How to identify BPL noise - general
BPL comes in several different varieties. Examples of how they sound are found on ARRL video recordings of interference in four BPL field test sites, available on the ARRL Web site.
First, you're only likely to hear a BPL signal if there's a BPL field test or system in operation in your area. You can check to see if BPL may be in your area by consulting the ARRL BPL database at http://p1k.arrl.org/~ehare/bpl/ex2.html or the BPL industry database at http://www.bpldatabase.org.
BPL signals occupy continuous blocks of spectrum. Above and below that spectrum, the noise will not be present. If you were to start tuning at 3 MHz, for example, and find it to be clear of noise, but then as you tune up the band see the noise increase dramatically, and stay that way for at least a MHz or so, that could be diagnostic of BPL. As you tuned higher, you may encounter notches (spectrum the BPL system doesn't use in which the noise is much lower), especially on the ham bands. At some upper limit, the BPL noise will disappear as rapidly as it started, sometimes as high as 35 MHz or so.
BPL noise sounds different than power-line noise, which often will have a strong 120-Hz component and generally are stronger on the lower bands, growing progressively weaker as one moves higher in frequency. BPL signals can be "noiselike," but with no significant 120-Hz component, or they can appear as a series of very close-spaced carrriers--about 1 kHz apart. The carriers can be modulated, often sounding something like a telephone ring. With many BPL modulations, if you switch your receiver to the AM mode, you will hear a distinct audio tone of around 1 kHz. This is generally diagnostic of BPL. If you hear a 120-Hz buzz or hum in the AM mode, it is not BPL, but some type of electrical devicel
The closed-space characteristic is a signature. Birdies every 15 kHz, for example, are not from BPL, although they could be from a nearby computer, switching-mode power supply or harmonics from a TV set's horizontal oscillator. BPL occupies considerable spectrum, but not in the same manner that power-line noise does. The onset of BPL vs frequency will be somewhat abrupt, perhaps inaudible or weak at 4.5 MHz, but increasing rapidly in signal strength by the time you get to 4.6 MHz, just as an example. The same effect is seen in reverse at the upper end of the spectrum in use. Some BPL systems use HomePlug modems, which notch most amateur bands. In those systems, WWV might be affected on 10 MHz, but by the time you get to 10.1, the noise is reduced, only to pick up again outside the amateur band.
Pictures of BPL Equipment
http://www.rarchams.org/images/bpl3.jpg - G1 Main.net pole hardware in Rochester, MN
http://www.mainnet-plc.com/mainnet/home/page.aspx?id=117 - Main.net underground coupler
www.utility.net/images/technology_img01.gif - Utility.net
http://www2.arrl.org/news/stories/2006/07/28/3/BPL-Cinergy-1-lrg.jpg - Current
http://media.nwcurrent.com/images/BPL_large.jpg - Current
Repeaters and other pole-mounted equipment