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Things that I have found to create wideband RF Interference (In order of occurrences):

Preface: There are many, many things that cause interference, and if you find the (suspected cause) of the problem, use some tact! There are many reasons for devices becoming unwanted receivers, and both sides may be "at fault". Both sides will have to work for a successful completion of the problem. Don't approach this with an "Attitude" or you'll have an Attitude all by yourself!

1. Dimmers... There are so many, with each company making them as inexpensive as possible, they reduce or remove the RFI filters to keep costs down, profits up. You can usually use a portable AM radio tuned to the upper band edge at an unoccupied station to be used as an RFI sniffer. It will sound like a raspy buzzing. Work around: Unplug it and replace with one that has better RFI suppression (Good Luck!)

2. Power transformers (Com ED)
Use the same RFI Sniffing procedure (Car or walkaround), then call and complain.

3. Electric devices (irons, frypans, heat lamps, etc). The noise they generate as they turn on, turn off, is considerable. AM radio (Powered off AC) will lead you right to it. Christmas light flashers will do same thing to a lesser extent.

4. Transmissions (Ham, CB, Cellphone, etc.) These are harder to find. Watch the modulation characteristics:

Is it a solid pattern, a repeating pattern on/off, or can you determine that something is modulating the interference?

A AM radio or cheap telephone may give you insight to the problem, but again, maybe not. Here is some generalizations that is guaranteed to get me in trouble for posting:

IF the interference IS or IS NOT detectable by a cheap phone, or AM radio, turn on the TV (not cable) to Channel 5 and then Channel 2. IF the same is visible on both 5 and 2 (sometimes CH 5 or 2 may be stronger), THEN it is a good chance it is CB radio. Transmissions will be intermittent in nature, and if you can detect it with a cheap phone or AM radio, you will hear the audio transmitted. 10-4 Good Buddy!

IF the interference is rapid in ON/OFF sequence (1/4 second pulses or 6-8 second bursts), followed by pauses of 1 to 3 minutes, AND there is no apparent modulation, THEN it is a good chance it is a paging system, or MORSE CODE, and origin is from a local ham, of paging company. Find by looking for antennas. Approach them without an attitude, and you'll get far.

IF the interference is random, with no modulation visible or audible in the interference, it could be many things, pagers, ham transmissions.

Take your time, be polite, and good luck.

Feedback received on this FAQ entry:
  • Devices with a resistive load, like heating devices, do not generate noticeable interference. Devices that have an inductive load, like motors, do cause radio interference. I've noticed that refrigerators, central air furnaces and washing machines give off interference. Some modern UPS systems give off a bit of radio interference. If you're trying to find the source of the interference with basic DSL, you must use an AM radio frequency less than 1104 kHz. It's probably best to start at the bottom of the radio dial (lowest frequency), and work your way up. If your modem comes with a signal to noise ratio graph or a bit loading graph interface, set the AM radio dial to the bin with the lowest signal to noise ratio. Each bin is 4.3125 kHz. So if you have no signal at bin 188, you would multiply that bin number by 4.3125 kHz to get the frequency, which is 810.75 kHz. This method will help you direct your attention to the exact frequency of the interference. This type of graph is available in the 2Wire modem's Management Diagnostic Console (gateway.2wire.net/xslt?PAGE=J42&THISPAGE=A02_POST&NEXTPAGE=J42).

    2010-03-26 14:49:24 (DiamondSky See Profile)

  • In the HDTV (ATSC) all-digital over the air TV world we are heading toward in 2009, Channel 2 and Channel 5 will no longer be suitable suggestions for interference troubleshooting. Using any TV channel for interference troubleshooting may become impossible if/when the FCC potentially approves the usage of frequency "white space" for broadband. It would be good to archive these troubleshooting methods in the future though, just for posterity. These older technologies interference patterns (period and duration) may still be recognizable with alternative (cheap spectrum analyzers?) or legacy equipment (AM/HAM) in the future.

    2007-09-23 21:48:06 (justbits See Profile)

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by Jan Janowski See Profile edited by Flippant See Profile
last modified: 2007-09-24 08:23:12