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Old August 15th 07, 03:51 PM posted to rec.radio.amateur.antenna,rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Aug 2006
Posts: 16
Default VSWR Meter and reactance

amdx wrote:

Is it possible for a VSWR meter to read 1 to 1 with reactance at the
antenna?...


The vswr is determined by the absolute value of the reflection
coefficient, being defined by:

1 + |ro|
vswr = ----------
1 - |ro|

where the greek letter "ro" indicates the complex reflection coefficient

Z - Zo
ro = ---------- .
Z + Zo

So the one and only case in which we can have unity value for vswr is
when |ro| = 0, or perfect match. In a * lossless * transmission line,
the vswr is the same in every point of the line (this is an easily
demonstrable general result, it comes right down from the above
expression), so (with normal lines) the answer is definitely no

Of course, as correctly pointed out by Cecil, in a lossy line the
reflection coefficient decreases as you move along the line away from
the termination, and the same happens to the vswr, so that a long lossy
line may simulate a good matching also when the matching isn't even near
to good (btw, using a lossy coax is an often used trick (especially in
microwaves), when you need to give to the generator a seemingly good load).

--
73 es 51 de i3hev, op. mario

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