SWR - wtf?
 

I don't see the original posting here on rec.radio.amateur, but there
are a few misconceptions in the followups which should be addressed.

Lancer wrote:
On Tue, 28 Jun 2005 19:13:35 GMT, james wrote:


On Tue, 28 Jun 2005 18:31:58 GMT, Lancer wrote:


To have the measured SWR change with coax length, means you have
current flowing on the outside of the coax. Your coax then becomes
part of the antenna, so changing its length is changing the antenna
length. This would change the feedpoint impedance and the SWR.

That's correct, except that coax loss will also cause the SWR to change
with coax length. Loss will cause the SWR at the antenna (load) to
always be greater than at the transmitter (source).


Unless the line is carrying common mode currents that affect antenna
impedance, changing coax length won't change the SWR, even if the
antenna isn't matched.

Again correct except for overlooking the effect of coax loss.

But there's a real problem in communicating this. If you hook a 50 ohm
SWR meter to the input of a 75 ohm, 300 ohm, or line of any impedance
other than 50 ohms, the meter reading won't be the SWR on the
transmission line. That can mislead people into thinking that the SWR is
changing with line length when it actually isn't.


********

BS

Common mode currents on the shield of coaxial cables do not alter the
feed impedance. Repeat ofter me. Common mode currents on the shield of
coaxial cables do not alter the feed impedance.

Why repeat it if it isn't true? The explanation given by Lancer was
correct. If you change the length of the antenna, the feedpoint
impedance will change. When you have common mode current flowing on the
feedline, the feedline is part of the antenna; changing its length is
changing the antenna's length.


The feed impedance of an antenna is solely determined by its physical
length and any load impedances within the antenna structure. Load
impedances can be stray capacitance with ground via metal objects
within the near field of the antenna or even a building.

You have to realize that a radiating feedline (one carrying common mode
current) IS part of the antenna structure.


The "Magic" of an electrical halfwave transmission line is at a
precise frequency, the reflection of the load to the transmistter is
equal to the characteristic impedance of the transmission line
irregardless of what impedance it is terminated with.

This is true only of a lossless line. If the load impedance isn't far
from the line's characteristic impedance (i.e., the line's SWR is low),
a small amount of loss won't make much difference. However, if the line
SWR is high, even a small amount of loss can make a major change in the
impedance seen at the line's input. The effect is to skew the impedance
toward the line's Z0.

Other lengths
have the load impedance reflected back and transformed by the length
of the coax. The coax then acts as a transformer. It will either step
up or step down the impeadnace of the load depending on the load
itself and the electrical length of the coax.

It's a little more complicated than that. The line doesn't simply
multiply or divide the impedance by a constant, like a transformer --
except in the special case of a quarter electrical wavelength line or
odd multiples thereof. In other cases, the line does transform the
impedance, but in a complex way in which the resistance and reactance
are transformed by different factors. And reactance can be present at a
line's input even when the load is purely resistive. A Smith chart is a
good visual aid in seeing what happens. Assuming a lossless line, the
impedance traverses a circle around the origin. The radius of the circle
corresponds to the line's SWR. With the chart, you can see all the
combinations of R and X which a given line can produce with a given load
by changing its length. Incidentally, loss causes the impedance to
spiral inward toward the origin as the line gets longer, showing how
loss skews the input impedance toward Z0.


All a tuner does is electrically lengthen or shoten the coax by
introducing a lumped LC constant that helps present a resistive load
to the transmitter. The SWR at the feedline does not change. By
placing various different lengths of coax inline, you do the same
thing a tuner does, add a lumped LC constant.

As can be seen from the Smith chart, you can produce only particular
combinations of R and X by changing the length of a line which has a
given load impedance. Unless you're unusually lucky or have planned
things carefully, none of these combinations will result in 50 + j0
ohms, the usual goal, at the line's input. In contrast, a tuner is able
to adjust both R and X to produce, if designed right for the
application, 50 + j0 for a wide range of load impedances.

It requires at least two adjustable components to achieve an impedance
match from an arbitrary load impedance, because there are two separate
quantities, R and X or impedance magnitude and phase, which have to be
adjusted. Changing the line length is only one adjustment, so it can't
be guaranteed to provide a match. If you could also change the line's
Z0, for example, or the length of a stub, you'd have two adjustments and
you could guarantee a match providing you have enough adjustment range.


james


So thats all my tuner does, lengthen or shorten the coax?

Are you sure about that?

Rest assured, that's not all it does.

Roy Lewallen, W7EL

(I've snipped parts of Roy's original posting, indicated by ..., that I
hope are not particularly relevant to my added comments.)

Roy Lewallen wrote:
I don't see the original posting here on rec.radio.amateur, but there
are a few misconceptions in the followups which should be addressed.

....


