VSWR question

[hannahs radios]

I'm having trouble really understanding VSWR I know how to read it and how to correct it which is probably the most important thing BUT what I would like to try to understand and get a grip on is this: They talk of reflectid waves does this mean that a whole software is reflected or is it just a voltage/current pulse at or near the zero crossing in a similar way to inductive and capacitive loads on 50 cycles. Hope someone can help

VSWR, best explained as the bit that doesn't get through going backward and creating a standing wave. Anything below 2:1 is fine in my book.

[hannahs radios]

Yes merlinmaxwell I get that but is it just a part of the power but a full sinewave OR just a fraction of the sinewave ie is it a full sinewave but a fraction of the power or a fraction of the wave wish I could explain it better

[GMB]

VSWR is a way of measuring how much energy is being reflected from the far end of a transmission line thus returning to the start. A fraction of the entire signal is reflected when it encounters any change of impedance.

Be careful how you interpret VSWR because if the transmission line is very long and hence has significant loss itself then the VSWR measured at the transmitter end is lower than you might expect because the returning power is itself reduced by the loss in the line.

Also the effect of it is not what it might seem, because it does not take into account how the transmitter reacts to the reflected power coming back into its mouth. I suspect this is the main real-world issue because otherwise a huge VSWR of 6 is only half the power being lost (so half an S-meter point down).

[Radio Wrangler]

There are several ways to explain waves in transmission lines, different explanations 'click' for different people.

For a piece of coax or an open-line pair, if you imagine a slice across the feeder there is only one voltage at a given instant across the line and there is only one current flowing down one conductor and back on the other one (we're ignoring the possibility of common mode stuff for the time being) The way this voltage and this current change with time and position along the cable is complicated. You can sit and do the maths and draw up equations.

Another way is to think of a longish cable and your transmitter sending a very fast burst of RF - a super fast Morse dot! The line is so long that the dot was over at the transmitter end before its beginning reached the end of the line. So just what did the transmitter think the line looked like while it was transmitting? It was pumping out the energy which will reach the load and do some work there. If you were using 50 Ohm line, the answer is simple. The transmitter thought it saw a 50 Ohm line. Because of the speed of light there hasn't been time for anything to get to the load end of the line to see what the actual load is like, and to get back to tell the transmitter about it. The transmitter was busy filling the line up, and all it could see was the line. Here is a neat way to understand the characteristic impedance of a line!

The transmitter has stopped before the burst reaches the load. So just what does the load think of it? The load only gets to see the line be the same argument. What comes out of the line is a voltage sinewave and a current sinewave in the ratio of 50 Ohms.... 50 volts of wave for every amp of wave..

If the load is itself 50 Ohms pure resistive, it says 'That'll do nicely!' and absorbs the lot. Maximum power transfer theorem, matching and all that jazz.

If the load isn't 50 Ohms pure resistive, the voltage and current hit it in the wrong ratio. It can't digest it all. It's too late that power is already coming down the line as if there was a good 50 Ohm system, but it doesn't all fit into the load. There is only one place left - to go back up the line. And the wave going back up is a voltage and current pair, again in the 50 Ohms ratio that the line insists on. Do the books at the load and all the voltages and currents all add up properly.

The reflected, smaller burst zooms up the cable and hits the transmitter. Whether it's absorbed here or a proportion gets reflected depends on the output impedance of the transmitter (the same as the reflection at the load end)

This model gives some understanding. But the transmission period is normally an awful lot longer than the line length's delay. So both the forward wave and the reflected wave can be going on simultaneously and continuously.

The reflected wave is a part of the forwards wave which turned back at the load. There may have been a bit of phase shift at the load if the load looked a bit reactive. Taking that into account looking at a place on the line, there is a time delay for the there-reflect-and-back trip to the same place. This means that down the line there are places where the forwards wave and the reflected wave add up, and places where they cancel.

VSWR is the ratio of the RF voltage at a hotspot on the line, to the RF voltage at a cold spot on the line.

Those equations describing the voltages and currents versus time and position happen to be exactly the same as equations describing the forwards and reflected wave model, and exactly the same as equations describing a moving wave form transmitter to load, plus a standing wave.

Three views, no waiting! You pays your money and you takes your choice. All three analyses work and are right, and they all come out to the same thing in the end.

You can think of things in any of the ways, but most people switch viewpoints to whichever is the one handiest for whatever they're trying to do.

Transmission lines are very very very badly taught. Half the students are scared off for life, and most of the other half are droned-off into sleep.

Most people that understand the things taught themselves!

And it's got nothing whatsoever to do with software. It existed long before software.


Hope this helps a bit.

David

[hannahs radios]

Hi David yes that helps as you say the theory we get taught is quit maths heavy and unfortunately I really struggle with maths I will read your explanation a few times and eventually my woolly brain mite get it. Thanks again

[Skywave]

VSWR, reflection coefficient and return loss are three ways of describing and measuring the same thing: the degree of mismatch between a generator and its load. There are mathematical equations which connect all three. I prefer Return Loss, since I find that to be the most easy to understand and determine of the three.

Al.

[G8HQP Dave]

You can see reflections of wave on water. You can hear reflections of sound waves. In both cases this is usually seen/heard most strongly when the reflector is hard and so reflects the incoming wave efficiently. A 'soft' reflector will not reflect much (think sound-absorbing panels). Hard and soft reflectors roughly correspond to different impedances at the end of a transmission line.

The theory of wave reflections necessarily has to include some maths, but some intuitive understanding can be had without maths.

[Radio Wrangler]

VSWR became popular a long time ago because it was easiest to measure. You got a length of transmission line at least half a wavelentth long and you ran a voltage indicating device along it. Microwave people used a length of waveguide with a slot along it and dipped in a diode detector probe. THe ratio of max to min volts = VSWR Simples!

If you are running a transmission line and are woried about voltage-caused breakdown, then VSWR is what you want to know.

Return loss is rather nice if youre thinking in terms of losses in lines.

THere are equations freely distributed, there are even little cardboard slide rules for converting these figures.

Just remember that they're just three different ways of expressing the same thing, and you'll be OK. Feel free to hop about from one form to another to suit whatever you're doing. Everybody does.

David

[ms660]

One of my favourites, similarities of wave behavior:

https://www.youtube.com/watch?v=DovunOxlY1k

Lawrence.