Question on dipole SWR problem
 

I think I misunderstood by what you meant by INPUT. The choke goes
between the antenna and the tuner. Generally at the antenna feed
point.

Tam/WB2TT


Tam,

Wouldln't that would be a very bad idea, at least in our particular
case? As I pointed out in the start of this thread, we are feeding the
antenna with about 14' of 600 ohm ladder line from the output of the SGC
antenna coupler. Isn't a choke balan used to prevent the flow of current
in the outer braid of a coax feedline? We certainly wouldn't want to be
choking any RF coming out of the SGC coupler going to the antenna!

Ed

Question on dipole SWR problem
 

"Tam/WB2TT" wrote in
:


You're right on the current in the shield. We have a choke balun,
currently located 85' down the coax toward the transmitter. I have
it scheduled, when it stops raining, to move it up to the input of
the SG- 237 coupler unit.

This has got to be a step in the right direction.

Tam/WB2TT

I think I misunderstood by what you meant by INPUT. The choke goes
between the antenna and the tuner. Generally at the antenna feed
point.

Tam/WB2TT



Tam,

Can you explain the difference between putting the choke adjacent to and
on either side of the tuner?

Owen

Question on dipole SWR problem
 

Owen Duffy wrote:
Can you explain the difference between putting the choke adjacent to and
on either side of the tuner?

How about a tutorial on choking common-mode
current on ladder-line?
--
73, Cecil http://www.w5dxp.com

Question on dipole SWR problem
 

"Ed G" wrote in message
. 192.196...

I think I misunderstood by what you meant by INPUT. The choke goes
between the antenna and the tuner. Generally at the antenna feed
point.

Tam/WB2TT


Tam,

Wouldln't that would be a very bad idea, at least in our particular
case? As I pointed out in the start of this thread, we are feeding the
antenna with about 14' of 600 ohm ladder line from the output of the SGC
antenna coupler. Isn't a choke balan used to prevent the flow of current
in the outer braid of a coax feedline? We certainly wouldn't want to be
choking any RF coming out of the SGC coupler going to the antenna!

Ed

The choke suppresses the common mode signal. There will still be current
flowing on the shield which will have the same magnitude as the current
flowing in the center conductor. You are not throwing away any energy by
doing this. If you put the choke at the antenna feed point, the transmission
line will not radiate and act as part of the antenna. The choke as you
described it could also be called a 1:1 current balun. You might want to
take a look at the ARRL Handbook.

As for the 600 Ohm line, it is not obvious that 14 feet of 600 Ohm line with
an SWR of 120:1 is better than 14 feet of RG8 with an SWR of 10:1. (I came
up with an impedance at 3800 of 5 Ohms, assuming a perfect ground. If the
roof is not that good a ground, you will want to add a couple of Ohms to
that).

You guys could solve a lot of the mystery by measuring the SWR as close to
the feedpoint of the antenna as possible.

Tam/WB2TT

Question on dipole SWR problem
 

"Tam/WB2TT" wrote in
:

....
The choke suppresses the common mode signal. There will still be
current flowing on the shield which will have the same magnitude as
the current flowing in the center conductor. You are not throwing away
....

This might just be really loose language, but assuming fully effective
skin effect (which is a reasonable assumption for most practical coaxial
cables at HF):

The current flowing on the outside of the inner conductor is accompanied
by a current equal in magnitude and opposite in direction flowing on the
inside of the outer conductor.

Skin effect isolates the inner of the outer conductor from the outer of
the outer conductor, but current on the inner of the outer conductor may
contribute to current on the outer of the outer conductor depending on
the treatment of the shield at the ends of the cable.

So, a choke formed by coiling the coaxial cable or placing ferrite
sleeves on the cable affects the impedance in the current path of the
outer of the outer conductor and does not directly affect what is
happening inside the coax.

Mind you, this concept is not universally accepted by hams.

In the case of coax, so-called common mode current flows only on the
outside of the outer conductor, and differential mode current flows only
on the inside of the outer conductor and outside of the inner conductor.

