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.

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.
********

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.

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.

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. 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.

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.

james

Pay attention. That's exactly what I typed. Just in different words.


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.

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.
********

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.

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.

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. 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.

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.

james

On Tue, 28 Jun 2005 14:36:23 -0400, Scott in Baltimore
wrote:

The speed of the signal INSIDE the coax (the velocity factor) is slower
then the speed of the signal OUTSIDE (on the shield). While 17.21 feet
is a quarter wave on the outside of the shield, the inside 1/4 wave is
shorter. If you want to see the actual SWR at the feedpoint, then use
a 1/2 wave electrical length of coax. This will shift the phase of the
mismatch back into it's original position at the other end of the feedline.

(I learned all this stuff while I was still a single bander, and still
laugh at all the ham's that still believe the coax length BS.)
*****

And I have the biggest laugh because most CBers as well as Hams have a
peanuts view of what a transmission line is or how signals act on and
in them.

First off, while the coax can be inside the field of radiation, the
signal from the transmitter to the antenna travels solely inside the
transmission line. That is between the center conductor and the
shield. The energy transmitted travels in the dielectric and it is the
dielectric that slows the wave down and casue loses. Even the worst
coax, RG-58 has sufficient shield as to not cause leakage through the
shield at 27 MHz. Maybe a 10 GHz. but not 27 MHz.

Common mode currents occur on the shield and are just that currents.
They can come from poor ground connection at the antenna feed point or
can be induced currents due to the coax being within the fear feild
energy of the antenna. Often common mode currents are also rich in
harmonic energy and that is what reradiates and cause TVI and
interference.


james

"Steveo" wrote in message
...
Vinnie S. wrote:
On Tue, 28 Jun 2005 01:11:31 -0400, Scott in Baltimore
wrote:

So ... I have an older CB - Cobra 21 LTD Classic with weather stns
etc. ... a 102" Shakespeare Antenna - 18' of cable - and this


What makes you think 18 feet of coax is even a half wave?

At 27.185 MHz (ch 19) a half wave is 17.21 feet.

At 66% velocity factor, an electrical half wave is 11.36 feet.

At 77% velocity factor, an electrical half wave is 13.25 feet.

What's so special about a half wavelength of coax?

The Mobile antenna websites practically tell you to keep the coax at 18
feet, or else. I thought that was true, until numerous people at this
group and several websites said that is nonsense.

Have you ever heard from the coax length police? Real sticklers when it
comes to that.

Hello, Mopar

Fact is that as long as the feedline is not lossy, SWR doesn't really
matter - so long as you present a proper 50 ohms to the rig via a tuner.

I used a long wire years ago from 1.8 MHz to 30 MHz and have no idea what
the SWR was on any frequencies. The pi network took care of that. The
thing was good for thousands of miles on milliwatts and anywhere at all on
50 watts or so )

Coax, however, can get lossy with high SWR, especially at the higher HF
frequencies (and virtually any frequency if the SWR is *really* high).

One possible warning - if the SWR is caused by a faulty connection or a bad
antenna, you can match the thing to your rig, but most of the power will
disappear as heat in the fault.


Best regards from Rochester, NY
Jim

On Tue, 28 Jun 2005 18:31:58 GMT, Lancer wrote in
42c18a0e.20437562@2355323778:

On Tue, 28 Jun 2005 01:11:31 -0400, Scott in Baltimore
wrote:

So ... I have an older CB - Cobra 21 LTD Classic with weather stns etc. ...
a 102" Shakespeare Antenna - 18' of cable - and this


What makes you think 18 feet of coax is even a half wave?

Where did he say he thought it was a 1/2 wave?


I missed that, too.






