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Transmission Line Question - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


mikerauh
Post subject: Transmission Line Question Posted: Fri Mar 07, 2008 2:46 pm

Lieutenant

Joined: Fri Mar 07, 2008 2:29 pm
Posts: 3
In his book, Prof. Guillermo Gonzalez writes:

V(x)=V+(x) + V-(x) and

I(x)=I+(x) - I-(x)

Why is there a minus sign in the second equation? The equations say the voltage at a point on the transmission line is the sum of the forward wave voltage V+(x) and the reflected wave voltage V-(x). But the current is the difference of the current of the forward wave I+(x) and the current of the reflected wave I-(x). It seems to me the voltage and the current are vector quantities, i.e. they have a phase. So why the minus sign? Why not just sum them?

Mike Rauh


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fred47
Post subject: Posted: Fri Mar 07, 2008 4:30 pm

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
If I understand it correctly, it's to make the direction of the power flow work out correctly. Power doesn't really have a phase, but power flow definitely has a direction, and I think the signs are set up to give that.

Hope this helps!
Fred


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mikerauh
Post subject: Posted: Fri Mar 07, 2008 5:18 pm

Lieutenant

Joined: Fri Mar 07, 2008 2:29 pm
Posts: 3
Fred,

You are correct. The apparent current has to be defined as the incident wave current minus the reflected wave current to get the power relationships right.

Thank you.

Mike







Posted  11/12/2012
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