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Re: [TowerTalk] Elevated vertical

To: "Towertalk" <>
Subject: Re: [TowerTalk] Elevated vertical
From: "Paul Christensen" <>
Date: Sat, 24 Dec 2011 12:35:26 -0500
List-post: <">>
> Besides, where did you get an SWR meter for a 600 ohm balanced line?   ;-) 
> Inquiring minds want to know!  LOL

Not needed.  Nor is any math involved in reaching the correct answer.  So, 
which is the correct answer?  If math or a 600-ohm VSWR meter is needed to 
answer the question, then the concept isn't understood.

Okay, I'll add some help here.  The correct answer is "D," VSWR is 1:1. 
Here's why:

An electrical half-wave of any characteristic Z line will repeat the 
termination impedance, even if complex on the opposite end of the line.  We 
started with an antenna feed-point Z of 50+j0.  Add an electrical half-wave 
of low loss 600-ohm feedline and the input impedance to the feedline is 
exactly 50+j0.  Now, connect low-loss coax with a characteristic Z of 50 
ohms to the end of the 600-ohm line.   The VSWR on *any* length of 50-ohm 
line will be 1:1 because the Z at the input to the 600-ohm section is 50+j0. 
We went from a VSWR of 12:1 on the 600 ohm line section to 1:1 on the 
coaxial section in just one simple termination without using external L or C 
components -- just using the half-wave section of line.

In this case, VSWR changes dramatically along the entire distance of line.

And, the bonus question.  Well, it was answered above.  We'll see 50+j0 at 
the input to the 600 line.

No 600 SWR meters, no math involved.  Just basic transmission line concepts.

Okay, I'm using this just to show an example of the absurdity that VSWR is 
always the same along the entire distance of the line, notwithstanding loss. 
So, probably that statement should be modified in all the related literature 
to read as follows:

"Notwithstanding line loss, VSWR on a line remains unchanged, provided the 
characteristic Z of the transmission line also remains unchanged."

Merry Christmas and Happy Hanukkah to all!

Paul, W9AC 


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