Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current
on the outside of the shield according to its characteristic impedance
(skin effect separates current flow on the inside of the shield from the
current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished.
One way is to run several turns of the coax through a toroid core so
that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

John

John S wrote:
Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current
on the outside of the shield according to its characteristic impedance
(skin effect separates current flow on the inside of the shield from the
current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished.
One way is to run several turns of the coax through a toroid core so
that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

John

The only nit I would pick with this is that the current inside the
shield is in the form of an electromagnetic field propagating down
the transmission line.

When it hits the end of the transmission line it becomes current
flowing via any conductors connected to the end. After the end,
it can only flow along the surface of the conductors and the
outside of the shield is just another conductor.


--
Jim Pennino

On 7/31/2015 10:49 PM, John S wrote:
Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current
on the outside of the shield according to its characteristic impedance
(skin effect separates current flow on the inside of the shield from the
current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished.
One way is to run several turns of the coax through a toroid core so
that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

The coil couples all currents in the coax so that the net current flow
(common mode) is impeded while differential current is not. In this
case there is no differential current so it doesn't matter. You could
just as well use a large resistor or in other words, disconnect the
shield from the generator and antenna.

--

Rick

rickman wrote:
On 7/31/2015 10:49 PM, John S wrote:
Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current
on the outside of the shield according to its characteristic impedance
(skin effect separates current flow on the inside of the shield from the
current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished.
One way is to run several turns of the coax through a toroid core so
that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

The coil couples all currents in the coax so that the net current flow
(common mode) is impeded while differential current is not. In this
case there is no differential current so it doesn't matter. You could
just as well use a large resistor or in other words, disconnect the
shield from the generator and antenna.

If you put a series of ferrite beads on the coax, there is no coil
and no coupling yet the current on the outside of the shield is
reduced because the outside of the shield becomes a high impedance
path.

The "generator" is the very end of the coax, i.e. where the center
conductor and shield become just wires, and doesn't actually exist.

The "generator" is a conceptual aid.

It is impossible to physically separate the inside and outside of the
shield.



--
Jim Pennino

wrote:

rickman wrote:
On 7/31/2015 10:49 PM, John S wrote:
Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current
on the outside of the shield according to its characteristic impedance
(skin effect separates current flow on the inside of the shield from the
current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished.
One way is to run several turns of the coax through a toroid core so
that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

The coil couples all currents in the coax so that the net current flow
(common mode) is impeded while differential current is not. In this
case there is no differential current so it doesn't matter. You could
just as well use a large resistor or in other words, disconnect the
shield from the generator and antenna.

If you put a series of ferrite beads on the coax, there is no coil
and no coupling yet the current on the outside of the shield is
reduced because the outside of the shield becomes a high impedance
path.

The "generator" is the very end of the coax, i.e. where the center
conductor and shield become just wires, and doesn't actually exist.

The "generator" is a conceptual aid.

It is impossible to physically separate the inside and outside of the
shield.


But, of course, electrically (at RF) they are entirely separate. Just
to emphasise the point you are making.


--
Roger Hayter

"John S" wrote in message ...

Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced now,
right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current on
the outside of the shield according to its characteristic impedance (skin
effect separates current flow on the inside of the shield from the current
on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it? What
if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished. One
way is to run several turns of the coax through a toroid core so that the
impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion and
learn.

John

This is similar to the way I always viewed the current.

In summary, at the balanced antenna, the center conductor path is
straightforward.
However on the shield half, the current has two possible paths: the antenna
half connected to the shield and the outside of the shield.

This would cause half of the dipole to radiate less, with the remainder
being radiated by the outer shield.

The proportion of the current flowing down the outer side of the shield
seems easier to stop than to calculate

flame shields up!

On 8/1/2015 1:57 AM, wrote:
rickman wrote:
On 7/31/2015 10:49 PM, John S wrote:
Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current
on the outside of the shield according to its characteristic impedance
(skin effect separates current flow on the inside of the shield from the
current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished.
One way is to run several turns of the coax through a toroid core so
that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

The coil couples all currents in the coax so that the net current flow
(common mode) is impeded while differential current is not. In this
case there is no differential current so it doesn't matter. You could
just as well use a large resistor or in other words, disconnect the
shield from the generator and antenna.

