In message , Jeff writes



A load in isolation without any transmission line connected cannot have
a standing wave, but it is still common to quote the mismatch as a VSWR
which is plain wrong, but still very common.

But as I've said (nitpickingly), any length of connection (no matter how
short) where the load is not a perfect match for its characteristic
impedance, will have a very tiny portion of a standing wave on it.

And as I've also said, the normal SWR meter DOESN'T measure (respond) to
SWR. It is a reflectometer, and it responds independently to the
forward-going signal and the reverse-going signal. It's really telling
you what the return loss ratio (RLR) is - but it's still perfectly
legitimate for it to be scaled in terms of SWR. It's a darned sight
easier way of finding out what the equivalent SWR would be than to try
and measure the Vmax and Vmin 'for real' along a long line.



--
Ian

On 7/11/2015 5:38 AM, Ian Jackson wrote:
In message , Jeff writes



A load in isolation without any transmission line connected cannot
have a standing wave, but it is still common to quote the mismatch as
a VSWR which is plain wrong, but still very common.

But as I've said (nitpickingly), any length of connection (no matter how
short) where the load is not a perfect match for its characteristic
impedance, will have a very tiny portion of a standing wave on it.

And as I've also said, the normal SWR meter DOESN'T measure (respond) to
SWR. It is a reflectometer, and it responds independently to the
forward-going signal and the reverse-going signal. It's really telling
you what the return loss ratio (RLR) is - but it's still perfectly
legitimate for it to be scaled in terms of SWR. It's a darned sight
easier way of finding out what the equivalent SWR would be than to try
and measure the Vmax and Vmin 'for real' along a long line.

Why don't we use the RLR in all these measurements instead of SWR?
Isn't that what we are really after?

--

Rick

On 7/11/2015 10:49 AM, Jeff wrote:

Why don't we use the RLR in all these measurements instead of SWR? Isn't
that what we are really after?


A very good question. One possible answer is that RL is normally quoted
in dB, and VSWR linear scales are perhaps easier to envisage.

eg 3:1 ~6dB RL
2:1 ~9.5dB RL
1.5:1 ~14dB RL
1.1:1 ~26dB RL

Personally I find log scales more intuitive for most things as they more
closely relates to factors of significance, no? But I see right away
that RL scales the non-intuitive way, a larger number is a less
significant value. While SWR scales the right way with 1 being no
effect. SWR can also be given in dB which would make the numbers very
intuitive.

--

Rick

In message , Jeff writes

Why don't we use the RLR in all these measurements instead of SWR?

Tradition!

Isn't
that what we are really after?

As long as you know what you're after, and get close to it, it doesn't
really matter.


A very good question. One possible answer is that RL is normally quoted
in dB, and VSWR linear scales are perhaps easier to envisage.

eg 3:1 ~6dB RL
2:1 ~9.5dB RL
1.5:1 ~14dB RL
1.1:1 ~26dB RL

Isn't there somehow something sort-of unnatural about trying to aim for
an infinite value?


--
Ian

Ian Jackson wrote:
In message ,
writes
Ian Jackson wrote:
In message ,
writes
Ian Jackson wrote:
In message ,
writes
Ian Jackson wrote:


Even when the only transmission line consists the output connector of
the SWR meter, and maybe an inch of internal coax, there will still BE a
standing wave - but it will only be a tiny portion of longer one.

There will NOT be standing waves and there will not be a voltage
maximum and a voltage minimum unless there is a transmission line.

Are you saying that for a standing wave to qualify as a standing wave,
the transmission line needs to be long enough for there to be a voltage
maximum a voltage minimum?

I am saying that if the connection between the two things of interest
is short enough in terms of wavelengths at the frequency of interest,
the connection no longer functions as a transmission line, there are
no standing waves, but the measurment we call SWR still exists.

Pray tell me exactly (in wavelengths) when something which is too short
to be a transmission line suddenly changes into something which IS long
enough to be a transmission line.

Sure.

A transmission line is distinguished from a wire by the fact that a
transmission line carries the energy in the form of an electromagnetic
field contained by the structure of the transmission line while a
wire carries the energy in the form of conduction in the wire.

This is true for all transmission lines, be they parallel, coaxial,
wave guide, microstrip, stripline, or any other type of transmission line.

A conducting structure becomes a transmission line when it's length in
wavelengths becomes long enough to allow the establishment of an
electromagnetic field within it's structure.

The general rule of thumb is that the dividing point is about 1/10 of
a wavelength.

For the pendatic, this does NOT mean that at exactly 1/10 of a wave
length things suddenly change, it means that in general transmission
line effects become negligable below 1/10 of a wave length.

A piece of coax will not function as a transmission line at 1/100
of a wavelength even though it is constructed to be a transmission line
because it is too small to establish an electromagnetic field between
the center conductor and the shield.

Note: This is a slightly simplified explaination, for details and
mathematical derivations:

https://en.wikipedia.org/wiki/Transmission_line
http://www.antenna-theory.com/tutori...ine.php#txline
http://www.ece.uci.edu/docs/hspice/h...001_2-269.html
http://www.allaboutcircuits.com/text...nt/chpt-14/lon
g-and-short-transmission-lines/

Standing waves only occur on a transmission line and are due to reflections
on the line.

If the line length is too short to act as a transmission line, there
can be no reflections and no standing waves.

I haven't checked those references yet, but regardless of what they may
say, if that 10' of coax between my TX and my 160m ATU is NOT a
transmission line - just what IS it? Do different laws of physics
apply?

No, it means that you can view the coax as just a wire and that the
transmission line effects are negligable.

About the only practical consequence of this that I can think of off
the top of my head at the moment, is that a very short, in wavelengths,
piece of coax does not have a characteristic impedance so it would
not matter what kind of coax you use.

