# transmission lines

Discussion in 'Wireless & RF Design' started by harshlogon, May 2, 2010.

1. ### harshlogon Thread Starter New Member

May 2, 2010
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plz tell me dat why current is unchanged in phase and only voltage gives phase reversal after reflection frm an an infinite transmission line???????????
why current is not reversed in phase?????????

2. ### retched AAC Fanatic!

Dec 5, 2009
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What? Do you think you can slow down, and try again? (Notice the use of a single question mark get the point across without the irritation.)

I am going to attempt to decipher your question, tell me if I get it wrong.

In an INFINITE transmission line:
What does voltage have to do with phase reversal?
What does current have to do with phase reversal?
Why would 10v 5a on a reverse phase NOT be -10v -5a?

If this is the question, I will try to answer.

Ampere(current) is defined as the number of electrons passing a point in time.

So, regardless of the direction it is going (Positive voltage is moving one way and negative voltage is moving the other way).
You are still counting the number of electrons passing one point. It doesn't matter which way, as long as they are passing a point, you are counting up.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Indeed - an infinite line has no reflections, since the incident wave never meets a discontinuity to reflect from.

4. ### harshlogon Thread Starter New Member

May 2, 2010
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sir, i was asking dat why only voltage has 180 degree out of phase after reflection from an infinite transmission line and why current is unchanged in phase after reflection ?

5. ### retched AAC Fanatic!

Dec 5, 2009
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There is no reflection in an infinite line. Thats why it is unchanged.

6. ### harshlogon Thread Starter New Member

May 2, 2010
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how phase changing takes place in an short circuited transmission line...?

i think dis can make clear my previous quest.

7. ### harshlogon Thread Starter New Member

May 2, 2010
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if we are counting up electons only so why reflection of current takes place in pen circuited transmission line and voltage is unchanged?

8. ### rjenkins AAC Fanatic!

Nov 6, 2005
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9. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Well - consider what happens at an open circuit connected to a source. There is voltage (potential difference) across the open circuit but no current flow. From the perspective of a travelling wave on a transmission line, how do we account for the zero current condition in the open circuit at the end of the line? The incident current wave coming from the source must be exactly cancelled by a reflected wave travelling back to the source. The only way for the two current waves to cancel is for them to have equal magnitude and to be in anti-phase, or 180° out of phase.

A similar argument applies to the case of the voltage wave encountering a short circuit at the end of a transmission line.

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10. ### harshlogon Thread Starter New Member

May 2, 2010
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thanx alot sir m quite satisfied by ur concept...but den also i hav so many doubts ..........lyk:if current fully cancel out den why it forms nodes and antinodes...?

11. ### Papabravo Expert

Feb 24, 2006
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I think what the OP meant by an infinite transmission line was not one of infinite length, but one that was unterminated. The lack of termination is modeled as an open circuit which of course in the ideal case has an infinite impedance.

12. ### t_n_k AAC Fanatic!

Mar 6, 2009
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That makes sense Papabravo. Curious how I can become fixated on a particular interpretation of a question - rather than trying to dig a little further to unearth the treasure being sought.

13. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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Hope y'all got a chance to read my article, "SWR Meters Make you Stupid." It's also been translated into Spanish. It goes into great detail on the infinite transmission line.

Eric