Owen

Question on dipole SWR problem
 

Let me add a little to Owen's excellent explanation.

We can mathematically separate any two currents into a "common mode" (or
even mode) current and a "differential mode" (or odd mode) current. If
the two currents are equal in magnitude and opposite in direction, the
common mode component is zero; if they're equal in magnitude and in the
same direction, the differential mode component is zero.

This mathematical trickery is very useful in analyzing transmission
lines, because superposition allows us to treat the effects of the two
mode currents separately and sum the results. In a transmission line,
the differential mode current is sometimes appropriately called the
"transmission line" current, and the common mode current the "antenna"
current. This is because the differential mode current conforms to all
the transmission line rules -- that is, it behaves as though it and it
alone is being carried by the transmission line, and its properties can
be found by applying normal transmission line equations and analysis. No
radiation results from the transmission line currents. (In practice, a
very small amount of radiation results from the differential current on
a non-coax line, but if it's significant, a poor choice of transmission
line was made.) And the common mode current behaves just like any other
current on a single conductor (or identical currents on two parallel
conductors) - it creates a radiating field. The conductor carrying the
current is, by any definition, an antenna. So if we want to eliminate
feedline radiation we need to eliminate (or, practically speaking,
reduce to a small value) the common mode current.

To do this analysis with a symmetrical line such as twinlead or open
wire line, we use the currents on the two conductors as the two currents
to separate into common and differential mode components. We can do
exactly the same thing with coax, using the current on the inner
conductor as one of the currents to be separated, and the vector total
current on the inside and outside of the shield to be the other. If we
do this, we find that the two types of line behave identically: If the
common mode current is zero, the line won't radiate (and can be
considered balanced). If it isn't, the line will. Equations and analysis
are identical. Either type of line can be balanced or unbalanced.

Coaxial lines do, however, have an interesting characteristic not shared
by other kinds -- the differential and common mode components aren't
simply a mathematical convenience, but are actually physically separate.
If we do the analysis described above, we find that the common mode
current equals the current on the outside of the shield and the
differential current equals the current on the inside. As Owen pointed
out, the differential current is solely on the inside of the shield and
the common mode current solely on the outside. While this makes the
effects of each mode current easier to visualize and sometimes to
measure, the net effects of common mode and differential currents are
exactly the same for coaxial and non-coaxial lines.

Roy Lewallen, W7EL

Owen Duffy wrote:
"Tam/WB2TT" wrote in
:

...
The choke suppresses the common mode signal. There will still be
current flowing on the shield which will have the same magnitude as
the current flowing in the center conductor. You are not throwing away
...

This might just be really loose language, but assuming fully effective
skin effect (which is a reasonable assumption for most practical coaxial
cables at HF):

The current flowing on the outside of the inner conductor is accompanied
by a current equal in magnitude and opposite in direction flowing on the
inside of the outer conductor.

Skin effect isolates the inner of the outer conductor from the outer of
the outer conductor, but current on the inner of the outer conductor may
contribute to current on the outer of the outer conductor depending on
the treatment of the shield at the ends of the cable.

So, a choke formed by coiling the coaxial cable or placing ferrite
sleeves on the cable affects the impedance in the current path of the
outer of the outer conductor and does not directly affect what is
happening inside the coax.

Mind you, this concept is not universally accepted by hams.

In the case of coax, so-called common mode current flows only on the
outside of the outer conductor, and differential mode current flows only
on the inside of the outer conductor and outside of the inner conductor.

Owen

Question on dipole SWR problem
 

"Roy Lewallen" wrote in message
...
[...]
Coaxial lines do, however, have an interesting characteristic not shared
by other kinds -- the differential and common mode components aren't
simply a mathematical convenience, but are actually physically separate.
If we do the analysis described above, we find that the common mode
current equals the current on the outside of the shield and the
differential current equals the current on the inside. As Owen pointed
out, the differential current is solely on the inside of the shield and
the common mode current solely on the outside. While this makes the
effects of each mode current easier to visualize and sometimes to measure,
the net effects of common mode and differential currents are exactly the
same for coaxial and non-coaxial lines.

