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HF/MW/LW General Discussion - General discussion on monitoring the HF (High Frequency), MW (Medium Wave), and LW (Long Wave) spectrum (0.5 - 30 MHz)

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  #1 (permalink)  
Old 11-04-2007, 7:55 AM
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Default Cable Internet interference on HF... a tip

I'd like to pass along a good tip for HF listeners with cable tv in their home. This tip may or may not apply to you, so I will tell you what my circumstances are.

After having Comcast Cable install their "package" of services:

1. Internet
2. TV channels
3. Cable telephone service

with the "Touchstone" Telephony Modem, I had horrendous noise generated all over the MW-SW bands at my location.

I suspected that this buzz/hash was coming from the new system installed but had to make sure. I also didn't want to modify or remove the equipment and lose the service.

Shutting off everything in the household, and powering it all up one circuit at a time, I found that the modem itself was the culprit. This modem is Touchstone model TM402P. This model has a 120VAC cord that plugs in the back of the unit to power the modem, so the unit has its own built in power supply. It also has an internal Li-Ion battery for backup when the power fails. It will "hold up" the telephone function in case of an outage for a number of hours.

I suspected that the internal power supply was generating the "hash" on HF when the modem was plugged into AC power. My first attempt to kill the interference was to wind the AC cord on a large ferrite toroid, trying to choke off the hash. I thought that the hash was traveling back out the cord and getting into the rest of the household wiring, where it was being radiated all over the home.

BTW, I cannot modify the modem as it is being "rented" from the company... it is theirs, not mine.

This choke failed to give any relief... the noise was still all over the spectrum covered by the HF receiver. I finally came to the realization that I could simply unplug the AC cord for the modem and see what happened. RELIEF!! YES!! The HF interference was nearly all gone, and I could finally copy weaker signals again!

What must be happening is that the internal power supply has little to no filtering, and all the switching transients are polluting the HF spectum household-wide. With no AC input, there is no hash generated. When I listen to my HF radio now, I simply unplug the AC from the modem and let the battery backup take over to keep the telephone working. When I am finished with HF listening, I just plug the cord back in to restore battery charging and normal modem operation.

I still notice some weaker digital noise on some freqs on the receiver. I assume that they will always be there due to the nature of the cable service and some "leakage". I can/must live with it as it isn't too severe... (hey, the wife needs her TV! ;-)

I hope this tip helps other poor souls who have just about given up the hobby in frustration.
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Old 11-04-2007, 8:02 AM
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Quote:
Originally Posted by exham
I'd like to pass along a good tip for HF listeners with cable tv in their home. This tip may or may not apply to you, so I will tell you what my circumstances are.

. . . .

I hope this tip helps other poor souls who have just about given up the hobby in frustration.
If it that bad, maybe it is a bad unit.

Have the cable company replace it and see if the next one is better.
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Old 11-04-2007, 2:02 PM
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Default interference

thats good that you were able to pinpoint the source but you should also notify comcast and make sure you keep and maintain all documentation and correspondence and keep a log. you may not need it right now but what if a neighbor gets one of these units and you can't simply unplug it when you would like to listen to HF, MANY neighbors don't understand nor do they care about part 15 devices which by the way are going to be the biggest threat to HF radio in the near future if we hav'nt already reach it.
I had a nightmare with a person that lived next door that had an "invisible fence" for his animal, the radio spectrum from VLF up to 7 mhz was wiped out by this interference. the people finally moved, weather it was from the FCC sending them several letters telling him what they were going to do if they sent an agent or what I don't know but they took a licking in the real estate market.
we just had verizon fios installed and I have not noticed any harmful interference yet.
Good job on finding the source.
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Old 11-04-2007, 4:49 PM
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N_Jay,

It's possible that the unit is bad, but since it functions well in its assigned duties, I'd bet the manufacturer just "cut corners" on the filtering to save money. It's somewhat of a hassle to take things back to the local office, so I'll just put up with the unit for now... (I may change my mind later).

comsec1,

Yes, if a neighbor gets one, I'd be "up a creek". I should notify Comcast of the situation... maybe by mail. Yea, there are SO many lousy part 15 devices creating nightmares for our hobby. Bottom line is:

Reality:

We are few, "they" are many. The FCC isn't really interested in making sure that these devices are "clean". It's easier to look the other way when so much money is involved, and since there are relatively few of us, who cares if a few folk with a hobby that few understand experience "grief".

Glad your "fence" problem went away. Man, I'll bet that was noisy!
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Old 11-05-2007, 10:44 AM
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Hi guys, my first post here. This forum was recommended by someone on candlepowerforums.com (a place for flashlight enthusiasts).

Anyway. I have my scanner and Ham radios on a 19-inch rack that is shared with my PC and cable modem. The PC and cable modem make a LOT of RF noise. I have solved 90% of this by using metal window screen to "plug" any RF leaks (in addition to the ferrite chokes).

On the cable modem, I had to disassemble it and "seal" the cooling vents with the window screen. The screen and metal case inside the modem are aluminum, so I couldn't solder it for electrical contact. So instead I used electrically-conductive epoxy.

On the PC, most RF noise was coming from the power supply through the hole for the cooling fan. I covered it with aluminum "honeycomb" and this worked most excellently well.

Lately I've also taken to buying copper shim stock and cutting it (with scissors) into flaps. The flaps join the ground path to any removable metal case covers. You basically solder or rivet the copper shim to a ground path and let it stick up so that it touches case covers when you close up the system.

If you try this yourself, just remember that any shielding metals have to be grounded...
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Old 11-05-2007, 8:58 PM
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That's good advice and information....

Dave
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Old 11-06-2007, 7:11 PM
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Quote:

"That's good advice and information"....



... and I would follow it if I owned the modem! :-)
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Old 11-07-2007, 9:37 AM
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Quote:
Originally Posted by exham
Quote:

"That's good advice and information"....



... and I would follow it if I owned the modem! :-)
I don't own mine... I guess I modified based on the opinion that nobody would care and/or nobody would notice hahaha....
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Old 11-10-2007, 1:20 PM
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Does anybody know what broadband over power lines interference sounds like over H.F.?? I ask this as I have an interference problem at my location that I can't get rid of So I am trying to identify the type of interference that I am hearing. can anybody help?? De NE9N
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Old 11-10-2007, 2:59 PM
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This page has some recordings but I have not listened to them
http://www.arrl.org/tis/info/HTML/plc/aud-vid.html
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Old 08-15-2017, 12:10 PM
   
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I had the same problem. I added shielding to my modem by placing it between three sheets of metal. I believe aluminum foil would do as well but remember to keep the front exposed for your remote control.
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Old 08-16-2017, 6:33 AM
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I have the same telephone modem mentioned in the original post and luckily I haven't had issues with it, but I also made sure to use one of those extremely short phone lines (6 inch?) out the back to my phone which actually sits on top of it. I'm guessing a real long one may act as a broadcast antenna for the interference as well...just a thought. As for my PC modem I had all sorts of interference issues until I ordered a nice short double shielded Ethernet cable from L-com to go between the modem and PC. It worked like magic eliminating all the trouble I was having. Cat5e Double Shielded Ethernet Cable - 24 AWG Solid LSZH Industrial Ethernet Patch Cords- Red
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