Unless the line is carrying common mode currents that affect antenna
impedance, changing coax length won't change the SWR, even if the
antenna isn't matched.

Again correct except for overlooking the effect of coax loss.

But there's a real problem in communicating this. If you hook a 50 ohm
SWR meter to the input of a 75 ohm, 300 ohm, or line of any impedance
other than 50 ohms, the meter reading won't be the SWR on the
transmission line. That can mislead people into thinking that the SWR is
changing with line length when it actually isn't.


In addition, most hams (and other non-professionals -- and even many
professionals) don't bother to check that their SWR meter is properly
calibrated to the impedance they think it is. Most are nominally 50
ohms, but they can be built for any practical line impedance. Checking
calibration is not all that difficult, if you take the time to do it.
In addition, your nominally 50 ohm line (or 75 or whatever) can have an
actual impedance 10% or more from the nominal value. If you have
properly calibrated your meter to 50 ohms, and your line is 60 ohms,
you would read 1.2:1 SWR when your line is actually 1:1. And if the
SWR on the 60 ohm line is 1.2:1, that 50 ohm SWR meter can read
anything between 1:1 and 1.44:1, depending on the line length and its
load. Finally, though you may have checked that the meter to reads 1:1
with a 50 ohm load and infinity to 1 with a short or open load, the
construction of inexpensive meters may cause them to have significant
errors at other load impedances.

....



The "Magic" of an electrical halfwave transmission line is at a
precise frequency, the reflection of the load to the transmistter is
equal to the characteristic impedance of the transmission line
irregardless of what impedance it is terminated with.

This is true only of a lossless line. If the load impedance isn't far
from the line's characteristic impedance (i.e., the line's SWR is low),
a small amount of loss won't make much difference. However, if the line
SWR is high, even a small amount of loss can make a major change in the
impedance seen at the line's input. The effect is to skew the impedance
toward the line's Z0.

The piece that Roy quoted is so outrageous that I can easily believe he
didn't read it right, but I've re-read it several times, and it keeps
coming out the same: the "magical" halfwave line does NOT reflect an
impedance to the source (transmitter) equal to the LINE impedance as
the quoted section says, but it reflects the LOAD impedance (altered by
line loss as Roy says).

....

about tuners, Roy went on to write:

It requires at least two adjustable components to achieve an impedance
match from an arbitrary load impedance, because there are two separate
quantities, R and X or impedance magnitude and phase, which have to be
adjusted. Changing the line length is only one adjustment, so it can't
be guaranteed to provide a match. If you could also change the line's
Z0, for example, or the length of a stub, you'd have two adjustments and
you could guarantee a match providing you have enough adjustment range.

In addition, two adjustable components in a particular configuration,
even if they are infinitely adjustable (and reasonably close to
lossless!!--a very tall order!) won't necessarily give you the ability
to transform any arbitrary impedance to 50 ohms. There may be whole
practical areas of the complex impedance plane left untransformable.
Also, the efficiency of a particular tuner topology for a given load
impedance may be very good or may be terrible, when using practical
components in the tuner. To reiterate what Roy wrote, it's important
to use the right topology for the job you need to do.

Cheers,
Tom




james


So thats all my tuner does, lengthen or shorten the coax?

Are you sure about that?

Rest assured, that's not all it does.

Roy Lewallen, W7EL

Thanks to Tom for the comments and additions.

. . .

[I've lost track of who said this:]

The "Magic" of an electrical halfwave transmission line is at a
precise frequency, the reflection of the load to the transmistter is
equal to the characteristic impedance of the transmission line
irregardless of what impedance it is terminated with.


[Roy:]

This is true only of a lossless line. If the load impedance isn't far
from the line's characteristic impedance (i.e., the line's SWR is low),
a small amount of loss won't make much difference. However, if the line
SWR is high, even a small amount of loss can make a major change in the
impedance seen at the line's input. The effect is to skew the impedance
toward the line's Z0.


[Tom:]

The piece that Roy quoted is so outrageous that I can easily believe he
didn't read it right, but I've re-read it several times, and it keeps
coming out the same: the "magical" halfwave line does NOT reflect an
impedance to the source (transmitter) equal to the LINE impedance as
the quoted section says, but it reflects the LOAD impedance (altered by
line loss as Roy says).
. . .

Wow, I certainly read that (top quote) too quickly. Tom is absolutely
right, as written it's very wrong, and I misread it. I retract my
statement about it's being "true only of a lossless line" -- of course
it's not true at all, but works as Tom says.

Roy Lewallen, W7EL

On 28 Jun 2005 17:51:10 -0700, "K7ITM" wrote in
. com:

snip
But there's a real problem in communicating this. If you hook a 50 ohm
SWR meter to the input of a 75 ohm, 300 ohm, or line of any impedance
other than 50 ohms, the meter reading won't be the SWR on the
transmission line. That can mislead people into thinking that the SWR is
changing with line length when it actually isn't.


