Some of the most important antenna parameters are return loss, S11, and VSWR.

All of these are connected to one another, but let's have a first look at each of them separately.

VSWR stands for Voltage Standing Wave Ratio.

If the impedance of an antenna and the cable are different, part of the feeding signal is reflected back.

The reflected signal adds up with the feeding signal into a single wave called a standing wave.

The ratio of maximum and minimum value of the standing wave is VSWR.

Ideally, VSWR is 1 to 1, which means the impedance of the antenna and the cable are exactly equal.

In the real world, the VSWR is for example 2 to 1, meaning there is a mismatch and some of the input signal is reflected back.

If it's 3 to 1, there is an even bigger mismatch, and so on.

Return loss tells us what portion of the input power is reflected from the antenna due to the mismatch.

For example, if return loss is 30 dB, it means the reflected signal is 30 dB weaker than the input signal.

S11, also known as the 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 minus infinity as possible.

In reality, minus 20 dB is considered a very good S11.

Minus 10 dB is a standard adopted by many RF engineers as an acceptable limit of good matching.

Anything above minus 10 dB is usually considered mismatched.

These parameters relate to each other through simple equations.

S11 can be described in terms of VSWR and vice versa.

The return loss is equal to minus S11.

The impedance of the antenna and the cable are never exactly equal, and it only depends on the real world application how much of a reflection is acceptable.

In the world of RF engineering, the unwritten standard of good matching is S11 equal to minus 10 dB, in other words, the VSWR of 2.

More serious manufacturers, though, have often better standards to deliver high quality products.

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