Knowledge Base

What is VSWR and how does it affect antenna performance?

Most antenna manufacturers include a VSWR figure in their antenna specifications. This article will explain, in simple terms, what VSWR is and how it can help you evaluate an antenna’s performance for your cell signal booster or cellular router.

A brief explanation of VSWR

An antenna sends and receives signal to and from a cellular booster, router, or other device. It does not, however, send all the signal it receives; some of that signal is reflected back to the source.

An antenna’s VSWR figure tells you how much of the signal it receives is reflected back. All other things being equal, an antenna with a lower VSWR number is going to deliver more signal to your booster or router.

A deeper explanation of VSWR

Impedance

Impedance is the opposition to the flow of electrical current as it moves through a circuit, a wire, a coaxial cable, or an antenna.

An antenna’s impedance differs—often substantially—by frequency; because of this, most antennas are designed to operate in a specified frequency range with acceptable impedance levels. Using an antenna outside of its intended frequency range results in overly strong impedance and signal reflection, so you should always use the right antenna for the right frequency.

Return loss

If the impedance of an antenna and the cable connected to it are different, part of the feeding signal (or forward power) is reflected (reverse power). Reflected power is almost always undesirable.

Return loss is the portion of the input power reflected from the antenna due to impedance mismatch.

VSWR

The feeding signal (forward power) and the reflected signal (reverse power) combine into a single wave called a standing wave.

In this animation, the green wave is the forward power, the red wave is the reflected power due to impedance mismatch, and the blue wave is the resulting standing wave:

SWR across a (lossless) line from 0x9900.com

Animation courtesy of Fred Cirera, 0x9900.com. Used with permission.

VSWR (Voltage Standing Wave Ratio, “viz‑whar”) is the ratio of the maximum (V_max) and minimum (V_min) voltage of the standing wave:

VSWR =

V_max

V_min

In the animation above, V_max = 1.5 and V_min = 0.5, so:

VSWR =

1.5

0.5

= 3

…or, expressed as a ratio, 3:1.

Ideally, the impedance of the antenna and the cable would be exactly equal, resulting in a VSWR of 1:1 with zero signal reflection. If VSWR is 2:1, there is a mismatch and some of the input signal is reflected. If it’s 3:1, there is an even greater mismatch, and so on.

An antenna’s VSWR figure will vary by frequency. Many manufacturers will give the highest VSWR figure across all the frequencies the antenna is designed for and state that the VSWR will be less than (<) or less than or equal to (≤) that number. For example:

VSWR figure of the Poynting MIMO-3-12 antenna

Other manufacturers will state the antenna’s VSWR at various frequencies

VSWR figure of the Wilson Electronics 314419 antenna

S₁₁ (reflection coefficient)

S₁₁—also known as the reflection coefficient and expressed as the Greek letter Gamma (Γ)—is the ratio of the voltage wave reflected from the antenna port to the input voltage wave.

We won’t go too deep into S₁₁; you can learn more about it in the Further reading section, below. We’re introducing it just so you can determine how much signal is reflected back at various VSWR figures.

First, we need to convert VSWR to S₁₁:

S₁₁ = |Γ| =

VSWR −1

VSWR +1

Using our example above of an antenna that has a VSWR of 3:

S₁₁ = |Γ| =

3 −1

3 +1

= 0.5

To determine what percentage of power is reflected, square the absolute value of S₁₁:

Reflected power = |Γ|²

Using our example:

|Γ|² = 0.5² = 0.25 = 25%

So, an antenna with a VSWR of 3 will reflect back 25% of the signal it receives. Keep in mind that’s the antenna’s VSWR varies across all the frequencies it’s designed for, so it’s possible that it’s reflecting less or more than 25% at a different frequency than the one we measured.

Using those formulas, we can determine how much signal is reflected back at different levels of VSWR:

VSWR

S₁₁ / Γ

Reflected power / |Γ|²

VSWR

S₁₁ / Γ

Reflected power / |Γ|²

1.00

0.00000

0.0%

2.50

0.42857

18.4%

1.10

0.04762

0.2%

2.60

0.44444

19.8%

1.20

0.09091

0.8%

2.70

0.45946

21.1%

1.25

0.11111

1.2%

2.75

0.46667

21.8%

1.30

0.13043

1.7%

2.80

0.47368

22.4%

1.40

0.16667

2.8%

2.90

0.48718

23.7%

1.50

0.20000

4.0%

3.00

0.50000

25.0%

1.60

0.23077

5.3%

3.10

0.51220

26.2%

1.70

0.25926

6.7%

3.20

0.52381

27.4%

1.75

0.27273

7.4%

3.25

0.52941

28.0%

1.80

0.28571

8.2%

3.30

0.53488

28.6%

1.90

0.31034

9.6%

3.40

0.54545

29.8%

2.00

0.33333

11.1%

3.50

0.55556

30.9%

2.10

0.35484

12.6%

3.60

0.56522

31.9%

2.20

0.37500

14.1%

3.70

0.57447

33.0%

2.25

0.38462

14.8%

3.75

0.57895

33.5%

2.30

0.39394

15.5%

3.80

0.58333

34.0%

2.40

0.41176

17.0%

3.90

0.59184

35.0%

Because the percentage of power reflected is equal to the square of S₁₁, power reflected is logarithmic:

Chart showing the percentage of reflected power of an antenna based on the square of S11

Why is VSWR important?

When comparing two antennas with the same amount of signal gain, the antenna with lower VSWR numbers in key cellular frequencies is going to deliver more signal to your booster or router.