rickman wrote:
On 7/9/2015 3:05 PM, wrote:
rickman wrote:
On 7/9/2015 1:58 PM, wrote:
rickman wrote:
On 7/9/2015 9:14 AM, Ralph Mowery wrote:
"Jeff" wrote in message
...

The SWR has to be the same at any point on the coax or transmission line
minus the loss in the line. A simple swr meter may show some differance
because of the way that kind of meter works. By changing the length of
the
line , the apparent SWR may be differant at that point.

There is no such thing as apparent SWR. It is what it is in a given
place.


By 'apparent SWR' he means as indicated SWR on the meter, and yes it can
change at various point on the line due to inadequacies in the meter; the
'real' VSWR will of course remain the same at any point on a lossless
line.

Jeff

That is what I mean Jeff. If there is any SWR, by changing the length of
the line, the voltage/current changes in such a maner that at certain points
you may get a 50 ohm match at that point.

https://en.wikipedia.org/wiki/Standi...dance_matching

"if there is a perfect match between the load impedance Zload and the
source impedance Zsource=Z*load, that perfect match will remain if the
source and load are connected through a transmission line with an
electrical length of one half wavelength (or a multiple of one half
wavelengths) using a transmission line of any characteristic impedance Z0."

This wiki article has a lot of good info in it. I have seen a lot of
stuff posted here that this article directly contradicts.... I wonder
who is right?

It has been my observation that when the subject matter is long established
science, such as transmission line theory, wiki is normally correct.

Why do you ignore it when it says Zo is the impedance of the
transmission line and not the source?

I don't; The transmission line in this case IS the source.

No, the source is the source. Even if you wish to consider transmission
line as the source in some example, the page clearly says Zo is the
impedance of the transmission line, without any context where you can
say it is a source or a load.

No, in THIS case the transmission is the source.

If you connect a transmitter to a power amplifier with cable so short
the transmission line effects are negliable, you STILL have a SWR
at the point between the two.

It is numerically irrelevant which end you designate the source and
which end you designate the load; the SWR at that point is the same
either way.

https://en.wikipedia.org/wiki/Charac...ion_line_model


The SWR in a system, any kind of system, is measured at a point in a system.

You mean a system that includes a source, a transmission line and a load?

I mean ANY system.

From a practical point of view, there is not much point in calculating
a SWR anywhere if there is not a source of energy somewhere that
eventually winds up in a load to be dissipated, but it is still a
perfectly valid calculation.

Here's an example of a more complex system one can actually find in
an Amateur station:

A transmitter is connected to a power amplifier through a coas transmission
line. The amplifier is connected through a coax transmission line to
a unbalanced to balanced transformer. The transmformer is connected to
a balanced transmission line to an antenna.

There is no SWR for the system. The SWR is meansured at a POINT in
the system.


One side of that point is the source and the other side of that point
is the load.

It also does not matter which side you declare the source and which side
you declare the load.


In that case the antenna is Zo? I don't think so.

Too bad; you are wrong.

A 50 Ohm source and a 100 Ohm load has the same SWR as a 100 Ohm source
and a 50 Ohm load.

By convention the load side is normally taken as the side which, when
the system is powered, the power is desired to be dissipated.

I don't know why you dig your heels in on every little point. Sometimes
you are just wrong and need to acknowledge that so the discussion can
move on. Zo (or Z0) is used to represent the characteristic impedance
of a transmission line. Zs (or Zsource) is used for the source. ZL (or
Zload) is used for the load. Read the wiki quote above and the many
other *clear* examples.

The source impedance is USUALLY a transmission line, but does not HAVE
to be.

The source impdedance could be the output of a matching section made
of lumped components.

WHAT the source and WHAT the load is physically does not matter.

If you are using a VNA to measure SWR, the source is whatever you connect
to the source port and the load is whatever you connect to the load port.


--
Jim Pennino