In addition, most hams (and other non-professionals -- and even many
professionals) don't bother to check that their SWR meter is properly
calibrated to the impedance they think it is. Most are nominally 50
ohms, but they can be built for any practical line impedance. Checking
calibration is not all that difficult, if you take the time to do it.
In addition, your nominally 50 ohm line (or 75 or whatever) can have an
actual impedance 10% or more from the nominal value. If you have
properly calibrated your meter to 50 ohms, and your line is 60 ohms,
you would read 1.2:1 SWR when your line is actually 1:1. And if the
SWR on the 60 ohm line is 1.2:1, that 50 ohm SWR meter can read
anything between 1:1 and 1.44:1, depending on the line length and its
load. Finally, though you may have checked that the meter to reads 1:1
with a 50 ohm load and infinity to 1 with a short or open load, the
construction of inexpensive meters may cause them to have significant
errors at other load impedances.


Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength. Regardless, I'm not a big fan of SWR
meters -- they are good for detecting a major malfunction but that's
about it. Antenna tuning/matching is best done with a field strength
meter.







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Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength.

Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.
--
73, Cecil http://www.qsl.net/w5dxp


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On Tue, 28 Jun 2005 15:53:03 -0700, Roy Lewallen
wrote:

To have the measured SWR change with coax length, means you have
current flowing on the outside of the coax. Your coax then becomes
part of the antenna, so changing its length is changing the antenna
length. This would change the feedpoint impedance and the SWR.

That's correct, except that coax loss will also cause the SWR to change
with coax length. Loss will cause the SWR at the antenna (load) to
always be greater than at the transmitter (source).


Would the changing of the coax lead to moving the SWR meter to a
different voltage point on the coax?

--
73 for now
Buck
N4PGW

On Tue, 28 Jun 2005 20:49:46 -0700, Frank Gilliland
wrote:

I'm not a big fan of SWR
meters -- they are good for detecting a major malfunction but that's
about it. Antenna tuning/matching is best done with a field strength
meter.


A local retired instructor of some sort (military, i believe) has the
same opinion. He doesn't like SWR meters but instead measures all his
antennas by field strength meter.

I used to tune my Swan with one. I found when I used an SWR meter,
the minimum SWR dip was NEVER the maximum field strength reading. I
always had to raise the SWR to about 1.3:1 or so.

Around here, most of us know not to mention the performance of an
antenna to him if we only used an SWR meter or antenna analyzer. His
first question is "How did it do with the FSM?"

I believe he is right. Radios drop power when they don't like the SWR
and raise it when it does.

73
N4PGW

--
73 for now
Buck
N4PGW

On Wed, 29 Jun 2005 01:04:59 -0400, Buck wrote:


I used to tune my Swan with one. I found when I used an SWR meter,
the minimum SWR dip was NEVER the maximum field strength reading. I
always had to raise the SWR to about 1.3:1 or so.

The probable reason for maximum power output not coinciding with the
plate current dip is imperfect neutralisation (whether or not the
stage is neutralised). If you have a neutralisation adjustment, you
can get the two to coincide by properly adjusting the neutralisation.

You can use this coincidence as a quite sensitive indication of
optimal neutralisation if you use a digital meter to monitor plate
current, and the power out indicator to monitor RF output power.
Adjust the tuning and loading for rated power into a dummy load, check
the tune cap is peaked for max Po, observe the plate current,
carefully dip the plate current, noting whether the dip was left or
right of max Po. Now tweak the neutralisation until the two coincide.

When it is all done properly, the "dip" of the plate current should be
symmetric, ie it should be as "sharp" approaching from one side as the
other. (Asymetric dips are another symptom of non-omptimal
neutralisation.

Owen
--

Buck wrote:

Would the changing of the coax lead to moving the SWR meter to a
different voltage point on the coax?

Sort of, but not exactly. Let's take an example. I'll keep the values
purely real to help folks who aren't familiar with complex math, but
keep in mind that these are special cases and a full treatment would be
somewhat more involved. I'll also make all the transmission lines
lossless to simplify things.

An SWR meter really just provides another way of reporting the impedance
it sees. You can verify this by connecting pure resistances of various
values to its output. For example, a (properly calibrated and operating)
50 ohm SWR meter will report 1:1 if you connect its output to a 50 ohm
resistor. If you connect it to either a 25 or 100 ohm resistor, it
reports 2:1. It does this despite the fact that there's no transmission
line at all connected to its output. Some people can put up a huge
smokescreen and waving of hands about reflected waves of one kind or
another, but at the end of the day the SWR meter can't tell the
difference between a resistor and a transmission line terminated with a
load, if the impedances the meter sees are the same. It's sensitive only
to impedance; it has no way of knowing even if a transmission line is
connected to its output, let alone what the transmission line's SWR or
even characteristic impedance is.

