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# Why do we need voltage standing wave ration?

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#### yorande

Why do we need voltage standing wave ration or the voltage reflection co-efficient when we just care for the power transport?
The maxim power transportation need conjunction impedance matching while the voltage reflection coefficient want the Zs=Zl as long as the Zl could not be treat as a resistant.
First I think that is because of the different reference plane.
But when the Zl under test is just not real, then the voltage reflection coefficient is definitely not zero if we do a conjunction impedance matching.
That's confusing.

Thanks..

VSWR is just a way to look at the mismatch. It is widely used years ago when mostly a slotted line was used for impedance matching. With slotted line you can easilly measure voltages along the line and calculate VSWR.

We need it, beacuse is VSWR was always 1.00, engineers would not have jobs.

>> Why do we need voltage standing wave ration?

...why we need impedance matching?[/quote]

The history behind this measurement is that it was easy to measure on the parallel wire transmission lines in the early days and in slot lines up to about 1965. Back then directional couplers and impedance bridges were expensive.

It has continued to be used in spite of reflection coefficient and load impedance being more directly usable parameters.

Well, the equation for Γ is
Γ=(Zs-Zl)/(Zs+Zl)
so when Γ is minimum, surely we are not doing conjunction impedance matching unless the system has only real parts like resistors.
That's what confuses me..

Op, I think I make a mistake.
Conjunction impedance matching is for maximum power transfer and VSWR is for minimum reflection. They are for 2 diofferent respect. Is it so?
Then when the imaginary part is large, we could not do the conjunction impedance matching directly, or even just can't do it?

Conjugate match is for lumped element circuits. For maximum power transfer on the load of a transmission line you need equal real and imaginary parts of the load to match the line values. Once you are above audio frequencies the line imaginary part is close to zero.

One more thing about the conjugate match is that most people fail to remember that it is for the case when the source impedance cannot be changed. In many design situations you can change the source impedance.

I think using VSWR is a tradition, from the old days. You can use reflection loss, it is the same. Traditions are everywhere, and personally I like them.
And why US uses the inch system instead of mm ? it is also a tradition. Why Britain and Australia use the ride left way, when almost all other countries use the ride right way on their roads ? There are no logical explanations for every aspect of life.

Some years ago some wag in England suggested that the switch to right side driving could be done in stages. For the first few months privately owned vehicles are driven on the left and commercially owned vehicles go on the right. Then after that time period the privately owned vehicles go on the right as well.

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