Combined power is the aggregated transmit power of radios on an AP. It is the input power for AP antennas rather than the actual transmit power after the antenna gain is generated.
Figure 1. Transmit power relationship
APs used to have only one radio; therefore, no combined power is involved. As Wi-Fi technology develops, 802.11n starts to support multiple-input multiple-output (MIMO). An AP can receive and send radio signals simultaneously through multiple radios. Each radio has its own transmit power. For the AP, its transmit power is the transmit power sum of all radios.
Combined power is used to evaluate the transmit capability of an AP and directly affects the transmit signal strength of the AP. Higher transmit power indicates a higher signal strength, larger coverage area, and stronger wall-penetration capability.
The actual transmit power is evaluated using the equivalent isotropically radiated power (EIRP). The calculation formula is as follows:
EIRP = P – Loss + G
P: transmit power of a radio. It is the input power for antennas. The transmit power sum of all radios is the combined power, in dBm. For example, a 3x3 MIMO AP supports three radios. Assuming that the transmit power of a radio is 20 dBm and around 5 dBm after being enlarged by three times, the combined power is calculated as follows: 20 + 5 = 25 dBm.
Loss: loss of lines such as antenna conversion adapters and feeders, in dB.
G: antenna gain, in dBi or dBd. For its concepts, see [From Beginner to Expert - WLAN Common Terms] Section 7 - Smart Antenna.
For example, if combined power is 25 dBm, line loss is 1 dB, and antenna gain is 3 dBi, the EIRP is calculated as follows: EIRP = 25 - 1 + 3 = 27
dBm
Common Misconception
Combined power is fixed. If the antenna gain increases infinitely, will the transmit power increase infinitely?
An antenna is a passive component. According to the law of conservation of energy, the transmit power does not change regardless of the antenna gain or the number of antennas.
Assuming an AP as an ideal radiation source, it evenly transmits signals in all directions. A sphere coverage area is displayed in space. Actual antennas are directional. After being sent from an antenna, signals are extended in some directions and shrank in other directions. The total volume is unchanged. Therefore, antennas only modify signal coverage but do not increase signal energy. An antenna radiation pattern shows energy distribution of signals. For details, see [From Beginner to Expert - WLAN Common Terms] Section 2 - Beamwidth.
Figure 2. Coverage comparison of an ideal radiation source and an antenna
