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Author Topic: Cable Modem RFI  (Read 9697 times)
K2DFC
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Posts: 40




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« on: December 07, 2011, 07:45:48 AM »

Getting very strong RFI from my cable modem. The model is a Motorola SB5101. Interference is strongest on 160 and 80 meters and the AM broadcast band. The question is, is it the wall wort or the modem? If I unplug the power line from the modem but leave the wall wort plugged in to the AC outlet the RFI is reduced by 80%. But the wort is still producing RFI. Is the modem amplifying the interference produced by the wort? If it's the modem not sure what the options are since it is provided by the cable company and they may not have anything better to replace it with. Anyone have any experience with this type of RFI?

Fred K2DFC
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KC9TNH
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« Reply #1 on: December 07, 2011, 08:18:26 AM »

Nice specific question Fred, thanks for posting. Have looked previously through some other threads on this topic and watching with interest, as will be having an install in the next few days.

As a clue, I have told the installer to bring a selection of chokes. (I have my own in case.) Have read here at eham.net in some cases a snap-on choke or some windings taken around, with one at EACH end of the wall-wart cord (typically the thinnest cheapest thing that'll conduct what they need) may mitigate things.

So watching with interest, no info as to whether that particular model is noisier than others. (Given some of the reviews of this relatively cheap device not surprised that ISPs would use them. I doubt anyone tests or certifies these devices much for compliance with anything nowadays.)

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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
W1AEX
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« Reply #2 on: December 07, 2011, 08:28:24 AM »

Hi Fred,

When I was using a Motorola SB5101 I found that the little switching supply it came with generated oscillations that wandered up and down the 75 meter band. I replaced the little switcher with a linear power supply and that was the end of the wandering oscillations. Eventually, I replaced the SB5101 with a Motorola SB6120 (DOCSIS 3.0) modem which came with a different switching power supply that was equally noisy, so it's running now with a linear supply as well. It's a shame that these crappy little switching supplies are so prolific as they add to the growing crud on the HF spectrum, but I guess that's the world we live in now!

73,

Rob W1AEX 
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KC9TNH
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« Reply #3 on: December 07, 2011, 09:24:38 AM »

It's a shame that these crappy little switching supplies are so prolific as they add to the growing crud on the HF spectrum, but I guess that's the world we live in now!
LOL Rob. Recently during a lull in the local deer season I tossed some wire up in a tree, hooked up the 817 behind the barn wayyyyyy out in the country and....
Holy Quietude!  Back home in RFI-HellSuburbia it's sadly a different story - jeez, we make alot of noise.
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
W1AEX
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« Reply #4 on: December 07, 2011, 10:55:58 AM »

Yes indeed, there are many dubious devices out there. We are pretty much at the mercy of whatever dubious devices our neighbors bring home from the shelves of Walmart and Best Buy. I cringe every time I see one of those clearance ads for cheaply priced and very crappy large-screen plasma televisions. All it takes is one neighbor within 1000 feet who didn't get the memo about those things...

:O)
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KC9TNH
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« Reply #5 on: December 07, 2011, 11:49:36 AM »

I told my neighbors not to even think about such a dirty thing as a plasma, and have offered to take them shopping to the nearby big city in my pickup truck to buy the LCD/LED of their choice if they want a chauffeur.
 Cheesy

I'm hoping the upcoming install doesn't turn into a lesson wherein I get to show grand-daughter the principal behind a Faraday cage...
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Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
VA2FSQ
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« Reply #6 on: December 10, 2011, 07:43:13 PM »

Well there may be some specific offenders in your home, but don't be surprised that even if you totally turn off everything and run your radio from a battery, the noise is still horrendous.  I am plagued by s4-s9 noise on many bands, and when I did the above I noticed zero difference.  Nothing.  It is all coming over the air in my QTH.
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VA2FSQ
W9CW
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« Reply #7 on: January 01, 2012, 09:59:07 AM »

I told my neighbors not to even think about such a dirty thing as a plasma, and have offered to take them shopping to the nearby big city in my pickup truck to buy the LCD/LED of their choice if they want a chauffeur.
 Cheesy

I'm hoping the upcoming install doesn't turn into a lesson wherein I get to show grand-daughter the principal behind a Faraday cage...


I know the post above is almost a month old, but I will relate my situation here.  My neighbor, less than 30 feet from my antenna, has a plasma flat screen, and when they're watching TV, I have S9 noise all over the HF bands, especially 80 thru 20m.  It makes operating not only a tremendous challenge, but somewhat depressing.

