[RFI] 40 meter intruder?

[Cortland Richmond]

The ARRL web site reports widely a heard intruder on 40 meters, consisting of closely spaced carriers at 
7238.063, 7238.150, 7238.237 and 7238.412 kHz.
http://www.arrl.org/news/stories/2004/09/09/103/?nc=1

These are 87 Hz apart.

063 + 87 = 150
150 + 87 = 237
237 + 87 = 324
324 + 87 = 411 (close enough)

What is significant about 87 Hz? 

"Some monitors, at this point, use a technique called interlacing to cheat a bit and allow themselves to display at a higher resolution than they otherwise could. Instead of redisplaying every line of the screen, when in an interlaced mode the electron guns sweep alternate lines on each pass. So the first pass, odd-numbered lines are refreshed, and the next pass, even-numbered lines, and so on. This allows the refresh rate to be "doubled" because only half the screen is redrawn at a time. The usual refresh rate for interlaced operation is 87 Hz, which corresponds to 43.5 Hz of "real" refresh given the half-screen interlacing."
source: http://www.pcguide.com/ref/crt/resRefresh-c.html

My conclusion: This is 87 Hz noise coupled from a computer monitor into the rig of someone east of Prescott. 

Cortland Richmond
KA5S