Now put a half wavelength piece of 50 ohm coax between the SWR meter and
those resistors. The SWR meter will still see the same impedances as
before, so it'll report the same SWRs. Now, though, there really is a
transmission line connected to its output. And because the meter is a 50
ohm meter and the line has a 50 ohm Z0, the SWR meter reading is the
same as the actual SWR on the line. When the load is 50 ohms, the line's
SWR is 1:1 and the meter sees 50 ohms so it reports 1:1. When the load
is 25 ohms, the line's SWR is 2:1, and the meter sees 25 ohms and
reports 2:1. When the load is 100 ohms, the line's SWR is 2:1, and the
meter sees 100 ohms and reports 2:1.

Next experiment: Connect the SWR meter through a *quarter* wavelength of
50 ohm line to a 100 ohm load. Now the impedance looking into the line
is 25 ohms instead of 100. But the SWR meter reads 2:1 when it sees 25
ohms as well as 100, so it still reads 2:1, which is also still the SWR
on the 50 ohm line. You can change the length of the 50 ohm line all you
want and, if it's lossless, the line's actual SWR stays the same -- but
the impedance at the input end of the line changes. For a 100 ohm load,
when the line is any even number of half wavelengths long, the input Z
is 100 ohms. When the line is any odd number of quarter wavelengths
long, the input Z is 25 ohms. At other lengths, the impedance is both
resistive and reactive, but the line's SWR is always 2:1. And the SWR
meter interprets all these possible impedances as 2:1, and that's what
it reads. The line SWR doesn't change as you change its length, and the
SWR meter reading doesn't change, either.

Now instead of a 50 ohm line, let's connect a half wavelength 100 ohm
line to the output of the same 50 ohm SWR meter and hook that to a 50
ohm resistive load. The line's actual SWR is 2:1 and, just like any
lossless line, the SWR stays the same regardless of its length. If the
transmission line is an even number of half wavelengths long we'll have
50 ohms at the input and the SWR meter will read 1:1, since it's a 50
ohm meter and interprets 50 ohms as 1:1. If we change the line length to
a quarter wavelength, the input impedance will be 200 ohms, which the 50
ohm SWR meter will interpret and report as 4:1. So by changing the line
length from a half to a quarter wavelength we've changed the SWR meter
reading from 1:1 to 4:1, even though the line's actual SWR was 2:1 all
along. That's what I was talking about. The SWR meter makes assumptions
about the SWR on the line from the impedances it sees. The line
transforms the load impedance in a different way than a 50 ohm line
would. The SWR meter then assumes an incorrect SWR value for the line,
and this incorrect value changes as the line length changes.

The same thing happens if the line has a 50 ohm characteristic impedance
and the meter is designed for some other Z0. The lesson is that an SWR
meter shows the actual SWR on a transmission line connected to its
output only if the SWR meter is designed for the same Z0 as the line.

Too often, people say "The SWR is. . .", but really mean "The SWR meter
reading is. . .". As you've seen, the two can often be very different.
When you see the SWR reading changing as you change the line length, it
doesn't necessarily mean that the line's SWR is actually changing.

Remember, in the preceding discussion I've assumed for simplicity that
all lines were lossless. In the real world, no line is, so the actual
line SWR will always be higher at the load than the source (unless of
course it's 1:1 at the load).

Roy Lewallen, W7EL

Roy, to cut things short, why don't you just say SWR meters don't
measure SWR on anything. All they do is indicate whether or not the
transmitter is terminated with its correct load resistance. So they
are quite useful.

They won't even tell you what the load resistance actually is unless
the load is exactly correct.

Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).
----
Reg, G4FGQ

Reg Edwards wrote:
Roy, to cut things short, why don't you just say SWR meters don't
measure SWR on anything. All they do is indicate whether or not the
transmitter is terminated with its correct load resistance. So they
are quite useful.

They won't even tell you what the load resistance actually is unless
the load is exactly correct.

Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).

Or - recalling that what the meter actually measures is the reflection
coefficient - why not go back to the old name of "Reflectometer"?


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Thanks very much to Owen for pointing out the following errors in my
recent posting:

Roy Lewallen wrote:
. . .
Next experiment: Connect the SWR meter through a *quarter* wavelength of
50 ohm line to a 100 ohm load. Now the impedance looking into the line
is 25 ohms instead of 100. But the SWR meter reads 2:1 when it sees 25
ohms as well as 100, so it still reads 2:1, which is also still the SWR
on the 50 ohm line. You can change the length of the 50 ohm line all you
want and, if it's lossless, the line's actual SWR stays the same -- but
the impedance at the input end of the line changes. For a 100 ohm load,
when the line is any even number of half wavelengths long, the input Z
is 100 ohms. . .

