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## [Imported] Calculating important reflected power values

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# [Imported] Calculating important reflected power values

>>Message imported from previous forum - Category:Trio Data Radios<<
User: joelw, originally posted: 2018-10-16 20:35:16 Id:16
This is a re-posting from the obsoleted (October 2018) "Schneider Electric Telemetry & SCADA" forum. The original posters' identities have not been included. Schneider Electric is not responsible for the content of these posts.

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When diagnosing problems in the feedline, surge arrestor (polyphaser) or antenna it's useful to know how to compare forward and reflected power. (if you don't have direct access to VSWR or a VSWR calculator) There are three VSWR points which are useful to remember:

**1.5:1 VSWR** VSWR in a well designed system will fall below this level
**2:1 VSWR** The point above which you need to start being concerned. Put in a bit of effort to fix this.
**3:1 VSWR** There is a definite problem and you need to fix it ASAP.

**NOTE:** TView+ Diagnostics just gives the number 1.5, 2, 3 or whatever. It does not include the ratio part ":1"

But how do we know where these points are if you don't have a VSWR calculator? There's a simple rule of thumb that can help. Start with the forward power in dBm, and subtract 6 dB. That will be the 3:1 VSWR point. Subtract 10 dB and you're at the 2:1 point. Or subtract 14 dB and you're at the 1.5:1 point. So for example in a K Series radio set to 30 dBm Tx power:

3:1 VSWR = 30 – 6 = 24 dBm (250 mW)
2:1 VSWR = 30 – 10 = 20 dBm (100 mW)
1.5:1 VSWR = 30 – 14 = 16 dBm (40 mW)

In a licensed radio system with the transmitter running at the usual 37 dBm here are the numbers to watch for:

3:1 VSWR = 37 – 6 = 31 dBm (1.25 W)
2:1 VSWR = 37 – 10 = 27 dBm (500 mW)
1.5:1 VSWR = 37 – 14 = 23 dBm (200 mW)

**VSWR = Voltage Standing Wave Ratio**, a comparison of forward wave voltage to reflected wave voltage. We don't really care how this is handled by calculators which use voltage. We're more interested in power because that's what our radios report.

The VSWR calculator available on the Times Microwave website can be really useful. There are lots of them online.

Here's another useful bit of information - the equation to **calculate dBm from power:**

dBm = 10 log (Power/1 mW) with Power measured in milliwatts. (the 1 mW point is 0 dBm)

If you want to calculate VSWR (eg in a program) from forward and reflected power, here's the equation:

VSWR = (1 + Sqrt (Prefl/Pfwd)) / (1 – Sqrt (Prefl/Pfwd)

And here it is more visually:
![]((see attachments below) aj/1yx2zrh5gsng.jpg "")

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Joel A. Weder
Schneider Electric

Attached file: (editor/aj/1yx2zrh5gsng.jpg), VSWR Calculation.jpg File size: 19448  