Jeff wrote:

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

Nope, it changes in the real world due to various line losses and in
the theoretical, lossless line world it can change for several reasons.

Ever heard of a transmission line impedance transformer, which is
multiple sections of transmission lines with differing impedance?

Ever heard of a tapered impedance transmission line which is also
used to transform impedances?

The SWR at any location in a system is what it is. Bringing up
equipment errors is a straw man.


--
Jim Pennino

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.

What do you mean "if"?

There is ALWAYS a SWR in a transmission line system.

--
Jim Pennino

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.


--
Jim Pennino

Jeff wrote:
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?


That is a very specific case where the source is not at the system
impedance and happens to be equal to the load impedance, there will also
be standing waves on the transmission line and associated losses as the
VSWR on the line will be equal to the magnitude of the mismatch between
the transmission line impedance and the load impedance.

Jeff

Most people take the source impdedance to be the system impedance, i.e.
the impedance for which everything else is designed for.

To put it another way, do we design transmitters to match the antenna
and feed line or design antennas and feedline to match the transmitter?


--
Jim Pennino

Jeff wrote:
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.


Absolutely NOT. By changing the length of a transmission you will NEVER
create the situation where you get a 50 ohm match from an initial mismatch.

How about a section of transmission line with one impedance of some
length attached to a section of transmission line with a different
impedance of random length?


--
Jim Pennino

Jeff wrote:
On 09/07/2015 17:03, Ralph Mowery wrote:
"Jeff" wrote in message
...
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.


Absolutely NOT. By changing the length of a transmission you will NEVER
create the situation where you get a 50 ohm match from an initial
mismatch.

This is clearly demonstrable on a Smith chart. Take any starting point
other than a pure 50 ohms and add a length of transmission line. What you
will find is that as you increase the length of line your point will
merely rotate around the chart at a fixed radius (known as a constant VSWR
circle), it will never spiral into the centre which is 50 ohms and where
it must be for a perfect match.

The only time that it will start to spiral inwards is if the line is
lossy, but you will need a very long length, and the spiralling inwards is
due to the loss in the coax NOT any matching characteristics due to the
length of line.

If such an effect as you are talking about is observed it is merely due to
the finite, and often poor, directivity of the SWR meter giving you a
false reading.

Also it is worth noting that achieving 50 ohms as a magnitude |Z| of the
complex impedance (Sqrt(R^2+X^2)) is not the same as getting a good match
with 50 ohms resistive. Even if |Z| = 50 ohms it will have a VSWR greater
than 1 if Z0. Again, plot the point on a Smith chart and you will see
that it can never be in the centre of the chart.

Jeff

That is easy to disprove Jeff.

If I have a 50 ohm load and use a 1/2 wave of any impedance line other than
50 ohms, the swr will be greater than 1:1, except at 1/2 wave multiplies of
the line. At this point there will be a 50 ohm match. The swr of the
line will not actually be 1:1 but some greater value.


That is correct, but not the situation that we are discussing, we are
talking about matching a load to a 50 ohm transmission line. In that
case changing the length of line will NEVER result in a match. Using a
*different impedance* length of coax as a transmission line transformer
is a totally different case, and as you say will result is a standing
wave on the line and associated losses.

Jeff

So you are only interested in special cases?

--
Jim Pennino

Jeff wrote:

No. It is defined as Vmax/Vmin on the line. Show an equation that
defines SWR as the matching of the source to the line.

OK, since you insist, one more time:

SWR = (1 + |r|)/(1 - |r|)

Where r = reflection coefficient.

r = (Zl - Zo)/(Zl + Zo)

Where Zl = complex load impedance and Zo = complex source impedance.

https://en.wikipedia.org/wiki/Reflection_coefficient

http://www.antenna-theory.com/tutori...nsmission3.php



And once more time, the formula linking VSWR to Reflection coefficient
uses ONLY the MAGNITUDE of the Reflection Coefficient |r|, taking the
magnitude removes the phase component.

The magnitude DEPENDS on the phase component.

Secondly the formula linking VSWR to Reflection Coefficient is NOT a
definition of VSWR, it is merely a formula that links 2 related
quantities.

So is power P=EI or P=E^2/R?

Which is the "official" definition?


--
Jim Pennino

On 7/9/2015 1:44 PM, wrote:
Wayne wrote:


wrote in message ...

John S wrote:
On 7/8/2015 7:27 PM, Wayne wrote:


"John S" wrote in message ...

On 7/7/2015 1:44 PM, wrote:
Ian Jackson wrote:
In message , Jerry Stuckle
writes


Sure, there is ALWAYS VSWR. It may be 1:1, but it's always there.

If there's no reflection, there can be no standing wave. So, being
pedantic, there's no such thing as an SWR of 1:1!

Despite the name, VSWR is defined in terms of complex impedances
and wavelengths, not "waves" of any kind.



Actually, VSWR is defined as the ratio of Vmax/Vmin.

That's also my understanding of the definition.
In fact since SWR is defined as the maximum to minimum voltage ratio,
the "V" in VSWR is redundant.

Sort of. There is also ISWR but it is not used frequently.

# Not sort of, but is.

# There is also PSWR.

And both go back to the Vmax/Vmin definition.

The PSWR is a tricky one because you can end up with a power ratio instead
of a voltage ratio.

Actually, no, PSWR has nothing to do with power ratios as in RF power,
rather it has to do with power ratios as in values raised to the second
power.

Lol!

--

Rick

On 7/9/2015 1:46 PM, wrote:
rickman wrote:
On 7/8/2015 7:43 PM, wrote:
Ralph Mowery wrote:

wrote in message
news Ralph Mowery wrote:


Can you show any place where the SWR definition mentions the Source
impedance ?

I have several times now, but once again:

SWR = (1 + |r|)/(1 - |r|)

Where r = reflection coefficient.

r = (Zl - Zo)/(Zl + Zo)

Where Zl = complex load impedance and Zo = complex source impedance.

https://en.wikipedia.org/wiki/Reflection_coefficient

http://www.antenna-theory.com/tutori...nsmission3.php

You might check that again. I don't see Zo being defined as the complex
source impedance, but rather as the transmission line characteristic
impedance... not the same thing at all.


YOu have just proven my point. Read carefully from your refernce to
Wikipedia :

"The reflection coefficient of a load is determined by its impedance and
the impedance toward the source."

Notice it says TOWARD and not THE SOURCE.

Notice it actually says "the impedance toward the source".

From the second referaence notice that it says load impedance and impedance
of the transmission line. Nothing mentions the source at all:

What the hell do you think the transmission line is in this case if
not the source?

"The reflection coefficient is usually denoted by the symbol gamma. Note
that the magnitude of the reflection coefficient does not depend on the
length of the line, only the load impedance and the impedance of the
transmission line. Also, note that if ZL=Z0, then the line is "matched". In
this case, there is no mismatch loss and all power is transferred to the
load."

Perhaps you would like the second link better as it has pictures.

Of maybe this one that explains it all starting with lumped equivelant
circuits.

http://www.maximintegrated.com/en/ap...dex.mvp/id/742

Notice that ALL the links talk about the source impedance.

How about this one?

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

I think this has some very interesting analysis, very specifically
referring to "purely resistive load impedance".

So what?

A purely resistive anything is a special case of the general problem.

So?

--

Rick

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?

--

Rick