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Transmission Line Question - RF Cafe Forums
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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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