Reflection coefficient / traveling wave coefficient / standing wave ratio / return loss definition, calculation and performance analysis

September 26, 2023

Take a two-port network as an example. For example, a single transmission line has four S parameters: S11, S12, S21, and S22. For a reciprocal network, there is S12=S21. For a symmetric network, S11=S22. For a no-cost network, there is S11. *S11+S21*S21=1, that is, the network does not consume any energy, and the energy input from port 1 is not reflected back to port 1 or transmitted to port 2.

The microstrip line or strip line used in high-speed circuit design has a reference plane, which is an asymmetric structure (but the parallel double-wire is a symmetrical structure), so S11 is not equal to S22, but it satisfies the reciprocity condition, and there is always S12=S21. Assuming Port1 is the signal input port and Port2 is the signal output port, there are two S parameters we care about: S11 and S21, and S11 represents the return loss, that is, how much energy is reflected back to the source (Port1). The smaller the better, the general recommendation is S11<0.1, ie -20dB. S21 indicates the insertion loss, that is, how much energy is transmitted to the destination (Port2). The larger the value, the better. The ideal value is 1, ie 0dB. The higher the efficiency of large transmission, it is generally recommended that S21>0.7, that is, -3dB. If the network is non-consumption, as long as the reflection on Port1 is small, the requirement of S21>0.7 can be satisfied, but the usual transmission line is consumeable. Especially above GHz, the loss is very significant. Even if there is no reflection on Port1, after a long distance transmission line, the value of S21 will become very small, indicating that the energy has not reached the destination during the transmission, and it has been consumed. On the way.

For a network consisting of two or more transmission lines, there will be mutual parameters between transmission lines, which can be understood as a near-end crosstalk coefficient and a far-end crosstalk system. Note that the S-parameter values under the odd-mode excitation and the even-mode excitation are different.

It should be noted that the S parameter indicates the information of the full frequency band. Generally, the attenuation of the high frequency is relatively large due to the bandwidth limitation of the transmission line, and the S parameter parameter satisfies the requirement within the EMI transmission bandwidth range indicated by the edge rate of the signal. That's it.

Return loss, reflection coefficient, voltage standing wave ratio, and S11 parameters are often encountered in RF microwave applications. Their respective meanings are as follows:

Return Loss: incident power / reflected power, in dB

Reflection coefficient (Г): reflected voltage / incident voltage, is a scalar

Voltage Standing Wave RaTIon: Antinode voltage / node voltage

S parameter: S12 is the reverse transmission coefficient, which is isolation. S21 is the forward transmission coefficient, that is, the gain. S11 is the input reflection coefficient, that is, the input return loss, and S22 is the output reflection coefficient, that is, the output return loss.

The relationship between the four:

VSWR=(1+Г)/(1-Г) (1)

S11=20lg(Г) (2)

RL=-S11 (3)

The definition and measurement of each of the above parameters has a premise that all other ports must match. What these parameters have in common: they are all parameters that describe how well the impedance is matched. Among them, S11 is actually the reflection coefficient Г, except that it specifically refers to the reflection coefficient of a network port No. 1. The reflection coefficient describes the ratio between the incident voltage and the reflected voltage, and the return loss is a problem from the power point of view. The original definition of the voltage standing wave is related to the transmission line. The two networks are connected together. Although we can calculate the value of the voltage standing wave ratio after the connection, in fact, if there is no transmission line here, there will be no standing wave at all. We can actually think that the voltage standing wave ratio is actually another way of expressing the reflection coefficient. As for which parameter to use for description, it depends on how convenient and how to get used to it.

The conversion relationship between return loss and VSWR can be calculated manually by the reader using Equations 1 and 2 above.

First, reflection coefficient / traveling wave coefficient / standing wave ratio / return loss 1, definition

Antenna line matching: The strength of impedance matching is generally measured by four parameters, namely reflection coefficient, traveling wave coefficient, standing wave ratio and return loss. There is a fixed numerical relationship between the four parameters. habit. The more commonly used are the standing wave ratio and the return loss.

Ratio: It is the reciprocal of the traveling wave coefficient, and its value is between 1 and infinity. The standing wave ratio is 1, indicating a perfect match; the standing wave ratio is infinity indicating total reflection, complete mismatch. In mobile communication systems, the standing wave ratio is generally required to be less than 1.5.

Return loss: It is the reciprocal of the absolute value of the reflection coefficient and is expressed in decibels. The value of the return loss is between 0 dB and infinity, and the larger the return loss, the better the match. 0 means total reflection, and infinity means complete match. In mobile communication systems, the return loss is generally required to be greater than 14 dB.