SWR - wtf?
 

So ... I have an older CB - Cobra 21 LTD Classic with weather stns etc. ...
a 102" Shakespeare Antenna - 18' of cable - and this
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=48 699

(not advertising it just using it for the picture) ... Anyone know how to
use the Land Matic LM-50 ... it was pretty cheap like $16.00 Cdn I think - I
got it new but it didn't have any instructions - I don't even know the
wattage I should be setting it on. I'm assuming that the antenna goes into
the back on the side with ANT and the radio goes into the side marked
TRANS - but I'm not sure - does anyone know of any webpages that provide
some sort of instructions on how to use SWR meters?

TIA
jdd

On Mon, 27 Jun 2005 13:48:34 -0400, "john d"
wrote in :

So ... I have an older CB - Cobra 21 LTD Classic with weather stns etc. ...
a 102" Shakespeare Antenna - 18' of cable - and this
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=48 699

(not advertising it just using it for the picture) ... Anyone know how to
use the Land Matic LM-50 ... it was pretty cheap like $16.00 Cdn I think - I
got it new but it didn't have any instructions - I don't even know the
wattage I should be setting it on. I'm assuming that the antenna goes into
the back on the side with ANT and the radio goes into the side marked
TRANS - but I'm not sure - does anyone know of any webpages that provide
some sort of instructions on how to use SWR meters?


They're pretty easy. The cables hook up just like you figured.

Start by setting your radio to ch 18 AM (not SSB). Switch the matcher
off (you won't even need it), set the power switch for 10W, the top
switch to FWD CAL and the SWR/CAL switch to CAL. Key up and adjust the
slider so the meter reads full scale. Then unkey, switch the SWR/CAL
to SWR, key up again and read the meter. Adjust your antenna to get
the lowest SWR -- the lower the better. Then check SWR at both ends of
the band. An SWR of 2:1 or lower across the band is fine. If you can't
get it below 3:1 on any channel then there's something wrong with your
system, like a bad ground or coax cable.

If you want to play with the matcher, make your initial adjustments
while listening to the noise in receive. Start with TUNE all the way
left and sweep LOAD until the noise peaks. Check SWR. Then turn TUNE a
little bit to the right, sweep through LOAD again for the noise peak,
and check SWR again. If the SWR is better then keep going through the
process until it starts getting worse. Eventually you will find a spot
for both knobs that work to give you the best SWR. But remember that
when the matcher is tuned it's tuned very sharply to that freq -- you
will need to retune the matcher every time you move more than a couple
channels away.




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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?

It's the point that the SWR at the feedpoint is reflected to the other
end of the coax. At any other point in the coax, the phase angle affects
the apparent SWR seen by a Voltage reading SWR bridge. You can swap in
different lengths of coax to see this in action for your self. If your
antenna feedpoint is 50 ohms NONREACTIVE, coax length does not matter.
If there is a reactive component to your antenna system, the reflection
travels back through the coax and at different points the voltage and
current will affect the reading on the meter. A matchbox does not fix the
mismatch of the antenna, it only fixes what the load sees the antenna as.

The mismatch does not go away just because you've adjusted a few knobs.

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.



It's the point that the SWR at the feedpoint is reflected to the other
end of the coax. At any other point in the coax, the phase angle affects
the apparent SWR seen by a Voltage reading SWR bridge. You can swap in
different lengths of coax to see this in action for your self. If your
antenna feedpoint is 50 ohms NONREACTIVE, coax length does not matter.
If there is a reactive component to your antenna system, the reflection
travels back through the coax and at different points the voltage and
current will affect the reading on the meter. A matchbox does not fix the
mismatch of the antenna, it only fixes what the load sees the antenna as.

The mismatch does not go away just because you've adjusted a few knobs.


Vinnie S.

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. :)

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.


It's just a convenient premade length of coax. My RadioShack 19-210 2 meter
antenna came with the same junky 18 foot length of crappy RG-58 with a crimped
on connector. After removing it from the magnet and drilling a hole to properly
mount it, I cut off the extra cable and put a solder on end on it. If it didn't
have a propietary connector going into the base of the antenna, I would have
replaced it with something better. RG-58 is barely good at 27 MHZ and even
leakier at 146 MHz. The shorter the better. Coax length does not matter if
the SWR is at 1.5:1 or less. I haven't bothered to check my SWR on my CB
antenna since I got my ticket. The CB still works just fine with the RG-8X
and 225 amp on it!!!

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.

For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.

I immediately noticed the antenna worked better with a real ground.

For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.


And one last thing,

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.)

I don't dislike CB. It's another band to use. I dislike all the noise on it now!
If I can't find someone to talk to on one band, I've got others to use now.

BTW, repeaters suck! I've only got to abide by Part 97, not what some control op
thinks his interpretation of the rules are. I'm simplex only these days.

(Sorry, no letters or warnings have been recieved by me! I just said **** it.)
And I didn't say that on the air!!!!!!!!!!!!!

On Tue, 28 Jun 2005 14:10:10 -0400, Scott in Baltimore
wrote:

For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.

I immediately noticed the antenna worked better with a real ground.
****

If the shield is the counterpoise for the antenna, then the antenna is
installed with no ground and thus inproperly installed period. In a
properly install antenna system you should not have common mode
currents residing on the shield of the coaxial transmission line.

All to often on cars today, there is far to much plastic and not
enough metal to offer a sufficient RF ground. The vehicle frame and
body, if metal, should be the uppper plate of a capacitor that is
formed with the Earth below. The metal under the antenna is important
for radiation. The more the better.

james

I immediately noticed the antenna worked better with a real ground.
****

If the shield is the counterpoise for the antenna, then the antenna is
installed with no ground and thus inproperly installed period. In a
properly install antenna system you should not have common mode
currents residing on the shield of the coaxial transmission line.


It worked good as a magnet mount, it just works better with a real ground!

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

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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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