That last sentence should be "For a 100 ohm load, when the line is *any
whole number* of half wavelengths long, the input Z is 100 ohms."

Likewise,

Now instead of a 50 ohm line, let's connect a half wavelength 100 ohm
line to the output of the same 50 ohm SWR meter and hook that to a 50
ohm resistive load. The line's actual SWR is 2:1 and, just like any
lossless line, the SWR stays the same regardless of its length. If the
transmission line is an even number of half wavelengths long we'll have
50 ohms at the input and the SWR meter will read 1:1, since it's a 50
ohm meter and interprets 50 ohms as 1:1.

"an even number of half wavelengths" should be "any whole number of half
wavelengths".

I appreciate the corrections, and encourage anyone who spots errors to
bring them to my attention, or the newsgroup's.

Roy Lewallen, W7EL

Steveo wrote:
if you have over a 2:1 standing wave you can do damage to your finals
or linear

Depends on what one is running. My IC-706 folds back
automatically and protects itself. My SGC-500 linear
is advertised to tolerate an SWR of up to 6:1.
--
73, Cecil http://www.qsl.net/w5dxp


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Roy Lewallen wrote:
Some people can put up a huge
smokescreen and waving of hands about reflected waves of one kind or
another, but at the end of the day the SWR meter can't tell the
difference between a resistor and a transmission line terminated with a
load, if the impedances the meter sees are the same. It's sensitive only
to impedance;

A 20K ohms/volt Simpson may yield an irrelevant screen voltage
reading for a pentode because it loads the circuit down. Hand
waving aside, any instrument can be misused.

An SWR meter designed and calibrated for a Z0=50 standing-wave
environment may yield an irrelevant reading when operated outside
of a Z0=50 ohm standing-wave environment.
--
73, Cecil http://www.qsl.net/w5dxp


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Reg Edwards wrote:
Roy, to cut things short, why don't you just say SWR meters don't
measure SWR on anything. All they do is indicate whether or not the
transmitter is terminated with its correct load resistance. So they
are quite useful.

Reg, how about my 450 ohm SWR meter? It just sits there
reading somewhere between 6:1 and 12:1.
--
73, Cecil http://www.qsl.net/w5dxp


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Ian White GM3SEK wrote:

Reg Edwards wrote:
Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).

Or - recalling that what the meter actually measures is the reflection
coefficient - why not go back to the old name of "Reflectometer"?

Trouble is, during steady-state, they only measure the virtual
reflection coefficient which is itself confusing since it is not
the same as the physical reflection coefficient measured by a TDR.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote:
An SWR meter designed and calibrated for a Z0=50 standing-wave
environment may yield an irrelevant reading when operated outside
of a Z0=50 ohm standing-wave environment.
___________________

Elaborating, an SWR meter will produce ~ accurate readings for an unknown
termination connected to it via a lossless transmission line of any length,
as long as that line has the same Zo as the sample section in the SWR meter.

It is only when the transmission line Zo varies from the Zo of the SWR meter
line section that accurate measurement of load SWR is problematic.

Selecting line lengths and line impedances to make an SWR meter and/or tx
"happy" when connected to an antenna doesn't necessarily mean that the all
components in the r-f output system have low SWR. The tx may be able to
deliver more power to the net load under those conditions, but SWR may still
exist on the transmission line capable of causing its failure.

RF

"Ian White wrote -
Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).

Or - recalling that what the meter actually measures is the
reflection
coefficient - why not go back to the old name of "Reflectometer"?

===================================

It does NOT read the reflection coefficient. It reads only half of
it. At least half of the information lies in the angle of the RC -
which is disregarded, ignored, by the so-called meter.

The magnitude of the RC without its angle is just another worthless
number. It can't be used for anything except to calculate a fictional
SWR.
----
Reg, G4FGQ

Reg Edwards wrote:
The magnitude of the RC without its angle is just another worthless
number. It can't be used for anything except to calculate a fictional
SWR.

Actually, it is pretty useful for a Z0-matched system
since there is one and only one unique solution at the
Z0-match point.

In a Z0-matched system, all forward and reflected voltages
and currents are at the reference zero degrees or at 180
degrees so all the phase angles are known without measuring
them. The physical reflection coefficient is a function of
Z01 and Z02 at a Z0-match point. The sign of the reflection
coefficient corresponds to either zero degrees or 180 degrees
and depends on whether (Z01 Z02) or (Z01 Z02).

Since the great majority of amateur radio systems are close
to a Z0-match, this becomes a useful analysis tool for the
most common cases.
--
73, Cecil http://www.qsl.net/w5dxp


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What is the reason a 2:1 SWR can cause such havoc?

How can I avoid this catastrophic condition?

