What Is a Standing Wave?

Standing wave refers to a distribution state of two waves (not necessarily radio waves) with the same frequency and opposite transmission directions, formed along a transmission line. One of these waves is generally a reflection of the other. Antinodes appear at the point where the voltage (or current) of the two are added, and nodes are formed at the point where the voltage (or current) of the two is subtracted. On the waveform, the positions of nodes and antinodes are always the same, giving the impression of "stand still", but its instantaneous value changes with time. If the amplitudes of the two waves are equal, the amplitude of the nodes is zero.

Standing waves are a very common phenomenon in nature. They are ubiquitous in life. For example, water waves, the sound of musical instruments, and treetop tremors are all related to standing waves. Standing waves are an important part of physics teaching. ^[1]

The two columns propagate in opposite directions with the same amplitude,

The end of the transmission line is broken, short-circuited or the impedance does not match, and reflection occurs;

The frequency and transmission speed of the two waves are exactly the same, but the directions are opposite.

The two simple harmonics with a period of T, a wavelength of , and an amplitude of A travel in opposite directions along the x-axis.

The wave propagating along the positive direction of the x-axis is called the right traveling wave, and the wave equation is

y1 = Acos2 (t / Tx / ) (1)

A wave traveling in the negative direction of the x-axis is called a left traveling wave, and the wave equation is

y2 = Acos2 (t / T + x / ) (2)

The synthesized standing wave equation is as follows:

y = y1 + y2 = 2Acos2 (x / ) cos2 (t / T) (3)

It can be seen that any point on the synthesized wave is doing the same period.

A broadband two-way coupler is connected in series at the multi-frequency signal combining port, and the input power and reflected power are respectively extracted and sent to the detector to convert the RF signal power into a DC signal. The standing wave ratio of this port. In this method, the signal detected by the detector is the superimposed value of the power of the previous multiple input signals. Since the standing wave ratio is a function of frequency, that is, the standing wave ratio changes with frequency, the above methods cannot distinguish the signal frequency. Therefore, the standing wave detection accuracy is extremely low, and often cannot meet the requirements for effective monitoring of equipment, and false alarms or no alarms often occur. ^[4]

At the input end of the multi-system access platform, input power detection is performed on a single frequency band to obtain accurate input power values. After coupling the multi-band combined power at the multi-band combined port, the radio frequency switch and filter are used to select the frequency band. The multi-band combined broadband signal is split into multiple narrow-band signals and sent to the detector for power detection. The standing wave detection of the combined port introduces frequency information to achieve the purpose of accurately detecting the standing wave ratio of the combined port. At the same time, compared with the traditional method, the coupling of the coupling port is changed from a bidirectional coupler to a unidirectional coupler, which can significantly reduce the difficulty of achieving high directivity of the directional coupler. ^[4]