On the subject of ferrite, using type 31 along the length of the Ethernet cable will have no negative effect. Each signal within the cable is on a balanced transmission line. The ferrite will only affect common mode current, not the intended differential mode current - Glenn W9IQ
I stand corrected. Glen is right (my "brain-fart" - sorry!) - unless there is a major imbalance in the ethernet cable due to issues with the driver/receiver or cable, the differential mode signals should be unaffected by any choke. No doubt there will be SOME imbalances - but they shouldn't be large.
However, since the balanced, twisted pair lines are tightly coupled in decent CAT5e or better cable, and the receivers and drivers in point to point connections are normally coupled to the cable by (balanced) transformers, common mode current shouldn't be much of an issue either and the chokes shouldn't do much to help OR hurt.
So why use them at all?
"Shouldn't" being the operative word. Since ham products exist specifically to address ethernet RFI issues, and some of these products are not compatible with POE (which can produce common mode current issues and RFI emissions FROM the cable), I presume there is SOME vulnerability to RFI from amateur radio transmissions coupling INTO the ethernet cable that ferrites/filters can help with. Gary, NA6O's comment backs this up with ferrite recommendations that seem to be based on his own experience, and
it would be very instructive to hear more regarding when he found chokes to be needed (and why) and in what level of ethernet (10/100/1G, etc.).
I've just reviewed Jim Brown's (KY9C) very good writeup on RFI and ferrites, which includes ethernet RFI protection. On Page 14 he makes the observation that unshielded, unbalanced home AC wiring can easily be the primary contributor to RFI entering the ethernet link via the power line into ethernet driven devices and discusses use of an AC line isolation transformer later in the text to prevent this. Exactly what solved the problem with my son's game console.
Regarding:
The commercial RFI devices are overpriced….
Perhaps, but $50 is not too bad for a pair of nicely packaged, easily moved line filters. The mix 31 toroids are about $10 each, with two needed to be comparable to the DXE kit, plus a new, longer ethernet cable (ballparked at $10) to absorb the 15 turns needed at each end (30 total). So its a difference of $20 per line, not $40, to have a pair of clean, movable filters vs. a pair of hand wound, bare (and conductive) mix 31 toroids that are fixed to one cable.
While I agree that in a very thorough treatment, each and every line into and out of a router or switch will need to be filtered - and that would also multiply the cost differential - it's probably also overkill. I'd do it for a commercial installation, but would experiment more for a lower budget ham application. Consider that a simple router/switch will have at least 6 lines to be protected, 4 of which, in the best case, will need two rather than one filter. Thats going to run more than $150 even with hand wound toroids.
Very possibly it's simpler and cheaper to get the easy, drop-in filters, and just move them around to see which lines they're most effective on rather than"shotgunning" all of the lines at once.
And per Jim Brown's writeup, spend some of the savings on mix 31 chokes or an AC line isolator (about $120 on Amazon) for the game console power lines.
Again, just some thoughts.
Brian - K6BRN