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

[Victor Rosenthal 4X6GP]

I don't have a guess as to the cause, but it might be worth trying a 
common mode choke at the base of the vertical. I know that some of these 
antennas (like the Cushcraft/MFJ versions) do have such chokes built in, 
but I found that an additional one helped isolate the line from the antenna.

Indeed, you can't even say for sure that the signal doesn't come from 
the K4 if it goes away when you switch to a dummy load, because if the 
antenna coax is not entirely isolated from the antenna, the signal could 
be flowing from the radio back to the antenna via the outside of the 
coax. The dummy load is probably shielded, so there's no way for this to 
happen.

73,
Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
https://www.qsl.net/k2vco/
On 08/06/2022 2:21, Alan Bloom wrote:
> 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
> 
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