I feed my dipoles with 450 Ohm ladder line, but the last 20 feet or so is 50
Ohm coax, I guess that makes it work ok. I haven't blown up my finals yet.

Lions and tigers and bears Oh my...

"Steveo" wrote in message
news:nceoaqqpc0a3yzz.280620052102@kirk...
if you have over a 2:1 standing wave you can do damage to your finals
or linear

On Tue, 28 Jun 2005 22:15:18 GMT, Lancer wrote:

So thats all my tuner does, lengthen or shorten the coax?

Are you sure about that?
****

Essentially yes. Without having to go into detailed mathematics, it is
the simplest form to explain what is happening.

james

On Tue, 28 Jun 2005 15:53:03 -0700, Roy Lewallen
wrote:

It's a little more complicated than that. The line doesn't simply
multiply or divide the impedance by a constant, like a transformer --
except in the special case of a quarter electrical wavelength line or
odd multiples thereof.
***

Roy

I believe this thread started on rec.radio.cb


and yes your correct here. I just did not want to get into great
details on quarter wave sections and uses of transmission lines as
lumped elements. I though that was beyond the scope of the original
post. I am kind of sorry that I even mentions what I did.

james

Balderdash!

"james" wrote in message
...
On Tue, 28 Jun 2005 22:15:18 GMT, Lancer wrote:

So thats all my tuner does, lengthen or shorten the coax?

Are you sure about that?
****

Essentially yes. Without having to go into detailed mathematics, it is
the simplest form to explain what is happening.

james

if you have over a 2:1 standing wave you can do damage to your finals
or linear

Oh brother. Here we go again with more nonsense and myths about SWR!

73 Tom

Reg Edwards wrote:

Roy, to cut things short, why don't you just say SWR meters don't
measure SWR on anything.

He did, he just felt compelled to add a page or so of explaination.

All they do is indicate whether or not the
transmitter is terminated with its correct load resistance. So they
are quite useful.

Well, to be picky all they do is indicate the divergence from the Zo of
the meter, in a certain way. If the TX is designed for that Zo then
they do what you say, and they're quite useful.

They won't even tell you what the load resistance actually is unless
the load is exactly correct.

Yes

Stop fooling and confusing yourselves. The solution to everybody's
problems is simple - just change the name of the thing to TLI.
(Transmitter Loading Indicator).

Hmm. I'll stick with SWR meter -- which it is if it's used properly.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Buck wrote:

I believe he is right. Radios drop power when they don't like the SWR
and raise it when it does.

That depends on the radio.

Without SWR protection one with a class C final, like the ones in the
little QRP rigs I have lying around, will deliver more power if it sees
a lower RF impedance at the final transistor than it was designed for.
It will also overheat said final transistor* and possibly damage it.
This will happen for some SWR mismatches but not all.

Again without SWR protection one with a class AB or B final, properly
tuned for a 50 ohm resistive load, will deliver less power for some
mismatches (the same mismatches that would be _higher_ power for the
class C final). With other mismatches it would exhibit higher gain but
more distortion. At some mismatch and level of drive you could probably
expose the finals to too much power dissipation, too high an RF current
or too high an RF voltage, and do damage. This depends _heavily_ on the
design of the final stage.

With SWR protection, of course, the radio will automatically back off,
perhaps even in a way that will do some good.

So I will use an SWR meter to keep the transmitter happy, and a field
strength meter to make sure my antenna is doing it's job.

* assuming I got the heatsink design right, neither overly conservative
nor overly optimistic.

-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

james wrote:
Lancer wrote:
So thats all my tuner does, lengthen or shorten the coax?

Are you sure about that?

Essentially yes. Without having to go into detailed mathematics, it is
the simplest form to explain what is happening.

If you include the possibility of changing the Z0 of the
coax as well as the length, you will be closer to the truth.
--
73, Cecil http://www.qsl.net/w5dxp


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Buck wrote:

I believe he is right. Radios drop power when they don't like the SWR
and raise it when it does.

This is an illustration of a common problem. It's really improper
*impedances* presented to the radio that disturb it; it doesn't know or
care about the actual SWR on whatever transmission line may or may not
be connected. Transmitters typically specify and show this load
impedance as "SWR". But they can't tell the difference between a half
wavelength 50 ohm line with 100 ohm load, which has a line SWR of 2:1;
any length of 100 ohm line with a 100 ohm load, which has a line SWR of
1:1; a quarter wavelength of 300 ohm line with a 900 ohm load, which has
a line SWR of 3:1; or a 100 ohm resistor. All these and an infinite
number of other combinations will present 100 ohms to the rig, all will
cause the rig's SWR meter to read 2:1, and all will have exactly the
same effect.

