> It's not only about SWR. Any line-to-load mismatch, no
> matter how small,
> will cause the complex impedance to change at the input end
> of the line with
> changing line length, even though the line SWR does not
> change with length.
> For example, if there's a 2:1 SWR load mismatch, then
> changing the line
> length may affect the operating performance of the driving
> stage by creating
> say...a 50+j0 or some other input Z, even though the line SWR hasn't
> changed. The input Z changed, the line SWR didn't.
I think the above needs additional clarification. If there is a 2:1 load
mismatch, and if the characteristic impedance (Z0) of the transmission line
is the same as the Z the transmitter is designed to operate into (typically
50 ohms in our ham radio world), then the transmitter will be looking into a
2:1 mismatch no matter what the line length is (ignoring losses of course).
That is, there does NOT exist a point along that 50 ohm line that is 50+j0
while there is a 2:1 VSWR on it. 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.
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. The VSWR is the same at all points along the line, and
if there is VSWR on a 50 ohm line, then by definition there does NOT exist a
point along that same line where the Z is 50+j0. As you change the length
of the line, the complex Z changes, but the VSWR is always the same. Some
seem to fall into the trap of assuming that a change of Z must also mean a
change in SWR. Quite the contrary when it comes to changing the length of a
transmission line; as the length of the line changes, the complex Z at the
source end does change, but those resulting Z's all correspond to the same
VSWR referenced to the Z0 of the line.
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.
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.
> In any line mismatch condition, SWR on the line remains the
> same provided
> the characteristic Z remains the same and the line is
> low-loss. If the line
> characteristic Z changes (e.g., RG-11 to RG-213 transition),
> then the SWR on
> each portion of the line is different. Changing the line
> length on one
> section of the transition will: (1) affect the SWR on the
> other section; and
> (2) cause the complex Z measured on each end to change.
>
> Paul, W9AC
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 true.
--- Jeff WN3A
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