Re: [IRCA] Cable connectors

[Rick Kunath <k9ao@xxxxxxxxxxx>]

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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