximum Antenna Gain and Maximum TX Chains Gain.
Tx Chain=static 2; Rx Chain=static 2;
And we can also tell from the CLI output info that this AP support 2x2 multiple chains with two transmitting and two receiving antennas. This will also help to boost the EIRP power with the chains gain.
Hope this article helps you understand further on WLAN transmission power local regulatory and EIRP calculation.
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