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 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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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

On Wed, 29 Jun 2005 14:07:26 -0700, Frank Gilliland
wrote:

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.

In a word, baloney. The error is independent of length. A zero length
bridge calibrated at 75 ohm is in error when measuring in a 50 ohm
system. Period.


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.

Often, field strength is of zero importance. What do you do when the
device under test isn't supposed to radiate? The simplest example of
this would be a CATV system, yet VSWR is *extremely* important in
cascaded networks.


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.

More baloney and it isn't even sliced.