RF/EM Interference

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Both RF (Radio Frequency) and EMI (Electro Magnetic Interference) can cause issues with DSL. This is most commonly seen in intermittent sync issues, but can cause permanent outages as well as just minor packet loss. A thing to look for when you are trying to troubleshoot environmental interference is the time of the outage. If there is a pattern to the outage, there is invariably something in the environment that is coming on at precisely the time of the outage. Identifying what that is becomes the next step.

The internal circuits of personal computers can generate RF fields. Also, cathode ray tube (CRT) displays (monitors, TVs, etc.) generate EM energy over a wide band of frequencies. These emissions can interfere with the performance of sensitive devices (such as your modem) nearby.

Listed below are just some of the more common causes of RF/EMI interference. This could easily be a much longer list.


An uninterruptible power supply (UPS) too close to the DSL modem will cause problems. If the modem is close enough to be plugged into the UPS it is probably too close. Separating the DSL modem from the UPS by six feet or more is advisable.

2.4GHz wireless signal

A wireless phone base station does put out a signal strong enough to affect DSL. The new 5.8 GHz phones do not appear to operate in a frequency that bothers DSL, but the slightly older 2.4 GHz phones do. Having these six feet away from each other is advisable. If possible move the base station to another jack altogether, as it is conceivable that the interference could cause a problem with the phone line coming into the modem.

Halogen Lights

A few years ago upright halogen lights gained popularity. These lights anywhere close to a DSL line will cause a problem. These lights are usually operated by a dimmer switch, and have even been found to cause a problem from a room away. If you turn the light off the DSL will come back in a minute or two. These are best kept far far away from the DSL line.

Dimmer Switch

High quality dimmer switches do not seem to cause this problem, so the issue is not universal to all dimmers. Some dimmer switches seem to cause a problem. It is unknown whether this is due to better shielding in the higher quality switches (likely), or if it the way the lower quality manages the power. If there is a dimmer switch in the DSL room, or an adjoining room, make sure it is off as you are trouble shooting.

Ham radio

Some Ham Radio installations can overpower DSL signals. The signal that they emit can over power a DSL signal. Usually, if this is the cause, will be a neighbor or someone relatively close by - within a one block radius.


The subwoofer that most gamers have under their desks are great for sound, but bad for DSL. If the modem is anywhere close to a speaker, the magnet could be affecting service. Moving the DSL modem six feet away from any speaker should prove helpful.

Flickering street light

This is a rarer cause for loss of sync, but really easy to identify. If the loss of sync occurs every night about dark, have the end user look on the street for a nearby street light that is having problems. If there is one they should be able to contact their local city government to get it fixed, this will fix their DSL issues as well.

Other possibilities include:

  • Flashing lights, such as Christmas decorations
  • A microwave oven
  • A timer motor for lights or watering the lawn
  • A television or computer monitor (CRT)
  • A motor (a fan in a bathroom, furnace, oven, or ceiling)
  • A transmitter (Ham or CB)
  • A darkness sensing outdoor light, even at a place down next door

One option for trying to find RF is to use an AM radio. DSL uses the same frequencies as the higher end of the AM radio band, so you can often use a portable AM Radio as a detection device to track down the source. Tune your AM radio near 1400 KHz in-between stations until you hear static. There must be no signal for this to work. You can then walk around the room with the DSL modem in it while the problem is occurring. If you hear feedback on the radio that is RF, and likely in the range that can affect DSL.

Please check and confirm if any of these situations exist in your environment.