I remember when I initially began in this hobby in 1961, there was NO noise anywhere on any bands.  The bands were dead quiet as to RFI/EMI, and you could copy signals that barely moved the S-meter off the peg.   This is not the case today, with so many cheap electronic devices, most of which have CPUs and switching power supplies.  Compared to 1961, we hams (who are in residential subdivisions or within city confines per se) live in RFI/EMI h*ll.  With the exception of my ham rig, and SWL receiver, the only electronics in my parent's house was one B/W CRT TV, one Westinghouse (when "Westinghouse" was really Westinghouse) portable stereo, one portable transistor radio made by the real Zenith, and one AM/FM table radio...  that's it.

I live in east central Illinois, and sometimes I wish I was a farmer with a couple hundred acres.  At least I'd have a fighting chance of being able to hear S2 to S7 signals again!
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K2DFC
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« Reply #8 on: January 01, 2012, 03:05:48 PM »

Don, you got that right. I live in the country. 30 years ago it was a QRN free zone. Now every house is full on noise generating devices. I just found out the noise that would wipe out the 10 meter band from time to time is coming from my neighbor's garage lights. Took me over a year to find it. Luckily he doesn't leave them on that often. But I still have to ind a way to get rid of it. But when I do something else will take it's place. My Son got a plasma TV one day and I had the same results. He has since sold it and replaced it with an LED one that is OK.

Fred K2DFC
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K1CJS
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« Reply #9 on: January 02, 2012, 08:38:45 AM »

Getting very strong RFI from my cable modem. The model is a Motorola SB5101. Interference is strongest on 160 and 80 meters and the AM broadcast band. The question is, is it the wall wort or the modem? If I unplug the power line from the modem but leave the wall wort plugged in to the AC outlet the RFI is reduced by 80%. But the wort is still producing RFI. Is the modem amplifying the interference produced by the wort? If it's the modem not sure what the options are since it is provided by the cable company and they may not have anything better to replace it with. Anyone have any experience with this type of RFI?

Fred K2DFC

What happens when youu pull the wall wart, Fred?  Does the noise completely go away?  If it does, you've more than likely identified the culprit.  Most of those modems run on 12 volts DC.  If yours does too, substitute another 12 VDC source and run the modem.  If the noise goes away completely, you've got your answer--and solution!  73!
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K2DFC
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« Reply #10 on: January 02, 2012, 07:58:57 PM »

Yes it's gone. I will try another 12 volt source. Just found a 12 volt linear wort and need a connector. Will see if that does the trick.
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KC9TNH
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« Reply #11 on: January 03, 2012, 04:04:38 AM »

Yes it's gone. I will try another 12 volt source. Just found a 12 volt linear wort and need a connector. Will see if that does the trick.
Best of luck to you. After getting net into the house as gift for the wife, took the old AM radio on "walkabout." My cable modem's wart is 10V but not quite as noisy as the 12V one that came with the Cisco e1200 router. A very aged (and bigger footprint) adapter left over from an old Uniden scanner (still in service) was substituted and life is good. At some point I may ask them to substitute a modem that uses 12V and just put the whole shebang on a little 3A power supply I have.

Must say that walking around a quiet house with the batt-powered AM radio was most enlightening. I could almost get the 'rhythm' of the house and offenders were easily ID'd. (Flourescents going bad are really loud.) Over the course of the holidays, mitigating this or that, my standard noise level on 75 or 40m went from S6+ down to S4. The last culprit will be the TV; and if it accidentally expires early - so I can replace with a newer LED model - OK with me. Other exterior residential stuff is largely out of my control. S4 on those 2 bands in town isn't bad compared to before. Am 'hearing' alot better now.
 Grin
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
K1CJS
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« Reply #12 on: January 04, 2012, 03:39:18 PM »

Yes it's gone. I will try another 12 volt source. Just found a 12 volt linear wort and need a connector. Will see if that does the trick.

Since you've found that that wall wart isn't good for your radios, how about just cutting the end off of it?  Even if the modem is a rental, the cable company usually doesn't require it back--just the modem.

Or better yet, the next time you see a cable company van on the street, stop and ask the tech if he has an old wall wart that is dead--or just ask for another.  Chances are you can get one from him for nothing!
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WD9DUI
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« Reply #13 on: November 07, 2012, 08:31:03 PM »

Thanks for the info on the cable modem RFI, I had been having problems for a while and it did turn out that it was the wall wart. I was getting noise on the AM radio, I had disconnected the cable from the back of the cable mode and the noise went a way, I thought that it was some leakage from the Motorola SB5101 but I was wrong. I changed out the wall transformer and the noise went away.
Gary WD9DUI
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KD8TZC
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« Reply #14 on: April 24, 2013, 10:10:43 AM »

Just came across this older post as I am having similar problems with a cable modem... can any recommend where I can find a quality wall wart (linear?)?

Thanks,

John
kd8tzc
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John - KD8TZC
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