Practical Guidance on VSWR & Return Loss

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When RF energy travels through feed or transmissions lines, it is susceptible to reflections or bounces. When we send RF down a coax cable, we want it to go into the antenna and then radiate out into the world. We do not want it to bounce back towards us like an echo in an empty room.

This bounce back reflection is called “return”. Return loss is the measure of how small the “return” or reflection/echo is. We want a small return, so a large loss on the return “echo” is good. Smaller return loss is bad, and means less energy is going into our antenna.

 

Return Loss & VSWR Table

Return Loss in dB

What It Means

VSWR Number (#:1)

0 dB

100% reflection, no power into the antenna, all reflected back

Infinite

1 dB

80% reflection, 20% power into the antenna

17

2 dB

63% reflection, 37% power into the antenna

9

3 dB

50% reflection, 50% power into the antenna

6

5 dB

32% reflection, 68% power into the antenna

3.5

6 dB

25% reflection, 75% power into the antenna

3

8 dB

16% reflection, 84% power into the antenna

2.3

10 dB

10 dB (10% reflection, 90% power into the antenna)

2

15 dB

15 dB (3% reflection, 97% power into the antenna)

1.4

20 dB

20 dB (1% reflection, 99% power into the antenna)

1.2

As you can see, higher return losses mean more power into the antenna. When someone says that the antenna is poorly matched, very often it means that the VSWR value exceeds 2 for a frequency of interest.

Other components in a transmission line system contribute to small reflections of the RF power. Below are typical return losses for common components. As the RF delivery system ages, the VSWR will increase.

 

Return Loss in dB

Component

VSWR Number (#:1)

-14 dB
Antenna at resonance
1.5
-25 dB
Connectors
1.12
-35 dB
Jumpers
1.04
-25 dB
Lightning Arrestor
1.12
-30 dB
Transmission Line
1.06

 

When installing a new RF transmission system or troubleshooting existing infrastructure, using a cable & antenna analyzer can perform return loss sweeps and a distance-to-fault measurements. Both plots, as shown below, can help quickly pinpoint areas of concern. 

Distance_to_Fault.PNG

 

 

 

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