Roy Lewallen, W7EL

On Tue, 28 Jun 2005 21:01:52 -0700, "Steveo"
wrote:

if you have over a 2:1 standing wave you can do damage to your finals
or linear

*****

Only if you are exceeding the power dissapation of the devices.
Considering the way most CBers use radios and amplifiers, your
statement maybe more ture than false.

Generaly if the power disapation of the finals is not exceeded and
there is sufficient margin to handle the reflected power, it will just
dissapate as heat in the output circuits and the fianls. Then this is
only true if the reflection coefficient of the radio is zero. If other
than zero then there will be some of the antenna power reflected back
to the transmitter reflected back to the load. Then the rest is
dissapated as heat. Then you get all kinds of funny things happening
inside the coax. But that is another subject.

james


"Frank Gilliland" wrote in message
.. .
On 28 Jun 2005 17:51:10 -0700, "K7ITM" wrote in
. com:

snip
But there's a real problem in communicating this. If you hook a 50
ohm
SWR meter to the input of a 75 ohm, 300 ohm, or line of any
impedance
other than 50 ohms, the meter reading won't be the SWR on the
transmission line. That can mislead people into thinking that the
SWR is
changing with line length when it actually isn't.


In addition, most hams (and other non-professionals -- and even many
professionals) don't bother to check that their SWR meter is
properly
calibrated to the impedance they think it is. Most are nominally 50
ohms, but they can be built for any practical line impedance.
Checking
calibration is not all that difficult, if you take the time to do
it.
In addition, your nominally 50 ohm line (or 75 or whatever) can have
an
actual impedance 10% or more from the nominal value. If you have
properly calibrated your meter to 50 ohms, and your line is 60 ohms,
you would read 1.2:1 SWR when your line is actually 1:1. And if the
SWR on the 60 ohm line is 1.2:1, that 50 ohm SWR meter can read
anything between 1:1 and 1.44:1, depending on the line length and
its
load. Finally, though you may have checked that the meter to reads
1:1
with a 50 ohm load and infinity to 1 with a short or open load, the
construction of inexpensive meters may cause them to have
significant
errors at other load impedances.


Impedance matching of an SWR meter is generally unimportant since
most
SWR meters used for HF have a directional coupler that is much
shorter
than the operating wavelength. Regardless, I'm not a big fan of SWR
meters -- they are good for detecting a major malfunction but that's
about it. Antenna tuning/matching is best done with a field strength
meter.







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On Wed, 29 Jun 2005 11:07:17 -0400, "Fred W4JLE"
wrote:

What is the reason a 2:1 SWR can cause such havoc?

How can I avoid this catastrophic condition?

I feed my dipoles with 450 Ohm ladder line, but the last 20 feet or so is 50
Ohm coax, I guess that makes it work ok. I haven't blown up my finals yet.

Lions and tigers and bears Oh my...
*****

Actually can happen if you push the finals to where there is
insufficeint margin to the maximum heat dissapation. Tubes are a bit
more forgiving. Transistor inadequately heatsinked and overdriven,
typical CB usage, often have little of no margin for heat dissapation.

If the transmitter has a refelction coefficient of zero and the load
say .3, then that reflected power from the load is dissapated as heat
in the output circuits and any final transistors or tubes. Now if the
radio has a reflection coefficient other than zero that will lessen
the heat dissapation on the transimiiter. Now you get load and source
reflections convoluting within the transmission line.

You ought to model a 400 Mhz square wave with source and load
refelctions coefficients other than zero. It can get ugly


james

On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength.

Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.


The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength. The error is even more
insignificant when there are a host of variables and confounds between
the SWR meter and the transmitted field that can (and frequently do)
affect the objective -- field strength. It's much simpler (and just
plain logical) to measure the field strength directly instead of
measuring an abstract value halfway towards the objective and relying
on nothing more than speculation that the rest is working according as
expected.





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"Frank Gilliland" wrote in message
...
On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength.

Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.


The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength.

It is the directional coupler that is balanced for a particular value of Z0.

Tam/WB2TT


The error is even more
insignificant when there are a host of variables and confounds between
the SWR meter and the transmitted field that can (and frequently do)
affect the objective -- field strength. It's much simpler (and just
plain logical) to measure the field strength directly instead of
measuring an abstract value halfway towards the objective and relying
on nothing more than speculation that the rest is working according as
expected.





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Frank Gilliland wrote:
Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.

The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength.

