Inside Wireless: VSWR, |S11|, Return Loss
Channel: RF elements s.r.o.
Date: 19/10/2020

There are many antenna parameters out there. In this Inside Wireless episode we will look at Voltage Standing Wave Ratio (VSWR), |S11|, and Return Loss. Do you know what is a standing wave and how it relates to these parameters? If not, make sure you check this episode.

VSWR stands for ‘voltage standing wave ratio’. Some of the feeding signal is always reflected back if impedances of the cable and the antenna are not equal. The reflected signal adds up with the feeding signal resulting into a standing wave. The ratio of maximum and minimum value of the standing wave is VSWR. Ideally, VSWR is 1:1, which means the impedance of the cable and the antenna are exactly equal. In the real world, the VSWR is for example 2:1, meaning there is a mismatch and some of the input signal is reflected, if it's 3:1, there is an even bigger mismatch, and so on. 

Return loss says what portion of the power is reflected from the input port due to impedance mismatch. The higher the return loss, the better. For example, if the return loss is 30 dB, it means the reflected signal is 30 dB weaker than the input signal. 

S11, also called reflection coefficient, is the ratio of the voltage wave reflected from the antenna port to the input voltage wave. Ideally, S11 is as close to -∞ as possible. In reality, -20 dB is considered a very good S11; -10 dB is a standard adopted by many RF engineers as an acceptable limit of good matching. Anything below -10 dB  is usually considered mismatched.

The impedances of the cable and the antenna are never exactly equal, so there is always some reflection. It only depends on the application or how strong of a reflection is acceptable. For WISP antennas, the reflection coefficient should be below 0.316, or in other words, below -10 dB, which corresponds to VSWR of 2. 

https://rfelements.com/

0:00 intro
0:21 VSWR
1:02 Return Loss
1:17 S11 ~ reflection coefficient
1:46 How these parameters relate to each other

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