[Imported] Reducing VSWR - and Understanding It !!

[sbeadle]

>>Message imported from previous forum - Category:Trio Data Radios<<
User: joelw, originally posted: 2018-10-16 21:28:04 Id:29
**_jweder:
What's the proper way to reduce VSWR in a radio? Well, a bit of background knowledge will help:_**

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**VSWR** = Voltage Standing Wave Ratio. If a signal is outgoing and hits any kind of obstacle (a change in impedance) then part of that signal will be reflected back instead of being absorbed into the new segment of the system. (eg polyphaser, new section of coax cable, antenna, whatever) The reflected signal, going in the opposite direction, sets up a second voltage on the conductor. So now you have two waveforms of different and varying voltages, overlapping.

We don't want there to be a change in impedance. It should be 50 ohms throughout the system. Each connector involved, each length of coax cable involved, any surge arrestor or filter, or the antenna itself, could be faulty. Perhaps the antenna is damaged, or is of the wrong frequency range so presents high VSWR at the frequency being used. Maybe one of the connectors was put on the cable incorrectly. Maybe one of the lengths of coax was damaged. Or maybe the polyphaser absorbed a lightning strike and is now faulty.

Each item in the link needs to be tested. This can be done either by swapping out each item in turn with a known-good replacement part, or by inserting an external watt meter at strategic points along the path from radio to antenna, then looking for forward vs reflected power. To understand this a bit better there's some background information below:

**The hard way...**
VSWR can be calculated by dividing the voltage going towards the antenna by the voltage reflected back. We don't typically do it this way, though, as that can require complex equipment to view. Instead we look at it in terms of power with some kind of measurement circuitry. There's a relationship between voltage and power if you know the current or impedance. Eg Voltage = Current x Impedance, or Voltage = (Current)2 x Impedance. But we don't need to worry about that, we can just stick a wattmeter in the cable and measure power, then do some math to work out VSWR.

Here is a calculation for VSWR (also known as SWR) if you have measured both forward and reflected power.
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But how can you get some idea of VSWR if you don't have a 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.
- Or subtract 10 dB and you're at the 2:1 point.
- Or subtract 14 dB and you're at the 1.5:1 point.

If there's NO reflected power the VSWR is perfect, at 1:1. But that's very unlikely in the real world!

**For example, in a radio set to 30 dBm Tx power:**
3:1 VSWR = 30 – 6 = 24 dBm (250 mW) (big problem)
2:1 VSWR = 30 – 10 = 20 dBm (100 mW) (little problem)
1.5:1 VSWR = 30 – 14 = 16 dBm (40 mW) (no problem)

**In a radio system with the transmitter running at 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)

**Here's the simple equation to calculate dBm from power:**
dBm = 10 log (Power/1 mW) where Power is the measured power in milliwatts. The radios refer to the 1 mW point as 0 dBm.

**The easy way...**
Trio radios (except M Series) can calculate VSWR if the forward power is set high enough for the measurement circuitry to work accurately. (in J/K series the forward power must be at least 20 dBm) You can then view the VSWR value (and other diagnostic information) by communicating with the radio using TView+ Diagnostics or ClearSCADA, or in the Ethernet-based radios by logging on via a web browser.

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