To put it another way, if your system is the common 50 Ohms and you
had the very best of lab grade test equipment, for very short lengths
you would see no difference between using 50 Ohm coax and 100 Ohm coax.

As you increase the length, you begin to see differences, and at some
length around 1/10 of a wavelength the differences become big enough
to be significant.


--
Jim Pennino

Jeff wrote:

There will NOT be standing waves and there will not be a voltage
maximum and a voltage minimum unless there is a transmission line.

Are you saying that for a standing wave to qualify as a standing wave,
the transmission line needs to be long enough for there to be a voltage
maximum a voltage minimum?



No, I think the point is that VSWR is the wrong quantity to be using
under those circumstances. It is possible to calculate what the swr
*would have been* IF the line had been long enough to observe a max and
min, but by its very name it is clear that it is not possible to
measure it directly and see a ratio of the standing wave due to the
shortness of the line.

No, the point is that VSWR, according to the laws of physics, can be
shown to be a voltage ratio under the conditions where such voltages
exist, AND and impedance ratio that has no dependance on line length.

In those circumstances a better solution would be to use return loss,
reflection coefficient or S11 etc. The fact that lots of people use VSWR
as a measure of a mismatch does not make it correct when it is not
possible to measure the VSWR directly by observing the ratio of the
standing wave.

It can be shown by the laws of physics the return loss, reflection
coefficient, or S11 etc. can be converted to VSWR.

Which convention you use for the measurement is relevant only to what
equipment you have on hand to do the measurement.

If you tell the grocer you want a pound of banannas and he gives you
2.2 kilograms of banannas because his scale is calibrated in kilograms,
are you going to get into a ****ing contest with him?


A load in isolation without any transmission line connected cannot have
a standing wave, but it is still common to quote the mismatch as a VSWR
which is plain wrong, but still very common.

A load in isolation has no source and is thus irrelevant to anything
in this discussion.

--
Jim Pennino

Ian Jackson wrote:
In message , Jeff writes



A load in isolation without any transmission line connected cannot have
a standing wave, but it is still common to quote the mismatch as a VSWR
which is plain wrong, but still very common.

But as I've said (nitpickingly), any length of connection (no matter how
short) where the load is not a perfect match for its characteristic
impedance, will have a very tiny portion of a standing wave on it.

I guess you could look at it that way, but the point is that such
effects are so small they are not measurable and irrelevant.

And as I've also said, the normal SWR meter DOESN'T measure (respond) to
SWR. It is a reflectometer, and it responds independently to the
forward-going signal and the reverse-going signal. It's really telling
you what the return loss ratio (RLR) is - but it's still perfectly
legitimate for it to be scaled in terms of SWR. It's a darned sight
easier way of finding out what the equivalent SWR would be than to try
and measure the Vmax and Vmin 'for real' along a long line.

Most scales do not measure weight, they measure the length of spring
extension, but they are calibrated to show pounds or kilograms.

Does that mean the measurement is not "real"?

The laws of physics allow one to both convert the forward and reverse
power measurements to VSWR and to convert spring deflection to weight.

As you say, it does not matter what a measurement device is actually
measuring, all that matters is that it is correctely calibrated to
display the information in the form you desire.


--
Jim Pennino

rickman wrote:
On 7/11/2015 5:38 AM, Ian Jackson wrote:
In message , Jeff writes



A load in isolation without any transmission line connected cannot
have a standing wave, but it is still common to quote the mismatch as
a VSWR which is plain wrong, but still very common.

But as I've said (nitpickingly), any length of connection (no matter how
short) where the load is not a perfect match for its characteristic
impedance, will have a very tiny portion of a standing wave on it.

And as I've also said, the normal SWR meter DOESN'T measure (respond) to
SWR. It is a reflectometer, and it responds independently to the
forward-going signal and the reverse-going signal. It's really telling
you what the return loss ratio (RLR) is - but it's still perfectly
legitimate for it to be scaled in terms of SWR. It's a darned sight
easier way of finding out what the equivalent SWR would be than to try
and measure the Vmax and Vmin 'for real' along a long line.

Why don't we use the RLR in all these measurements instead of SWR?
Isn't that what we are really after?

What we are really after is a convenient way to determine the quality
of an impedance match.

VSWR is about as convenient as there is.


--
Jim Pennino

Jeff wrote:

Why don't we use the RLR in all these measurements instead of SWR? Isn't
that what we are really after?


A very good question. One possible answer is that RL is normally quoted
in dB, and VSWR linear scales are perhaps easier to envisage.

eg 3:1 ~6dB RL
2:1 ~9.5dB RL
1.5:1 ~14dB RL
1.1:1 ~26dB RL

Jeff

A lot easier for most people, which is why it is so commonly used.


--
Jim Pennino

rickman wrote:
On 7/11/2015 10:49 AM, Jeff wrote:

Why don't we use the RLR in all these measurements instead of SWR? Isn't
that what we are really after?


A very good question. One possible answer is that RL is normally quoted
in dB, and VSWR linear scales are perhaps easier to envisage.

eg 3:1 ~6dB RL
2:1 ~9.5dB RL
1.5:1 ~14dB RL
1.1:1 ~26dB RL

Personally I find log scales more intuitive for most things as they more
closely relates to factors of significance, no? But I see right away
that RL scales the non-intuitive way, a larger number is a less
significant value. While SWR scales the right way with 1 being no
effect. SWR can also be given in dB which would make the numbers very
intuitive.

Perhaps the world is ready for the Rickman, where Rickman = 10 * log (VSWR).

0 Rickman = 1:1 VSWR.
1.76 Rickman = 1.5:1 VSWR.
3.01 Rickman = 2:1 VSWR.

At the very least, it would eliminate any arm waving about standing waves.


--
Jim Pennino