From:
http://www.qrpis.org/~k3ng/bpl.html
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Q: What is intermodulation?

A: Intermodulation is the mixing of radio signals which produces new radio signals. Think of
it as radio waves having children. But just how do radio waves have children ?

This mixing is caused by what are called non-linearities. One non-linear electronic component
that you find in most any electronic device is a diode. When multiple radio signals are run
through the diode, they mix together. Let's say we have a 4 Mhz signal and a 6 Mhz signal
going into the diode. We would then get:

4 + 6 = 10 Mhz

6 ? 4 = 2 Mhz

4 and 6 Mhz had two ?children?, 2 and 10 Mhz !

Now, non-linearities are usually good. This phenomenon is used in just about every radio
device to either create a signal to be transmitted, or receive a signal that you hear or see.
But, non-linearities can occur where you don't want them and then in causes problems. One such
place is in power lines. Bad, corroded connections or dissimilar metals touching can create
natural diodes that act like mixers and produce this intermodulation.

So, let's take a BPL signal and for the sake of discussion, say it's a grossly simplified
consisting of radio signals at 1, 5, 8, 9, and 12 Mhz. Some of the intermodulation products
that could be created would be:

1 + 5 = 6 Mhz

8 + 9 = 17 Mhz

9 + 12 = 21 Mhz

12 ? 9 = 3 Mhz

But you could also have what is known as third order products:

1 + 9 + 12 = 22 Mhz

8 + 9 + 12 = 29 Mhz

8 ? 5 + 12 = 15 Mhz

Or even:

2 * 12 = 24 Mhz

(9 ? 5) * 12 = 48 Mhz

You can do the math and figure out each permutation, but you get the idea. If we took a real
BPL signal that has signals from 1 ? 80 Mhz the number of products and where they would fall
are mind-boggling. The resulting intermodulation products in a system could extend well above
the band BPL proponents want, falling into FM broadcast, VHF TV, Aeronautical, and more public
safety bands. This is just another reason why BPL is so problematic.

It's arguable that such non-linearities in power lines are exhibited as arcing connections,
something that most power companies are actively searching for these days as the RFI (radio
frequency interference) effects are well understood. These maintenance issues will be
addressed quickly by well run utilities. However, non-linear loads are common in homes, light
dimmers being the first devices that come to mind. Theoretically, these devices could create
intermodulation that would in turn be radiated by the house wiring and outside power cabling.