Owen Duffy wrote:
In the case of coax, so-called common mode current flows only on the
outside of the outer conductor, and differential mode current flows only
on the inside of the outer conductor and outside of the inner conductor.

I assume that you are talking about a length of coax that is attached to a
free-space antenna. What about the case where the coax shield is grounded
at both ends? (make it a non-ideal ground if you like.) Wouldn't this
create a ground-loop that will cause some of the signal current to flow
through the ground-connection, thus unbalancing the center-conductor/shield
current? In this case, the common-mode current isn't necessarily flowing on
the outside of the shield. (I am asking a question here).

Also, consider the case at frequencies low enough that skin-effect doesn't
apply. Here there is no inside or outside of the coax shield. Still, the
magnetic fields caused by imbalance between center-conductor and shield
currents are the same, with or without skin effect.

At least these are the thoughts I had while I was discussing the
installation of antennas and tuners on boats. There is no end to the
controversy surrounding the grounding of radio equipment on a boat. There
the antennas are typically end-fed wires (usually part of the rigging), and
some combination of radials and seawater connection for the RF counterpoise.

-Paul

Question on dipole SWR problem
 

Paul wrote:
"Roy Lewallen" wrote in message
...
[...]
Coaxial lines do, however, have an interesting characteristic not shared
by other kinds -- the differential and common mode components aren't
simply a mathematical convenience, but are actually physically separate.
If we do the analysis described above, we find that the common mode
current equals the current on the outside of the shield and the
differential current equals the current on the inside. As Owen pointed
out, the differential current is solely on the inside of the shield and
the common mode current solely on the outside. While this makes the
effects of each mode current easier to visualize and sometimes to measure,
the net effects of common mode and differential currents are exactly the
same for coaxial and non-coaxial lines.

Owen Duffy wrote:
In the case of coax, so-called common mode current flows only on the
outside of the outer conductor, and differential mode current flows only
on the inside of the outer conductor and outside of the inner conductor.

I assume that you are talking about a length of coax that is attached to a
free-space antenna. What about the case where the coax shield is grounded
at both ends? (make it a non-ideal ground if you like.) Wouldn't this
create a ground-loop that will cause some of the signal current to flow
through the ground-connection, thus unbalancing the center-conductor/shield
current? In this case, the common-mode current isn't necessarily flowing on
the outside of the shield. (I am asking a question here).

A tricky part in answering this is determining what you mean by "shield
current". There are separate and distinct currents on the inside and
outside of the shield. I'll assume that by "shield current" you mean the
vector sum of these two currents.

The first part of the answer is that the current on the outside of the
inner conductor is always equal to the current on the inside of the
shield, and in the opposite direction (that is to say, they comprise a
pure differential current), provided that the shield is at least several
skin depths thick. This is a consequence of the confinement of the field
by the shield, and has nothing to do with what we connect the cable to.
Connections only impact the current on the outside.

Now consider what happens when the coax is connected to a free-space
dipole, for example. All the current from the center conductor flows
into one half the dipole. But the current on the inside of the shield
has two possible paths: to the other half of the dipole or around the
end of the shield to the outside of the shield. I won't go into more
detail about this, since I've already done so -- you can see what I've
written at http://eznec.com/Amateur/Articles/Baluns.pdf.

If you "ground" both ends of the coax, that is, connect them to
conductors which provide another path between the two ends, you have a
third path the inner shield can follow -- along the "ground" path. So it
splits three ways instead of two. If you use a "pigtail" wire for
grounding or connecting to the load, it adds inductance to the desired
path to the load, which makes the path back along the outside of the
coax more desirable, so you end up with more common mode current than
you would with a low impedance connection.

Also, consider the case at frequencies low enough that skin-effect doesn't
apply. Here there is no inside or outside of the coax shield. Still, the
magnetic fields caused by imbalance between center-conductor and shield
currents are the same, with or without skin effect.

I'm not sure I follow this. When the frequency gets low enough that the
field can penetrate the shield, the line behaves more like a twinlead
line behaves at HF. As I mentioned in my earlier posting, the line can
still have common and differential mode currents -- they're just no
longer physically separated.

