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Author Topic: Router, Modem, and Power Supply RFI Elimination  (Read 4352 times)
NT0A
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Posts: 93




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« on: September 07, 2011, 04:21:50 AM »

Router, Modem, and Power Supply RFI Elimination

Executive Summary
To minimize computer generated RFI in your ham shack:

  • 1. Never use unshielded cable in your home network.
  • 2. Put a Fair-Rite type 31 and a Fair-Rite type 61 toroid at each end of your cable.
  • 3. Never use an unfiltered power supply in your computer.
  • 4. You can rely on power supplies manufactured by PC Power. They cost more, but they eliminate power supply RFI.

Background
Those of you who have read my earlier posts know that my son’s entry into ham radio in May marked the start of my trek back into amateur radio. All-in-all, I’ve probably spent as much time and effort getting re-established as I did getting into the hobby some 35 years ago, but on entirely different tasks. Those tasks have included:

  • 1. Helping my son in 6-land get on the air.
  • 2. Cleaning up 15 years’ worth of dust and grime from ham shack and from the inside and outside of the equipment.
  • 3. Putting up a new 80m dipole antenna.
  • 4. Finding and eliminating hobby killing RFI across the ham bands.
  • 5. Rejuvenating my CW fist and ear.*
  • 6. Adapting my TS-830S to the explosion of digital modes over the last 20 years.
    * An on-going and seemingly never ending effort.

It’s Like an Onion
Each of those have presented a series of challenges to overcome, but the most frustrating task has been finding the source of the RFI and eliminating it or at least reducing it to an acceptable level. One of the things that I learned in the process is that eliminating RFI is like peeling an onion. You peel off one layer only to discover that there is yet another layer below it to be peeled or eliminated.

Broadband RFI from Computer Power Supply
The most objectionable RFI was from the computer in my ham shack. From what I learned on the Internet, the best solution was to replace the ham shack desktop computer with a laptop. Considering that I had just spent several fistfuls of money upgrading the computer, I refused to spend another hunk of change on another laptop exclusively for the ham shack. The next best solution appeared to be replacing the original computer power supply with one that was fully filtered. The one with the most recommendations was by PC Power, but that was still more than I wanted to spend. I made the obligatory trip to Radio Shack to buy a bag of ferrite beads to put on every electrical wire, cord, and cable that came from the computer. I was rewarded with a reduction of approximately 1dB or 1/3 of an S-unit.

Reluctantly I ordered the PC Power model 760W from Amazon fully expecting it was going to be $120 down the drain, but when I installed the power supply, all of the computer noise disappeared! In its place were multiple birdies of undetermined origin.

Bird Hunting
Some of the birdies chirped in at more than an S-7. The birdies were tracked to the CAT5 UPT (unshielded twisted pair) cables hooked to the Linksys router. Based upon Jim Brown’s (K9YC) exhaustive treatise on using ferrite to solve RFI problems I ordered the mix of toroids recommended by Jim. (Arrow Electronics had the best price.)

Toroids – Phase I
I put at least one type 31 toroid with 12-14 turns on the end of each cable. The impact was minimal. Additional troubleshooting revealed that the culprit was the Cisco cable modem and the primary radiator was the CAT5 cable between the modem and the router. Since the impedance of multiple toroidal chokes in series is additive, I put seven toroids on modem-router CAT5 cable. The router/modem RFI on 80m and 40m was greatly reduced, but on 20m and above the reduction was only 6dB. That left discrete S-5 noise spikes across the upper HF bands. Still looking for an inexpensive solution I researched shielded cables. CAT9 STP (shielded twisted pair) would have provided the greatest attenuation, but I could not find any readymade CAT9 cables, and the smallest length of CAT9 cable I could find was a 1,000 foot spool. The specs on CAT6 STP looked very good and I found some 5-meter CAT6 STP cables with shielded RJ-45 connectors on Amazon.com for  $3.09 each.