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On Tue, 28 Jun 2005 19:36:53 GMT, james wrote
in :

On Tue, 28 Jun 2005 14:36:23 -0400, Scott in Baltimore
wrote:

The speed of the signal INSIDE the coax (the velocity factor) is slower
then the speed of the signal OUTSIDE (on the shield). While 17.21 feet
is a quarter wave on the outside of the shield, the inside 1/4 wave is
shorter. If you want to see the actual SWR at the feedpoint, then use
a 1/2 wave electrical length of coax. This will shift the phase of the
mismatch back into it's original position at the other end of the feedline.

(I learned all this stuff while I was still a single bander, and still
laugh at all the ham's that still believe the coax length BS.)
*****

And I have the biggest laugh because most CBers as well as Hams have a
peanuts view of what a transmission line is or how signals act on and
in them.


You can say -that- again.....


First off, while the coax can be inside the field of radiation, the
signal from the transmitter to the antenna travels solely inside the
transmission line. That is between the center conductor and the
shield. The energy transmitted travels in the dielectric and it is the
dielectric that slows the wave down and casue loses.


The energy in a coax travels on the conductors -and- in the dielectric
-and- within the magnetic fields. The propogation delay of a line is
the combined phase delays of distributed capacitance -and- distributed
inductance in the line. The dielectric constant only -seems- to be the
determining factor of coax propogation delay because the conductors
are straight. IOW, if you replace the center conductor with a coil you
will introduce an additional propogation delay into the coax which is
-independent- of the dielectric constant (and will have constructed a
device known to us old farts as a 'helical resonantor'). Regardless,
it has no relevance to this discussion.


Even the worst
coax, RG-58 has sufficient shield as to not cause leakage through the
shield at 27 MHz. Maybe a 10 GHz. but not 27 MHz.

Common mode currents occur on the shield and are just that currents.
They can come from poor ground connection at the antenna feed point or
can be induced currents due to the coax being within the fear feild
energy of the antenna.


One of the most misunderstood terms in radio is "common-mode current".
It simply means that current is moving in the same direction, and in
phase, on two or more conductors. It occurs in a coax when current on
the -inside- of the shield is in phase with the current on the center
conductor. Any RF current on the -outside- of a coax has -nothing- to
do with common-mode currents -- it's simply the result of RF spilling
out of the coax or being induced onto it from an external field.


Often common mode currents are also rich in
harmonic energy and that is what reradiates and cause TVI and
interference.


Hogwash. Harmonics don't just appear because of common-mode currents.
They must come from a source -- i.e, the transmitter. And conductors
of common-mode currents don't have any magical properties that let
them conduct or radiate harmonics any better than the fundamental
frequency. That's RF voodoo.







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

snip
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.


I think you have that a little misconstrued..... reflection of the
load to the transmitter by a half-wavelength coax is equal to the
-load- regardless of the characteristic impedance of the -coax-.

And Lancer was right, RF on the shield at the feedpoint -will- change
the input impedance of the coax because the shield is no longer
grounded, which is a necessary condition for proper operation of the
coax.







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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

On Tue, 28 Jun 2005 22:52:51 GMT, Lancer wrote in
gtk3c11b9q6nhs69mr9r6rftv1rkur1v70@2355323778:

On Tue, 28 Jun 2005 15:40:40 -0700, Frank Gilliland
wrote:

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

snip
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.


I think you have that a little misconstrued..... reflection of the
load to the transmitter by a half-wavelength coax is equal to the
-load- regardless of the characteristic impedance of the -coax-.

Thanks Frank;
I missed that, so if I have a 100 ohm load and and feed it with a
1/2 wave of 50 ohm coax, I'll see 100 ohms at the radio, not 50 ohms?


Yep. And I should add that 18' of coax is recommended not because of
it's propogation characteristics -inside- the coax, but because of
it's velocity factor on the -outside- of the shield which is nearly 1.
IOW, when the shield of an 18' length of coax is grounded only at one
end, that ground will be reflected at the other end of the coax. At
least that's the theory. In practical use it's not perfect, but it's
still better than a fully ungrounded radio or antenna.






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(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