[Elecraft] OT Cat 7 Ethernet Cable

[Jim Brown]

On Mon,3/7/2016 6:47 PM, Clay Autery wrote:
> Any device putting out RFI as common mode are NOT in spec

Perhaps, but such devices are VERY common in the consumer world. I have 
no contact with the pro world of IT.

> ....  Whether
> they can be "fixed" or not has a lot to do with what "mistake" in design
> caused the problem in the first place.  That has to be determined by
> device.  Depending on the cause of the RF generation, there are a number
> of ways to mitigate/eliminate the problem...  Shielded cable is only ONE
> possible component in the overall solution.

Yep. Suitable common mode chokes can make a big dent in the RF trash 
radiated by the cable.

> A non-inclusive list of causes:
>
> Badly designed and/or shielded switch mode power supply (wall wart OR
> internal).

The Ethernet birdies/carriers I listed are from the data stream, NOT 
from the PSU. Of course the PSU often generates trash, but it's 
DIFFERENT trash. :)

>   A wall wart can radiate via the DC cord to the device and/or
> send the signal into the device via the DC cable and then radiate via
> the chassis or signal cables..  USUALLY internal PSs are adequately
> shielded... but poor input filters or none at all can let the bad stuff
> through.

Trash that radiates to our antennas is COMMON mode, not differential 
mode, and common mode is caused by improper return paths, and it cannot 
be "filtered." Filtering affects differential mode trash.

> Wireless radios that are improperly designed or shielded can also be the
> culprit.

I can't see any way that a 900 MHz or 1.8 GHz radio can cause RFI at HF. 
It is the baseband digital signals that have HF components.

> Not having a separate logical and chassis ground can be a problem...

WRONG -- that is a CAUSE of problems, not a solution. See Henry Ott's 
classic text on EMC.

> Shielded cables work best in my experience when you can create an
> isolated chassis (or supplemental Faraday cage type superior to the
> chassis (or internal to a plastic case) where the outer shield of the
> cable and connector housing can be connected in one continuous shield
> around the device(s) and the cable, etc, etc...

EXACTLY WRONG. For shielding to be effective, it must be continuous, 
including a termination to the shielding enclosure at both ends. Again, 
see Ott.
> The benefit to CAT 7/7a would be the double shielding and the superior
> connector termination and housings... make for easier continuous shield
> external connections to drain/ground, et al.
>
> Typically, it would be cheaper/more reliable to buy a properly designed
> device in the first place...

Of course. :) IF you can identify one. But that requires a published, 
competent, technical review of products on the market that include RFI.  
Call me collect when you find such a thing. :)
> CAT 7(+) would make an excellent 4 circuit control wire for remotes and
> the like since it has shield per pair...

Those working in the world of pro audio deal every day with microphone 
signals in the range of -140 dBu, with required dynamic range of at 
least 110 dB, and typically have 24-32 mics active in a given recording 
or production. Any coherent noise that is present in multiple inputs 
will degrade s/n by 3 dB per doubling of the number of inputs, and 
modulation of an RF signal present on multiple inputs will be coherent 
after detection. My point is that pro audio faces a s/n problem FAR 
worse that we do in the radio or computer networking world. In that 
world, we have learned that TWISTING is at least as important as 
shielding, and often MORE important.

That said, serious contesters with extensive IT background have reported 
that STP does kill Ethernet birdies, and I believe them. I've never 
heard a suggestion that CAT7 was needed, nor have I ever read a spec for 
any of these cables.

73, Jim K9YC