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