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

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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