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
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