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Re: [Amps] Ten-Tec Titan 10 Meter Input SWR

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Subject: Re: [Amps] Ten-Tec Titan 10 Meter Input SWR
From: "Paul Christensen" <>
Date: Sun, 11 Jun 2017 21:51:19 -0400
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>"It is only when the Z0 of the line is NOT the same as the source Z that we
want to achieve that the line has any chance of acting as a transformer to
yield an ideal source match.  I'm sure you know that, but it wasn't stated
in what you wrote above."

Yes, that's absolutely correct.

>"This relates to a common myth/misconception that you can "trim" your 50
ohm coax to find a sweet spot where you get a 1:1 VSWR even though the
antenna is presenting something other than a perfect match.  It just plain
does not, and can not, happen."  

Agreed, again.  As an example, a 100+j0 load fed with low-loss 50-ohm line
results in radically changing R and X values (from Z=25 to Z=100) as line
length changes on a Smith Chart but always stays in a 2:1 arc at the line
input.  To your point, a change to 50+j0 requires a different line Zo to
effectuate the transformer function.

>"With that in mind, if you have a VSWR meter that reads a different VSWR
depending on where you insert it along the line, then you need a new VSWR
meter.  Or perhaps you have a meter designed for some impedance other than
the line you're using."

More often it's neither cause but the result of the presence of RF current
on the outer surface of the outer conductor that results in SWR reading
changes along the line.  

And let's not fall into the trap of using the term "VSWR" when we're really
talking about impedance.  An antenna doesn't have "VSWR"; at any given
frequency the antenna presents some IMPEDANCE at its terminals.  If that
impedance is not the same as the transmission line connected to it, there
will be VSWR on the line while transmitting, but not necessarily when
receiving.  When transmitting, the transmitter is the source and the antenna
is the load.  When receiving, the antenna becomes the source and the
receiver the load.  VSWR on a transmission line can only be affected by a
LOAD mismatch, not a source mismatch.

>"Not quite.  If there are two sections of line with different Z0's in
series, changing the length of the section closest to the load can affect
the VSWR on the section further from the load, but the opposite is not

Yes, although I was referring to a reversal of the source and load that
probably wasn't apparent.

Good edit suggestions, Jeff!

Paul, W9AC 

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