At least these are the thoughts I had while I was discussing the
installation of antennas and tuners on boats. There is no end to the
controversy surrounding the grounding of radio equipment on a boat. There
the antennas are typically end-fed wires (usually part of the rigging), and
some combination of radials and seawater connection for the RF counterpoise.

Grounding would be much easier to understand if people would realize
that calling a conductor or connection "ground" doesn't impart magical
qualities. And that currents flow wherever the impedance dictates.

Roy Lewallen, W7EL

Question on dipole SWR problem
 

"Roy Lewallen" wrote in message
...
Paul wrote:
I assume that you are talking about a length of coax that is attached to
a free-space antenna. What about the case where the coax shield is
grounded at both ends? (make it a non-ideal ground if you like.)
Wouldn't this create a ground-loop that will cause some of the signal
current to flow through the ground-connection, thus unbalancing the
center-conductor/shield current? In this case, the common-mode current
isn't necessarily flowing on the outside of the shield. (I am asking a
question here).

A tricky part in answering this is determining what you mean by "shield
current". There are separate and distinct currents on the inside and
outside of the shield. I'll assume that by "shield current" you mean the
vector sum of these two currents.

The first part of the answer is that the current on the outside of the
inner conductor is always equal to the current on the inside of the
shield, and in the opposite direction (that is to say, they comprise a
pure differential current), provided that the shield is at least several
skin depths thick. This is a consequence of the confinement of the field
by the shield, and has nothing to do with what we connect the cable to.
Connections only impact the current on the outside.

Now consider what happens when the coax is connected to a free-space
dipole, for example. All the current from the center conductor flows into
one half the dipole. But the current on the inside of the shield has two
possible paths: to the other half of the dipole or around the end of the
shield to the outside of the shield. I won't go into more detail about
this, since I've already done so -- you can see what I've written at
http://eznec.com/Amateur/Articles/Baluns.pdf.

If you "ground" both ends of the coax, that is, connect them to conductors
which provide another path between the two ends, you have a third path the
inner shield can follow -- along the "ground" path. So it splits three
ways instead of two. If you use a "pigtail" wire for grounding or
connecting to the load, it adds inductance to the desired path to the
load, which makes the path back along the outside of the coax more
desirable, so you end up with more common mode current than you would with
a low impedance connection.

Yes, this agrees with what I have been thinking, and what I was trying to
say.

Also, consider the case at frequencies low enough that skin-effect
doesn't apply. Here there is no inside or outside of the coax shield.
Still, the magnetic fields caused by imbalance between center-conductor
and shield currents are the same, with or without skin effect.

I'm not sure I follow this. When the frequency gets low enough that the
field can penetrate the shield, the line behaves more like a twinlead line
behaves at HF. As I mentioned in my earlier posting, the line can still
have common and differential mode currents -- they're just no longer
physically separated.

Grounding would be much easier to understand if people would realize that
calling a conductor or connection "ground" doesn't impart magical
qualities. And that currents flow wherever the impedance dictates.

Again, I agree. The reason I suggested a "non-ideal ground" was to de-magic
it. We do need to recognize some non-zero impedances on the grounds and
shields, and their connections, if we are to analyze how the current splits.

The reason for the low-frequency question is that on the boat installations
I've been discussing, there are multiple signal sources and some very
non-ideal grounds. The signal sources include DC (due to how the electrical
equipment is usually wired), and relatively low frequency signals, sometimes
carried on cables in very close proximity to the coax. There is also the RF
field from the close-in antenna and RF grounding system. I've been stating
that the coax shield does *not* provide a magic shield. Your comparison to
twinlead at low frequencies (for current, and thus for inductive coupling),
confirms what I have been saying. These superimposed low-frequency currents
don't affect the RF situation, but they can affect the equipment itself.

Regards,
-Paul (wb6cxc)

Question on dipole SWR problem
 

Getting back to the original problem.

Most autotuners can't cope with impedances of less than 5 or 6 ohms.
As you are near that limit even a slight increase in antenna height
may facilitate correct tuning.