Two days later I plugged in a raw cable and ran a test. The result was slightly less attenuation than the CAT5 UTP cable with 7 12-14 turn toroids. A type 31 toroid with 12 turns at each end of the CAT6 cable provided minimal additional attenuation.

Toroids – Phase II
Perhaps toroids of a different material would do the trick. I reviewed Jim Brown’s graphs, and using two different types of ferrite toroids appeared to offer the best hope. I ordered a half dozen type 61 toroids. When they arrived, I added a type 61 toroid to the end of the CAT6 STP cable that already had a type 31 toroid at each end. Voilà! The modem/router RFI dropped to an S-0. It’s still there, but it it doesn’t hide weak signals unless it’s on the exact same frequency.

Lessons Learned
To minimize computer generated RFI in your ham shack:

  • 1. Never use unshielded cable in your home network.
  • 2. Put a Fair-Rite type 31 and a Fair-Rite type 61 toroid at each end of your cable.
  • 3. Never use an unfiltered power supply in your computer.
  • 4. You can rely on power supplies manufactured by PC Power. They cost more, but they eliminate power supply RFI.

73 es gud hunting de Bob NT0A
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WB4BYQ
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Posts: 179




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« Reply #1 on: September 07, 2011, 06:29:06 AM »

Thanks for the information.  I have a book from a fiber optic to cat 5/6 converter that reads that in order to fit part 15 rules for residental use, that shielded cable must be used.  i have fixed the router/ethernet switch with type 43 and type 31 ferrite material as well.  i have seen that in Europe that all ethernet cabling should be done in shielded cable for rfi control.  thanks for the report and i hope many will find help from your work.

richard
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NT0A
Member

Posts: 93




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« Reply #2 on: September 07, 2011, 06:55:28 AM »

Richard,

Thanks for the information.  I have a book from a fiber optic to cat 5/6 converter that reads that in order to fit part 15 rules for residential use, that shielded cable must be used.  i have fixed the router/ethernet switch with type 43 and type 31 ferrite material as well. I have seen that in Europe that all Ethernet cabling should be done in shielded cable for rfi control.

The longer that I am in ham radio the more I wish that the FCC would enforce Part 15 of the FCC Regulations. They repeatedly bend under pressure of industry cries that, "We can't do that because it costs too much." or conversely give Part 15 certification for a well designed electronic appliance and then allow the manufacturer to market the device with the components that ensure Part 15 compliance left out.

Quote
Thanks for the report and i hope many will find help from your work.

Me too!
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K3AN
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Posts: 787




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« Reply #3 on: September 27, 2011, 10:09:32 AM »

Problems with modems and routers might be brand-specific. I have a "WebSTAR" cable modem supplied by the cable company and a Netgear router. Unshielded CAT 5 cables run from the modem to the router and the router to the desktop. This equipment is in a room directly above the hamshack, and no more than 40 feet from either of my HF antennas. I've installed no toroids anywhere but all I can find are occasional spots on 10M and 15M where the noise is a bit higher than what I presume is the background level. Nothing moves the S-meter.

There's also a box supplied by the cable company for our telephone service. I did have to wrap the modular cable through a toroid about five turns to suppress some RFI.

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KF7CG
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Posts: 812




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« Reply #4 on: September 27, 2011, 11:06:25 AM »

Remember for higher network speeds the length of sheilded cable runs without regeneration (switch, router, amplifier) should be limited to approximately 50 feet. Sheilded cables put a large strain on computer cable drivers. Back when I did a fair amount of ASCII cabling the cables were rated up to 1500 feet for UTP or 150 feet sheilded. This was for 19.2 KB signaling. The transmission speeds have gone up sharply but the transmission lengths have gone down with the ratio between sheilded and unsheilded remaing about the same. Cable quality, especially the rate and evenness of the twist in the twisted pairs, has a great deal to do with the noise from digital cabling. Don't sheild unless you have to, but don't use cheap or unrated cable either.

KF7CG
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