How effective would the screen of a shielded twisted pair be if it was left floating and the pair used as flow and return in a single ended application?
It is not acting as a faraday cage because there are connections to other things outside of the cage. If the shield is connected to the enclosure at each side of the lead then that whole system becomes the faraday cage.
For unbalanced connections a single core screened cable offers the best rejection and containment.
Leaving the shield unconnected invites disturbances into your equipment by antenna action and capacitive coupling.
For unbalanced connections a single core screened cable offers the best rejection and containment.
Leaving the shield unconnected invites disturbances into your equipment by antenna action and capacitive coupling.
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Not true. read here Design Techniques for EMC: cables and connectors
(note not going to enter the 'can you hear it' discussion. Just pointing out that the experts don't agree. ).
(note not going to enter the 'can you hear it' discussion. Just pointing out that the experts don't agree. ).
Is the shape an issue or the small holes either end or the proximity to the conductors?
A floating shield may not be very effective. It all depends on exactly what you are trying to shield from. In some cases it could be worse than no shield.
While Henry Ott wrote:
A shield does not have to be grounded or have its potential controlled in any way to act as an effective shield. Many shields are grounded for other reasons, such as electrical safety.
There is a lot of fine print that goes with that quick answer (that was not linked to the answer)
Grounding of Shields
A shield does not have to be grounded or have its potential controlled in any way to act as an effective shield. Many shields are grounded for other reasons, such as electrical safety.
There is a lot of fine print that goes with that quick answer (that was not linked to the answer)
Grounding of Shields
I don't follow the logic...I see what you say, but, if the two enclosures weren't connected by the screen wouldn't you just have 3 Faraday cages?
I just found this, is it credible? The Antenna Myth - Screened and Shielded Network Cabling Standards Guide, By Siemon
Well they blow it in the first paragraph
Now the UTP they are talking about is good old cat5, which has the twists carefully defined to optimise the performance from a cheap cable. But it's digital.
Audio as you have realised is generally exceedingly resilient to crud with the cable lengths that we generally have. stadiums and theatres and recording studios have issues. But to get useful screening you need 360 degree coverage, so at least at radio frequencies you want everything linked together. aircraft aren't grounded but are Faraday cages.
This doesn't actually make sense as written.
Now the UTP they are talking about is good old cat5, which has the twists carefully defined to optimise the performance from a cheap cable. But it's digital.
Audio as you have realised is generally exceedingly resilient to crud with the cable lengths that we generally have. stadiums and theatres and recording studios have issues. But to get useful screening you need 360 degree coverage, so at least at radio frequencies you want everything linked together. aircraft aren't grounded but are Faraday cages.
That was linked to from this discussion Shielding -- Is it effective when floating? - Page 1
Jerry in reply #25 explains why the shield should be connected each end, I think? This is what I'm trying to understand.....why it has to be connected, why it can't function as a faraday cage (tube, as you like) on it's own.
PS, I'm ignoring the "grounding" part, just that they have to be connected together
Jerry in reply #25 explains why the shield should be connected each end, I think? This is what I'm trying to understand.....why it has to be connected, why it can't function as a faraday cage (tube, as you like) on it's own.
PS, I'm ignoring the "grounding" part, just that they have to be connected together
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It's only a cage when it totally encloses the signal wires. Otherwise there is a big hole in the cage at each end. It will give 'some' screening, but how much is unknown unless you measure it in situ.
Terminating at one end with a capacitive coupling at the other does have advantages in some cases and Bill Whitlock discusses this in his presentations.
Terminating at one end with a capacitive coupling at the other does have advantages in some cases and Bill Whitlock discusses this in his presentations.
This is what I suspected, and then the bandwidth over which it is effective would be governed by the size of the hole.....and the length of the tube? I do intend to measure it at some point, wi-fi should make a good interference source, the only problem is I only have a 20MHz scope........
It's 868MHz up to 2.6GHz that you have to worry about. 5GHz wifi is becoming more prevalent, but its pretty weedy. Old school 2G phones are a good test as you get the 216.7Hz frame rate coming through esp on call setup. We all remember TVs where you could hear the 'beep-dada-beep-da-da-beep-beep-beep' just before the phone rang if it was too close 🙂
An ungrounded shield may provide some protection from RF to a balanced twisted pair. It could make things worse for an unbalanced twisted pair, by increasing the surface area and thus the stray capacitance. Post 1 suggests that an unbalanced twisted pair is being considered.
I take the approach that a floating screen makes things unpredictable / chaotic. Actual effect depends on numerous other factors - electrical balance / impedances / mechanical orientation etc...etc...
In fact any floating conductor can be problematic in a system of any complexity and likely cost you time / money in any lab testing .
It's also worth noting that 'grounding' doesn't have to mean 'earthing' - your car chassis is 'grounded' to the battery 0V but not connected to earth (ignoring static dissipating 'straps' !)
If you really want to get into it have a think about the impedance of free space (20pi Ohm) !!!
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