In order to improve the antenna balance and RX S/N ratio. Put a good
quality ferrite common mode chokes on the Coax and control cables
going into the tuner (I suggest either 50 ferrite beads over the coax
or 10 turns of coax on a ferrite ring as a minimum) If you wish to
earth the coax do it on the transmitter side of the choke. A few turns
of coax will not provide sufficient choking impedance at 3.8MHz.

The purpose of this is to 'float' the tuner above RF earth so that the
output apears to be balanced. By putting the choke on the input side
of the tuner, it is always working at a constant impedance thus
minimising losses.

UKM



On Oct 30, 2:42 pm, "Paul" wrote:
"Roy Lewallen" wrote in message

...





Paul wrote:
I assume that you are talking about a length of coax that is attached to
a free-space antenna. What about the case where the coax shield is
grounded at both ends? (make it a non-ideal ground if you like.)
Wouldn't this create a ground-loop that will cause some of the signal
current to flow through the ground-connection, thus unbalancing the
center-conductor/shield current? In this case, the common-mode current
isn't necessarily flowing on the outside of the shield. (I am asking a
question here).

A tricky part in answering this is determining what you mean by "shield
current". There are separate and distinct currents on the inside and
outside of the shield. I'll assume that by "shield current" you mean the
vector sum of these two currents.

The first part of the answer is that the current on the outside of the
inner conductor is always equal to the current on the inside of the
shield, and in the opposite direction (that is to say, they comprise a
pure differential current), provided that the shield is at least several
skin depths thick. This is a consequence of the confinement of the field
by the shield, and has nothing to do with what we connect the cable to.
Connections only impact the current on the outside.

Now consider what happens when the coax is connected to a free-space
dipole, for example. All the current from the center conductor flows into
one half the dipole. But the current on the inside of the shield has two
possible paths: to the other half of the dipole or around the end of the
shield to the outside of the shield. I won't go into more detail about
this, since I've already done so -- you can see what I've written at
http://eznec.com/Amateur/Articles/Baluns.pdf.

If you "ground" both ends of the coax, that is, connect them to conductors
which provide another path between the two ends, you have a third path the
inner shield can follow -- along the "ground" path. So it splits three
ways instead of two. If you use a "pigtail" wire for grounding or
connecting to the load, it adds inductance to the desired path to the
load, which makes the path back along the outside of the coax more
desirable, so you end up with more common mode current than you would with
a low impedance connection.

Yes, this agrees with what I have been thinking, and what I was trying to
say.

Also, consider the case at frequencies low enough that skin-effect
doesn't apply. Here there is no inside or outside of the coax shield.
Still, the magnetic fields caused by imbalance between center-conductor
and shield currents are the same, with or without skin effect.

I'm not sure I follow this. When the frequency gets low enough that the
field can penetrate the shield, the line behaves more like a twinlead line
behaves at HF. As I mentioned in my earlier posting, the line can still
have common and differential mode currents -- they're just no longer
physically separated.

Grounding would be much easier to understand if people would realize that
calling a conductor or connection "ground" doesn't impart magical
qualities. And that currents flow wherever the impedance dictates.

Again, I agree. The reason I suggested a "non-ideal ground" was to de-magic
it. We do need to recognize some non-zero impedances on the grounds and
shields, and their connections, if we are to analyze how the current splits.

The reason for the low-frequency question is that on the boat installations
I've been discussing, there are multiple signal sources and some very
non-ideal grounds. The signal sources include DC (due to how the electrical
equipment is usually wired), and relatively low frequency signals, sometimes
carried on cables in very close proximity to the coax. There is also the RF
field from the close-in antenna and RF grounding system. I've been stating
that the coax shield does *not* provide a magic shield. Your comparison to
twinlead at low frequencies (for current, and thus for inductive coupling),
confirms what I have been saying. These superimposed low-frequency currents
don't affect the RF situation, but they can affect the equipment itself.

Regards,
-Paul (wb6cxc)- Hide quoted text -

- Show quoted text -