[Elecraft] I need a Sherlock Holmes (weird spurs on 40m)

[Alan Bloom]

As part of christening my new QTH/antenna/rig here at N1AL, today I did 
the test where I recorded all off-the-air spurious signals on all bands 
and then threw the main circuit  breaker for the house and did the 
measurement again, powering the K4 from a battery.  This is to identify 
any spurs that are coming from my house so I can do further sleuthing to 
figure out what is causing them.

One spur (or set of spurs) has me mystified.  It is a series of 
harmonics, with very stable frequencies, spaced at precisely 24 kHz, 
that extend from roughly 6.6 MHz to 7.4 MHz.  Each spur consists of a 
main carrier and a secondary carrier approximately 150 Hz lower in 
frequency and approximately 8 dB lower in amplitude.  The spurs are all 
the same amplitude, around -90 dBm (S6), dropping off as you approach 
6.6 or 7.4 MHz.  I don't see these spurs on any other band.

The spur amplitudes did not change when I turned off AC power, so it 
can't be the rig's switching power supply or any other electronic device 
in the house.  It's nothing internal to the radio because if I switch to 
a dummy antenna the spurs go away.

So it's coming in through the antenna.  The antenna is a 6-band trap 
vertical about 30 feet from the house, with the coax coming underground 
to the shack.  We're on a large lot, there is a canyon (i.e. no houses) 
behind the property, and there is a vacant lot on the side where the 
antenna is located so the nearest houses in the neighborhood are about 
150 feet away from the antenna.

The electric utility power lines switch from overhead to underground at 
our property line, about 150 feet away from the antenna. Internet is via 
cable, which is underground also.  Both power and Internet enter at the 
far end of the main house, which is over 100 feet from the shack, 
located in a granny unit.

I believe the exact fundamental frequency is 7007.03 kHz / 292 = 23.9967 
kHz, in case that's a clue.

Anyone have any ideas of what could be causing this?

Alan N1AL