Why AM radio and ADSL are at odds

AM radio interferes with ADSL because they try to use the same electromagnetic frequencies at the same time.

The nearly 5,000 AM radio stations licensed in the U.S. broadcast at frequencies between 540 KHz and 1.7 MHz. ADSL service providers use the 138-KHz to 1.1-MHz range to download data to customers.

So as you can see, there's a sizable overlap.

It would seem that AM radio would wipe out most of the ADSL range, but because stations transmit at discrete frequencies, each station affects just a targeted area of the ADSL spectrum. AM stations in a given area don't generally fill up the entire available spectrum.

ADSL modems have the ability to just stop using that segment of the frequency spectrum occupied by any nearby AM station. ADSL transmissions are broken into frequency chunks called "carriers." There are 256 carriers per ADSL line and 128 per line for G.lite, a lower-speed version of ADSL. When an AM signal interferes with a carrier, one remedy is to stop using that carrier and to drop the bandwidth available to carry data.

Only the download speed is affected, and that's because the frequencies used to send data to customers are the ones that overlap with AM radio.

The longer the wire to the customer site, the more susceptible an ADSL line is to interference. That is because the signal gets weaker as it travels down the wires and is therefore more easily disrupted. The effect is particularly pronounced if the AM transmitter is near the customer at the end of a long line.

Twisting wires around each other makes the signal on them less sensitive to interference, and phone companies use twisted pairs of wire to reach customers.