### Author Topic: How Reflections are generated in unmatched transmission line?  (Read 2016 times)

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#### Kedar264

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##### How Reflections are generated in unmatched transmission line?
« on: May 24, 2015, 03:28:18 pm »
I perticularly want to know about joints in tratransmission line ,how sigals reflection occure at joints . Thinking a lot ...

Please let me know Any related contents .

#### blueskull

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##### Re: How Reflections are generated in unmatched transmission line?
« Reply #1 on: May 24, 2015, 04:09:59 pm »
When applying a signal to a tline, since the tline has L and C, the tline will be charged, so there will be current flowing in. The square root of L/C is called characteristic impedance, the exact moment when it charges, it looks like a resistor which has a resistance of the tline's characteristic impedance.

Since it charges, energy flows into it. Since EM wave propagates at a specific velocity, when it reached an end which is not properly terminated, the exact charging current can mot be perfectly transfer to the load, so the mismatched part reflects back at the same velocity.

The reflected wave superimposing with the transmission wave created standing wave, so at some point, there will be higher voltage, while at some points there will be lower voltage.

For a short enough tline, say, much shorter than wavelength, the standing wave is not obvious, so you can ignore it. When frequency increases, the standing wave is more and more noticeable. When the tline is long enough, the standing wave reaches its max value, where VSWR equals to the ratio of |Zin| and |Zout|.

#### miguelvp

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##### Re: How Reflections are generated in unmatched transmission line?
« Reply #2 on: May 24, 2015, 04:23:06 pm »
These two videos might help:

#### hamster_nz

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##### Re: How Reflections are generated in unmatched transmission line?
« Reply #3 on: May 24, 2015, 04:42:18 pm »
Watch  and understand...

Oldie but a goodie
Gaze not into the abyss, lest you become recognized as an abyss domain expert, and they expect you keep gazing into the damn thing.

#### AlfBaz

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##### Re: How Reflections are generated in unmatched transmission line?
« Reply #4 on: May 24, 2015, 04:47:23 pm »
you can also have a look at this previous thread
https://www.eevblog.com/forum/chat/how-are-signals-reflected-in-a-wire/

#### John_ITIC

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##### Re: How Reflections are generated in unmatched transmission line?
« Reply #5 on: May 24, 2015, 04:53:45 pm »
I perticularly want to know about joints in tratransmission line ,how sigals reflection occure at joints . Thinking a lot ...

Please let me know Any related contents .

I would suggest studying the first 5 hits on Google.

#### Rupunzell

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##### Re: How Reflections are generated in unmatched transmission line?
« Reply #6 on: May 24, 2015, 05:40:51 pm »
Adding a bit of math and theory.

Spend the time to watch lecture series 5, then lecture series 6. This will give some foundational knowledge to how this transmission line stuff works.

Bernice

#### miguelvp

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##### Re: How Reflections are generated in unmatched transmission line?
« Reply #7 on: May 24, 2015, 06:21:56 pm »
Watch  and understand...

Oldie but a goodie

I love those wave machines.

#### Jeff1946

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##### Re: How Reflections are generated in unmatched transmission line?
« Reply #8 on: May 25, 2015, 06:02:32 am »
You may want to check out this web site on vibrations,

http://www.acs.psu.edu/drussell/Demos/reflect/reflect.html

to compare a wave on a string to a transmission line.  Note how the wave is inverted with a hard reflection (transmission line shorted) vs a soft reflection (transmission line open).  With perfect termination, typically 50 ohms, there is no reflection as if the transmission line went out to infinity.

Smf