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Author Topic: Modems and mains cables - interference  (Read 760 times)

parkdale

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Re: Modems and mains cables - interference
« Reply #15 on: June 17, 2018, 10:25:09 AM »

Have you seen this for making up a Faraday cage, Aluminium would be easily worked although mild steel can be painted.
https://www.ebay.co.uk/i/291025744754?chn=ps&var=590190624927&adgroupid=55998037190&rlsatarget=pla-412322736374&abcId=1133916&adtype=pla&merchantid=101769991&googleloc=20342&device=c&campaignid=1057744529&crdt=0

I have also seen similar to this in Wickes and other diy stores
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tubaman

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Re: Modems and mains cables - interference
« Reply #16 on: June 17, 2018, 10:39:06 AM »

@Weaver,

I've just taken a look at my 1312 and the top definitely does have ventilation too - it's in the vertical sides of the curved patterning if that makes sense.
So they are vented top, bottom and on two sides.
I would personally be very cautious about obstructing any of the ventilation as excess heat and electronics are not good bedfellows.
I appreciate that you are after any small gain in speed that you get, but would honestly be very surprised if any homemade screening will make a worthwhile difference.
Happy to be proved wrong of course.
 :)
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Weaver

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Re: Modems and mains cables - interference
« Reply #17 on: June 17, 2018, 10:40:17 AM »

I take Burakkucat et al’s advice seriously because I skipped most classical electromagnetism in the second year I am ashamed to say. Here is an extract from The Myths and Realities of Screened Shielded Cabling:

Quote
A second antenna myth is that common mode signals appearing on a screen or shield can only be dissipated through a low-impedance ground path. The fear is that an ungrounded screen will radiate signals that are “bouncing back and forth” and “building up” over the screen/shield. In fact, left ungrounded, a screen/shield will substantially attenuate higher-frequency signals because of the low-pass filter formed by its resistance, distributed shunt capacitance, and series inductance.
The effects of leaving both ends of a foil twisted-pair cable ungrounded can also be verified by using the abovementioned experimental method. The coupling between two UTP cables is still a minimum of 20 dB worse than the interaction between two ungrounded F/UTP cables. (Note that 20 dB of margin corresponds to 10 times less voltage coupling.) Even under worst-case, ungrounded conditions, the UTP cable behaves more like an antenna than the F/UTP cable.
Modeled and experimental results clearly dispel these antenna myth. Screens and shields offer substantially improved noise immunity compared to unshielded constructions above 30 MHz, even when improperly grounded.

But I am nowhere near as high as 30MHz, so this does not apply. I am assuming that at x 100 kHz frequencies the value of dV/dy is going to be small because the wavelength is so long compared with the separation distance between conductors in a twisted pair. The article says that good balance should handle the lower frequencies.

Quote
Importantly, the overall susceptibility of twisted-pair cables to electric field disturbance depends on both the balance performance of the cabling and the presence of a screen or shield. Well-balanced cables (Category 6 and above) should be immune to electromagnetic interference up to 30 MHz. The presence of a shield or screen is necessary to avoid electromagnetic interference at higher frequencies, which is an especially critical consideration for next-generation applications.

Quote
At frequencies below 30 MHz, noise currents from the environment can penetrate the screen/shield and affect the twisted pairs; however, the magnitude of these signals is substantially smaller (and mostly attenuated due to the absorption loss of the aluminum foil), meaning that unshielded twisted pairs in the same environment are actually subjected to a much higher electric field strength. The good news is that the balance performance of the cable itself is sufficient up to 30 MHz to ensure minimum susceptibility to disturbance from these noise sources, regardless of the presence of an overall screen/shield.

Any thoughts?
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parkdale

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Re: Modems and mains cables - interference
« Reply #18 on: June 17, 2018, 10:52:28 AM »

https://backyardbrains.com/products/faradaycage    did make me laugh ;D

But it does demonstrate even simple cage can reduce noise
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spring

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Weaver

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Re: Modems and mains cables - interference
« Reply #20 on: June 17, 2018, 11:04:14 AM »

Thanks Spring, I had forgotten about those threads.
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4candles

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Re: Modems and mains cables - interference
« Reply #21 on: June 17, 2018, 04:09:02 PM »

I remember back in the 80s using conductive spray paint on the inside of a BBC B case to stop crud getting out.
Presumably it would stop it getting in too. I've never tried taking out the innards of a router, so I don't know whether that would be practicable.
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Weaver

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Re: Modems and mains cables - interference
« Reply #22 on: June 17, 2018, 05:04:14 PM »

That really is an excellent idea! I need to get the stuff from somewhere and find out how to take a modem apart without wrecking it. Anyone have any ideas about how these things are out together? Has anyone themselves taken one apart?

I could practice taking something that is knackered apart .
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burakkucat

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Re: Modems and mains cables - interference
« Reply #23 on: June 17, 2018, 07:17:58 PM »

To open a ZyXEL VMG1312-B10A turn it over so that the label is uppermost. Cross-head screws are then visible. Remove the screws, turn the device label side downwards and lift it apart.

Don't think of the xDSL patch lead as a cable carrying DC or 50 Hz AC but as a radio frequency transmission line.
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Weaver

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Re: Modems and mains cables - interference
« Reply #24 on: June 17, 2018, 07:35:41 PM »

Thanks for that! I might look around to see if I can find some conductive paint stuff.

I did read about someone painting an entire office with some kind of gunk and the author remarked that it came to quite a lot of money given the large areas that they had to cover.
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4candles

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Re: Modems and mains cables - interference
« Reply #25 on: June 17, 2018, 08:35:27 PM »

In the case of the BBC B, a short wire was run from an earthing point and clamped to the paint surface with a washer, nut and bolt. I don't recall whether I ever tested the efficacy with and without the earth, but effective it certainly was in preventing the computer interfering with the packet radio TNC.
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