On Monday, September 06, 2010 12:21:22 pm Mark Durenberger wrote:
Got a question Rick. Given the shielding properties of well-built
RG-6...do you think the ferrite choke placed over the shield can be
effective?
Mark,
The ferrite over the shield won't improve the shielding performance of the
foil/shield combination in well made RG-6 cable.
What it will do is prevent any energy on the outside of the shield from
getting on the inside surface. The ferrite choke prevents any energy that
is
riding on the cable's outer shield (as the outer surface acts like an
antenna)
from getting past any connector imperfections and onto the inner surface
of
the shield, and via that into the receiver as leakage.
Remember that in coaxial cable there are 3 conductive surfaces when we are
talking about radio frequency energy. Because of the skin effect, RF flows
on
the surfaces of conductors and not the entire cross-section. So we have"
1 - The outer surface of the center conductor
2 - The inner surface of the foil or shield braid, whatever the cable has.
(The electromagnetic field of the desired energy carried inside the cable
is
here in this space consisting of the inner surface of the shield, the
dielectric, and the outer surface of the center conductor.)
3 - The outer surface of the shield.
So if you can make it harder for any currents flowing on the outer surface
of
the shield to get to the inside surface, either via connector issues,
enclosure leakage or non-shielding, improperly installed or wrong
connectors,
etc. You help reduce leakage around the shielding effect. The ferrite
chokes do
that.
A good test was what Craig did. Lay out the cable and connect it to a
shielded
dummy load. Then the other end to the receiver and tune the bands of
interest
looking for leakage. Any signals there point to some failure of the
shield/receiver system. Choking the antenna end deals with leakage at that
end, choking the receiver end deals with that end. Again, this could be
connector choice or installation, or enclosure or case issues, etc.
It's important to know the impedance that the chokes you are going to use
on
the outside of the shield will add to common-mode currents flowing on the
outside surface of the shield. Too little added impedance and the effects
aren't all they should be. Usually, paying attention to the core material
for
the band in question, and getting large enough over-the-cable tubular
chokes,
or winding an appropriate number of turns of cable through a toroid core,
will
do the trick.
No connector or case or installation technique is perfect. If we have
cable
that is well shielded, and can keep undesired signals picked up on the
outer
surface of the cable's shield from getting to the inner surface of the
shield,
and then to the receiver, we can make a big improvement in coaxial cable
leakage performance.
Balanced transmission lines depend on these common-mode signals being
equal on
both conductors, while the desired signals are differential. So
terminations on
either end, if done right, allow the common-mode signals to cancel and the
differential mode signals to remain. But the key here is *balance*. This
means
you have to pay attention to where the balanced feedline is run, what it
gets
near to, and keep it away from the ground, metal, etc. essentially
anything
that can distort the field between the conductors and unbalance them.
Since the
electromagnetic field of the balanced transmission line extends some
distance
outside the physical space of the wires (a few times the line spacing or
more), close bundles of twisted pairs will not exhibit the same isolation
that
you might expect. Twisting does help make sure that any environmental
anomalies affecting the balanced transmission line affect each conductor
in a
similar manner. But you still have to pay attention to where you run it.
If
you get it too close to absorptive surfaces, other losses can also occur,
even
if the balance is not upset.
Hope that made sense.
Rick Kunath
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