The radio frequency signal output by the wireless transmitter is transmitted to the antenna through a feeder (cable) and radiated out in the form of electromagnetic waves by the antenna. After the electromagnetic wave reaches the receiving location, it is received by the antenna (only receiving a very small part of the power) and sent to the radio receiver through the feeder line. Therefore, in wireless network engineering, it is very important to calculate the transmission power of the transmitting device and the radiation capability of the antenna.
Tx stands for Transmission. The transmission power of radio waves refers to the energy within a given frequency range, and there are usually two measures or measurement standards:
Power (W) - Linear level relative to 1 watt. For example, the transmission power of WiFi wireless network cards is usually 0.036W, or 36mW.
Gain (dBm) - Proportional level relative to 1 milliwatt. For example, the transmission gain of a WiFi wireless network card is 15.56dBm.
Two expressions can be converted into each other:
DBm=10 x log [power mW]
MW=10 [gain dBm/10 dBm]
In wireless systems, antennas are used to convert current waves into electromagnetic waves, and during the conversion process, they can also "amplify" the transmitted and received signals. This measure of energy amplification is called "gain". The unit of measurement for antenna gain is "dBi".
Due to the fact that the electromagnetic wave energy in wireless systems is generated by the superposition of the transmission energy of the transmitting device and the amplification effect of the antenna, it is best to measure the transmission energy using the same metric - gain (dB). For example, if the power of the transmitting device is 100mW or 20dBm; If the gain of the antenna is 10dBi, then:
Total transmitted energy=transmitted power (dBm)+antenna gain (dBi)
＝ 20dBm ＋ 10dBi
＝ 30dBm
Or:=1000mW
＝ 1W
In "low-power" systems (such as wireless local area network devices), every dB is very important, especially remembering the "3 dB rule".
For every 3 dB increase or decrease, it means doubling or halving the power:
-3 dB=1/2 power
-6 dB=1/4 power
+3 dB=2x power
+6 dB=4x power
For example, a wireless transmission power of 100mW is 20dBm, while a wireless transmission power of 50mW is 17dBm, and a transmission power of 200mW is 23dBm.