Nope, that's not the point at all. It is true that a 50 ohm
SWR meter designed for HF may not work on 70 cm but the error
I'm talking about is the calibration error in a 50 ohm SWR meter
designed for HF and used on HF in, for instance, a Z0 = 450 ohm
environment instead of its calibrated-for 50 ohm environment. It
works perfectly in a 50 ohm environment at the HF frequency of
operation. Here's the proof using a 50 ohm SWR meter:

XMTR--1/2WL 450 ohm line--SWR meter--1/2WL 450 ohm line--50 ohm load

The 50 ohm SWR meter will read 1:1, nowhere near the actual SWR

XMTR--1/4WL 450 ohm line--SWR meter--1/4WL 450 ohm line--50 ohm load

The 50 ohm SWR meter will read 81:1, nowhere near the actual SWR

An SWR meter calibrated for 450 ohms will correctly read 9:1
in both cases.
--
73, Cecil http://www.qsl.net/w5dxp


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james wrote:
If the transmitter has a refelction coefficient of zero and the load
say .3, then that reflected power from the load is dissapated as heat
in the output circuits and any final transistors or tubes.

Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.
--
73, Cecil http://www.qsl.net/w5dxp


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On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:

Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.
*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james

On Wed, 29 Jun 2005 17:28:01 -0400, "Tam/WB2TT"
wrote:

It is the directional coupler that is balanced for a particular value of Z0.

Tam/WB2TT
*****

Correct

james

Frank Gilliland wrote, among other things, "The point is that the error
is insignificant when the directional coupler is much shorter than the
wavelength."

Certainly "directional couplers" for HF may be built at essentially
zero length, and ideally would have exactly zero length, monitoring the
current and voltage at a single point on a line. Then SWR or
reflection coefficient magnitude or even complex reflection coefficient
may be calculated under the assumption we know the desired reference
impedance. But if the equipment combines the voltage and current
samples in the wrong ratio, you will get the WRONG answer. Even if the
coupler looks like a perfect 50 ohms impedance section of transmission
line (with some attenuation), the error _in_measurement_output_ can be
significant indeed. Just because the coupler looks like a 50 ohm line
to the line it's hooked to doesn't mean it will read zero reflection
when IT's presented with a 50 ohm load.

And by the way, not everyone who measures and cares very much about SWR
(or reflection coefficient) cares a whit about field strength. Not all
loads are antennas.

Indeed, as Reg says, we might do better in amateur applications to
consider the SWR meter as an indicator of the degree to which we're
presenting a transmitter with the desired load. That's really what
we're using it for, most of the time. It may ALSO be interesting to
know the field strength, but please be aware that a transmitter's
distortion products may be significantly higher if it's presented the
wrong load impedance, even though the power output may be increased.
Field strength alone is not acceptable to me as a means to adjust an
antenna load to a transmitter, or as a way to adjust the operating
point of the transmitter.

Cheers,
Tom

james wrote:
On Wed, 29 Jun 2005 11:07:17 -0400, "Fred W4JLE"
wrote:


What is the reason a 2:1 SWR can cause such havoc?

How can I avoid this catastrophic condition?

I feed my dipoles with 450 Ohm ladder line, but the last 20 feet or so is 50
Ohm coax, I guess that makes it work ok. I haven't blown up my finals yet.

Lions and tigers and bears Oh my...

*****

Actually can happen if you push the finals to where there is
insufficeint margin to the maximum heat dissapation. Tubes are a bit
more forgiving. Transistor inadequately heatsinked and overdriven,
typical CB usage, often have little of no margin for heat dissapation.

If the transmitter has a refelction coefficient of zero and the load
say .3, then that reflected power from the load is dissapated as heat
in the output circuits and any final transistors or tubes. Now if the
radio has a reflection coefficient other than zero that will lessen
the heat dissapation on the transimiiter. Now you get load and source
reflections convoluting within the transmission line.

You ought to model a 400 Mhz square wave with source and load
refelctions coefficients other than zero. It can get ugly


james



Consider the MRF 140, a 150 Watt 2.0 - 150.0 Mhz N-Channel
linear RF power fet. From the technical data sheet: "100% Tested
For Load Mismatch At All Phase Angles With 30:1 VSWR." You'd have
a tough time damaging this device with a mere 2:1 VSWR.
How do load and source reflections convolute within the
transmission line? That's a new one on me. My old dictionary
defines 'convolute' as "Rolled or folded together with one part
over another; twisted; coiled." The rest of the post is pretty
fanciful, too. A trip to the library would do wonders.
73,
Tom Donaly, KA6RUH

james wrote:

On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:


Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.

*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james


Cecil was talking about current, not power. You can't add
power the way you can voltage and current. If you could, you
could build a very nice perpetual motion machine just by using the
reflections in a transmission line to add power so that the output
was greater than the input.
73,
Tom Donaly, KA6RUH

The rig has no way of detecting any alleged "reflected power". It can't
tell the difference between a feedline with a lot of "reflected power",
a feedline with no "reflected power", and a plain resistor. It behaves
exactly the same in all cases, provided only that the impedance that
each provides to it is the same.

Anyone not convinced of this should put a couple or more dummy loads in
series or parallel, make up a few lengths of transmission line of
various impedances, and see for himself.

Roy Lewallen, W7EL

james wrote:
On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:


Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.

*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james