(I) Understanding of standing wave:
To understand voltage standing wave ratio (VSWR), we must first explain the "standing wave". In a transmission link, when we transmit two signals of the same frequency at the same time from the opposite direction, the test is an overlay signal. Since the direction of the two signals is opposite, the superposition signal is no longer transmitted in a certain direction, but shows a "standing and standing" state. At this time, we call this signal a "standing wave", as shown in Figure 1.
Figure 1 The formation of standing wave
(II) Voltage standing wave ratio:
refers to the ratio of the voltage peak value of the standing wave to the voltage valley value, VSWR = Umax/Umin. VSWR is essentially used to measure whether the matching between components is good. The closer the standing wave ratio (VSWR) is 1, it means that the higher the matching degree, the smaller the reflection power, the higher the transmission efficiency, and vice versa. As shown in Figure 2
Figure 2 The relationship between standing wave ratio and energy transmission
(III) The relationship between VSWR value and antenna performance:
First of all, VSWR = 1 does not mean that all are good antennas
1. If VSWR = 1
In this case, it can only mean that the energy of the transmitter can be effectively transmitted to the antenna system (no reflection). But whether these energy can effectively radiate into space is another question. A dipole antenna made of theoretical length and a shortened antenna with a length of only 1/20 can achieve VSWR = 1 as long as appropriate measures are taken, but the emission effect is definitely very different. Even a 50 ohm resistor has a VSWR ideally equal to 1, but its emission efficiency is 0 (all energy is eaten by the resistor and becomes thermal energy)
2. If VSWR 1
is on the impedance circle diagram, each VSWR value is a circle (VSWR = 2 on the blue circle in Figure 3, and the circle is <2),>VSWR value is the same, there are many possibilities for the state of the antenna system , so it is not very meaningful to use the VSWR value to simply compare each other between the two antennas. As shown in Figure 3,
, etc. VSWR circle
3, the hazard of excessive VSWR
When VSWR is too high, it is mainly because the antenna system does not resonate , so impedance has a large reactance component, and the transmitter's last stage device may need to withstand a large instantaneous overvoltage. High VSWR can easily cause damage to RF final power devices. Therefore, it is necessary to control VSWR to a lower value, such as <3.>
