Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
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design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
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Transmission Line Question - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
platforms dominating public commenting venues. RF Cafe Forums began sometime around
August of 2003 and was quite well-attended for many years. By 2010, Facebook and
Twitter were overwhelmingly dominating online personal interaction, and RF Cafe
Forums activity dropped off precipitously. If the folks at
phpBB would release a version with integrated
sign-in from the major social media platforms, I would resurrect the RF Cafe Forums,
but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
Post subject: Transmission Line Question Posted: Fri Mar
07, 2008 2:46 pm
Joined: Fri Mar 07, 2008
In his book, Prof. Guillermo Gonzalez writes:
V(x)=V+(x) + V-(x) and
I(x)=I+(x) - I-(x)
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?
Post subject: Posted: Fri Mar 07, 2008 4:30 pm
Joined: Wed Feb 22, 2006 3:51 pm
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!
subject: Posted: Fri Mar 07, 2008 5:18 pm
Joined: Fri Mar 07, 2008 2:29 pm
correct. The apparent current has to be defined as the incident wave
current minus the reflected wave current to get the power relationships