If you put a series of ferrite beads on the coax, there is no coil
and no coupling yet the current on the outside of the shield is
reduced because the outside of the shield becomes a high impedance
path.

Uh, there is still a "coil" and there is plenty of coupling. That is
the entire purpose of the ferrite bead. The coil may only be a partial
turn, but it is exactly the same concept.


The "generator" is the very end of the coax, i.e. where the center
conductor and shield become just wires, and doesn't actually exist.

The "generator" is a conceptual aid.

It is impossible to physically separate the inside and outside of the
shield.

You *can* physically separate the inside and outside current flow in
the shield. Coax is made with *two* shields, i.e. triax. The inside
flow would be on the inside of the inner shield and the outside flow
would be on the outside of the outer shield. Not sure what your point
is about this.

--

Rick

On 8/1/2015 11:34 AM, Wayne wrote:


"John S" wrote in message ...

Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the
left side is equal to the sum of the currents on the right side. This
is the message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a
current on the outside of the shield according to its characteristic
impedance (skin effect separates current flow on the inside of the
shield from the current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be
accomplished. One way is to run several turns of the coax through a
toroid core so that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

John

This is similar to the way I always viewed the current.

In summary, at the balanced antenna, the center conductor path is
straightforward.
However on the shield half, the current has two possible paths: the
antenna half connected to the shield and the outside of the shield.

This would cause half of the dipole to radiate less, with the remainder
being radiated by the outer shield.

The proportion of the current flowing down the outer side of the shield
seems easier to stop than to calculate

flame shields up!

Give the man a cigar! I think your entire post is correct.

rickman wrote:
On 8/1/2015 1:57 AM, wrote:
rickman wrote:
On 7/31/2015 10:49 PM, John S wrote:
Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current
on the outside of the shield according to its characteristic impedance
(skin effect separates current flow on the inside of the shield from the
current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished.
One way is to run several turns of the coax through a toroid core so
that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

The coil couples all currents in the coax so that the net current flow
(common mode) is impeded while differential current is not. In this
case there is no differential current so it doesn't matter. You could
just as well use a large resistor or in other words, disconnect the
shield from the generator and antenna.

If you put a series of ferrite beads on the coax, there is no coil
and no coupling yet the current on the outside of the shield is
reduced because the outside of the shield becomes a high impedance
path.

Uh, there is still a "coil" and there is plenty of coupling. That is
the entire purpose of the ferrite bead. The coil may only be a partial
turn, but it is exactly the same concept.

Nope, no coupling, just a higher series impedance.

The "entire purpose of the ferrite bead" is to increase the impedance.


The "generator" is the very end of the coax, i.e. where the center
conductor and shield become just wires, and doesn't actually exist.

The "generator" is a conceptual aid.

It is impossible to physically separate the inside and outside of the
shield.

You *can* physically separate the inside and outside current flow in
the shield. Coax is made with *two* shields, i.e. triax. The inside
flow would be on the inside of the inner shield and the outside flow
would be on the outside of the outer shield. Not sure what your point
is about this.

You can have a 5 conductor transmission line, but that isn't what is
being discussed, it is coax.

The point of it being impossible to physically separate the inside and
outside of the shield is that the inside and outside of the shield are
two different current paths at RF and are connected together at the
point where the coax stucture ends, i.e. the where you connect something.

Ergo there will always be a parasitic current path on the outside of
the shield.

Whether or not there is significant current flow on the outside of the
shield is a separate issue.


--
Jim Pennino

Wayne wrote:


"John S" wrote in message ...

Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced now,
right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current on
the outside of the shield according to its characteristic impedance (skin
effect separates current flow on the inside of the shield from the current
on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it? What
if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished. One
way is to run several turns of the coax through a toroid core so that the
impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion and
learn.

John

This is similar to the way I always viewed the current.

In summary, at the balanced antenna, the center conductor path is
straightforward.
However on the shield half, the current has two possible paths: the antenna
half connected to the shield and the outside of the shield.

This would cause half of the dipole to radiate less, with the remainder
being radiated by the outer shield.

The proportion of the current flowing down the outer side of the shield
seems easier to stop than to calculate

flame shields up!

It is trivial to calculate the current on the shield and the effect it
has on the antenna pattern with any antenna simulation software.

If you do that, you find that, just as you would expect, the effect is
dependant on how long the shield portion is.


--
Jim Pennino