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Author Topic: Cable Modem  (Read 4371 times)
AJ8MH
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« on: March 26, 2014, 06:35:44 AM »

I installed my Charter provided modem and wireless router last night and I now have low level whine on 160 and 80.  It's just noticeable at the 80 meter CW noise floor, but if you select AM, the noise is clearly audible.  Interference is worse on 160.  However, the noise isn't really a BIG issue at the moment.

I tried ferrites and no change.  Noise increases if I unplug the Cat 5 cable between the modem and wireless router.  Noise will also increase if I put my hand on top of the modem.  If I shut off the router, the noise is still there, so the problem is the modem.

The modem is an Arris TM822 with DOCSIS 3.0 capabilities.  (There is a Part 15 disclaimer in the manual.)  It's located in the next room about 5 feet from the shack and about 10 feet from the base of the inverted-L antennas.

Charter is trying to replace their DOCSIS 2.0 modem/wireless router single box systems with these new 2 box systems.  Overall, a speed increase is noticeable (except with Charter e-mail and that's a server problem).  Progress is great!
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W8JX
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« Reply #1 on: March 26, 2014, 02:34:32 PM »


The modem is an Arris TM822 with DOCSIS 3.0 capabilities.  (There is a Part 15 disclaimer in the manual.)  It's located in the next room about 5 feet from the shack and about 10 feet from the base of the inverted-L antennas.


Not sure why you would locate modem only 10 feet from base of antenna...
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KD0REQ
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« Reply #2 on: March 26, 2014, 04:13:06 PM »

what kind of power does it have?  wall wart?  by any chance, a no-weight slimline wall wart with a switcher supply inside?

if so, shake the junkbox until a suitable analog wall wart comes up with the same plug and polarity.  I bet you notice a difference.

if not, read Part 15 to them and say they can replace it now, or be herded out by the FCC waving fines and imprisonments all the way behind them...
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AJ8MH
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« Reply #3 on: March 26, 2014, 04:44:50 PM »

Not sure why you would locate modem only 10 feet from base of antenna...

When we bought the house, we placed the combo modem/wireless router in the same place the old owner had one.  The cable company used the first tap on the first splitter for the Internet cable run.  The second tap feeds all the TV's in the house through a filter.  I believe it's called a back-feed filter.  (I have three additional splitters off the second feed, too.)  Never had any issues until the change.

When I put the tower in, I found it convenient to place it near the shack.  The inverted-L's were an afterthought.

To answer another question...the modem power supply is built into the box.  Yup, first one I've seen without a wall-wart.  The router has one, but it's quiet.  But, a switching power supply could be inside the box.

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WA2ISE
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« Reply #4 on: March 28, 2014, 12:11:15 PM »

Thought comes to mind: placing the cable modem inside a metal box.  The CATV would enter the box via an F connector barrel, the shield grounded to the box.  The ethernet outlet usually have isolation transformers so hopefully the RFI won't leak out thru them.  If it does, make a common mode choke, using a large ferrite core wrap many turns of ethernet cable thru it, having this ferrite core close to the modem inside the box.  Look to see if the cable modem ethernet jack has side metal prongs that would make using shielded ethernet cords possible, and if so, use one.  As for the power line, use one of those combination EMI filter and IEC connector mounted to the box.  Then find a power cord that fits the cable modem and wire it to the terminals of that IEC connector. 
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K1CJS
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« Reply #5 on: March 29, 2014, 07:06:49 AM »

Have you tried contacting Charter and telling them you have an RFI problem with their equipment?  See if you can switch it out and if the replacement is any better.  If not, contact them again.  If no joy then, you can always send a letter to the regional FCC office.  It may help you, it may not, but you'll never know unless you try.
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AJ8MH
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« Reply #6 on: April 02, 2014, 08:19:57 AM »

I banished the Charter Arris cable modem and Netgear wireless router boxes to the wall in the basement next to the old Ariss telephone modem.  Wireless coverage seem OK.
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