rickman wrote:

On 8/2/2015 4:37 AM, Roger Hayter wrote:
rickman wrote:

On 8/1/2015 4:31 PM, Roger Hayter wrote:
rickman wrote:

On 8/1/2015 3:29 PM, Roger Hayter wrote:
rickman wrote:

On 8/1/2015 1:38 PM, Roger Hayter wrote:

But your second point is unhelpful in some circumstances. For
instance, if the type of balun is the inductive coil of the feeder
with or without ferrites, then there simply *is no* current path
down the outside of the feeder from the junction of the balun and
the feeder, Except from the outer of the cable in the balun coil,
and it is this that is decoupled by the inductance.

Your description is not clear to me.

I am looking at the junction of the feeder with the balun, and the only
source of current on the outside of the feeder is connected by a very
high inductance to the source of signal at the antenna end.

How is the inner surface of the shield not connected to the outer
surface of the shield?


At the point the balun joins the feeder, the only way RF can get from
inside the coax braid (skin effect deep) to the outside of the coax
braid is to go all the way up to the antenna and down the outer surface
of the balun.

I think this is our first point of disagreement. There is nothing to to
stop the current flowing on the shield inside surface from moving to the
shield outside surface other than a tiny amount of resistance in the
shield wire. Unless the current flow sees a lower impedance path to
follow through the balun, it will travel back on the shield outside
surface.


At RF, the coax braid is an impenetrable Faraday screen. That is what
it is for, after all. The current simply can't go through the
fractional mm of copper from inside to outside.

This is one of those cases where you have forgotten the details and
underlying premises. A Faraday cage can't stop currents from flowing
through the "cage". It stops fields from penetrating the cage by action
of the resulting currents in the cage. Your car is largely a Faraday
cage but you can still be electrocuted if a live wire is in contact with
the chassis and you touch it while inside. There can be a high
potential across different parts of the car from the current and that
current can pass through you if you touch the cage.

DC certainly. VLF probably, certainly 60Hz! But any RF where the skin
effect depth is much less than the thickness, no. This is probably
iteself a field effect, but said field is inseparable from an RF
voltage.



This is a high inductance path.

I think you are applying this term without appreciating the full
meaning. It is a high impedance path for common mode currents, but a
low impedance path for differential currents. Since the current in the
shield inner surface balances the current on the center conductor, it is
a very low impedance path for the full current on the shield.

If it were accurate to say the balun was "a high impedance path" without
the qualifications, the balun would prevent the desired signal from
reaching the load.

Which is why I said it is a high impedance path for the current on the
outside of the screen. "Common mode" isn't really quite the right name
for this current, as the inner is totally uninvolved, but it is
certainly not differential, which satisfies your point.

Actually, I quoted you accurately above. You omitted the qualifier and
that was my point.

Fair enough.


--
Roger Hayter