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Basic VSWR question

 
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slk23
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Joined: 11 Sep 2013
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PostPosted: Thu Aug 14, 2014 4:30 pm    Post subject: Basic VSWR question Reply with quote

One thing I've been pondering about return loss testing and VSWR: if standing waves are present on the transmission line, where one measures on the line affects the result, correct? I mean, there could be a large standing wave, indicating a lot of reflected energy, but if I measure at a node in the standing wave it looks okay. Or am I missing something?

These pages are what I'm referencing:
http://www.allaboutcircuits.com/vol_2/chpt_14/6.html
http://www.allaboutcircuits.com/vol_2/chpt_14/7.html

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admin
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PostPosted: Sat Aug 16, 2014 11:27 pm    Post subject: Reply with quote

No - pretend you are looking at an open circuit (or infinite impedance) and want to measure the VSWR. You will have a standing wave - which oscillates it time at the frequency you are measuring the VSWR. So what is the result?

If you just look at the end of the line (the open circuit part), you would just see a voltage oscillating in time at the frequency f. What's the VSWR? You can't tell. But if you back up a quarter-wavelength, you will see a node - a place where the voltage has amplitude zero for all time. Hence you have a pretty good Voltage Standing Wave Ratio (VSWR) here - it is infinite.

So VSWR is not measured at a single point on the line, but rather it looks at a section of the guide to determine the VSWR. Hence, (on a lossless line) the VSWR measurement is invariant of position.
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slk23
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Joined: 11 Sep 2013
Posts: 13

PostPosted: Sat Aug 23, 2014 7:12 pm    Post subject: Reply with quote

Thanks for the reply. I'm still struggling to understand but making some progress.

In Wikipedia SWR is defined as:
Quote:
standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.


That seems to suggest that amplitude measurements need to made at different locations (antinode and node) on the line. Or, perhaps "partial standing wave" is the key. If the "standing" wave is actually moving is space and time then any point on the transmission line would see the antinode and node pass by.

This video has helped me visualize the standing wave. However, the animation at 2:15 confuses me. I follow it until 3:00 when it looks like the antinodes and nodes are actually standing (not moving in space). In that case it does matter where the VSWR measurement is made on the line and some points will have better VSWR than others.

Here's another video I found helpful.


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la7no
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Joined: 05 Jul 2014
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Location: Bergen, Norway

PostPosted: Sun Aug 24, 2014 2:26 pm    Post subject: Reply with quote

Please keep in mind that we are dealing with *waves*. That is, the voltage is always varying between max and min values.VSWR is defined as Vmax/Vmin.

If you measure the VSWR at a high voltage point with Vmax = 5V and the corresponding Vmin = 2.5V, VSWR = 5/2.5 = 2.
If you then measure VSWR at a point with a lower voltage of Vmax = 4V, the corresponding Vmin will be 2V. Giving VSWR = 4/2 = 2.

So, the VSWR will be the same everywhere on the line, provided it is loss-less. For any practical line with losses, the VSWR will always be lowest at the feed-point and highest at the load-point.

Per-Tore
LA7NO
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