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