radio transmission power
The radio frequency signal output by the radio transmitter is sent to the antenna through the feeder (cable), and is radiated by the antenna in the form of electromagnetic waves. After the electromagnetic wave reaches the receiving location, it is received by the antenna (only a small part of the power is received), and sent to the radio receiver through the feeder. Therefore, in wireless network engineering, it is very important to calculate the transmitting power of the transmitting device and the radiation capability of the antenna.
Tx is the abbreviation of Transmits. The transmitted power of radio waves refers to the energy within a given frequency range. There are usually two measures or measurement standards:
power (W) - a linear level relative to 1 watt (Watts). For example, the transmit power of a WiFi wireless network card is usually 0.036W, or 36mW.
Gain (dBm) - Relative to 1 milliwatt (milliwatt) proportional level. For example, the transmit gain of WiFi wireless network card is 15.56dBm.
The two expressions can be converted to each other:
or
dBi is the amount of gain relative to a spot circle antenna. dBi and power (dBm) can be directly added and subtracted, making the calculation very convenient.
In wireless systems, antennas are used to convert current waves into electromagnetic waves. During the conversion process, the transmitted and received signals can also be "amplified". The measure of this energy amplification is called "Gain". Antenna gain is measured in "dBi".
Since the electromagnetic wave energy in the wireless system is produced by the amplification and superposition of the transmitting energy of the transmitting device and the antenna, it is best to measure the transmitting energy with the same measurement-gain (dB), for example, the power of the transmitting device is 100mW, or 20dBm; the gain of the antenna is 10dBi, then:
Total transmit energy = transmit power (dBm) + antenna gain (dBi)
= 20dBm + 10dBi
= 30dBm
or: = 1000mW
= 1W
In "low power" systems (such as WLAN equipment) every dB counts, especially remembering the "3 dB rule".
Every increase or decrease of 3 dB means doubling or reducing the power by half:
-3 dB = 1/2 power
-6 dB = 1/4 power
+3 dB = 2x power
+6 dB = 4x power
For example, a 100mW wireless transmit power is 20dBm, a 50mW wireless transmit power is 17dBm, and a 200mW transmit power is 23dBm.
Question 1: . Question 2 and Question 3 can use the 3dB rule. Or calculate directly.