Sigh... This wasn't meant to be an exercise in ignoring the context!
I'm talking about a single feedline with a uniform characteristic impedence and
resonable loss, with reasonable defined as a level that does not significantly
attenuate the reflected wave. I fully realize that I've now left the term
"significantly" just dangling out there waiting to picked apart!
With respect to the context busting scenario described below: If I correctly
understand the setup, the answers are D and 50 ohms.
Merry Christmas to all!
From: Paul Christensen [mailto:firstname.lastname@example.org]
Sent: Saturday, December 24, 2011 10:38 AM
To: Al Kozakiewicz; email@example.com
Subject: Re: [TowerTalk] Elevated vertical
> "Adding a matching circuit at the transmitter (tuner) or adjusting the
> length of the feedline such that a 50 ohm resistive impedence is
> presented to the source does not change the VSWR in the feedline.
> Yes, the VSWR between the source and the tuner will be 1:1, but the
> VSWR in the feedline will remain unchanged."
Sometimes VSWR does change significantly along a line -- and not always for
reasons of loss nor common-mode RF current on the outside of a coaxial line.
Consider this example:
At the operating frequency, a dipole at its input terminals is exactly 50 ohms
resistive (50+j0). Let's feed the dipole with an electrical half-wave of
low-loss 600-ohm open line. VSWR on the line is 12:1. Correct?
Finally, let's connect a random length of 50 ohm, low-loss coax to the input of
the 600 ohm line.
What is the VSWR on the 50 ohm section of line?
A. Is it still about 12:1 because VSWR does not change on a low-loss line.
B. Not enough information because you didn't state the coax length.
C It's now about 6:1
D. It is now 1:1
E. You can't terminate a coaxial line into a 600-ohm balanced line without a
current balun and get an answer.
F. None of the above.
Bonus question: What is the impedance at the input to the 600-ohm line section?
So, here we have one transmission line composed of two types. Ignoring loss,
is the VSWR really the same at all points on the transmission line?
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