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

[Alan Bloom]

Mystery solved!  The spurs appear to be coming from a solar installation 
on a house about 1/2 mile (3/4 km) from my house.

One interesting point is that the spurs do not go away at sundown, but 
continue until fairly late in the evening.  Apparently that solar 
installation must have some kind of battery to store the energy.

Solar panels are DC devices and do not themselves generate 
interference.  Rather it is the inverter(s) and other electronics that 
are the problem.  There are two kinds of solar systems -- the ones with 
all the panels in series feeding a single inverter and the kind with a 
separate inverter for each panel.  The latter is the kind I had on my 
house in California before it was destroyed in a fire and I never had a 
noise problem.  I have heard that the single-inverter systems are more 
troublesome from an RFI standpoint.  There was an article in April 2016 
QST magazine about how to mitigate RFI from solar systems.  ("Can Solar 
Power and Ham Radio Coexist?" by Tony Brock-Fisher K1KP)

------------------------------

The story:  I finally go around to walking around the neighborhood with 
my KX2.  I only have the AX1 antenna for it, which is not resonant on 
the 40 meter band, and I was not using a counterpoise so I could barely 
hear the signal from in front of my house.

I started walking south down the street but the signal seemed to be 
getting weaker.  So I turned around and walked north.  The signal was 
getting slightly stronger the farther I went.  I turned right at the end 
of the street onto another street and it kept gradually getting a little 
stronger.  At one point I suspected it might be coming from a Montessori 
school, but when I walked down the access street toward it it didn't get 
any stronger.  Plus with everything in the news these days I didn't 
think it would be a good idea for a strange man holding a strange 
contraption to be walking around the school grounds.  :=)

So I kept walking down the main street and within a couple blocks the 
signal started to rapidly increase in strength.  It peaked in front of a 
certain house, strongest at the right side of the house. Sure enough 
there are solar panels on that side of the roof.  The signal was peaking 
about S4 or S5 on the KX2 S-meter.  Again, this is with a non-resonant 
antenna with no counterpoise.

As I mentioned, the spurs are about S6 on the ground-mounted trap 
vertical at my house and they are almost buried in the noise when the 
band opens up at night.  So I'm not going to bug the neighbor about it.  
But I bet they would have trouble trying to listen to AM radio at their 
house.

Alan N1AL



On 6/7/22 17:21, 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