And even if each WERE FCC Class A compliant, together, they could --
switching synchronously -- far exceed the limit. A cheap switcher might
lock to noise on a common power source, whether generated elsewhere or
on one or more switchers.
Cortland
KA5S
On 2/5/18 12:09 PM, Hare, Ed W1RFI wrote:
That system would be a Class A Part-15 device, so would have to meet the higher
industrial limits. Locally, ie near the motors, controller or wiring, I could well
imagine a 50+ dB increase in noise from a "legal" device. If that were 55 dB
at a nearby Amateur antenna, it is possible that the system exceeded the Class A limits.
Ed, W1RFI
-----Original Message-----
From: RFI [mailto:rfi-bounces@contesting.com] On Behalf Of David Eckhardt
Sent: Saturday, February 03, 2018 12:50 PM
To: Cortland Richmond
Cc: RFI
Subject: Re: [RFI] RFI every 15 KHz on 160 meters, suspect source is a
manufacturing facility.
Look for AC/AC or AC/DC switching converters. I won't relate the story, but we had
an incident where 208/3-phase was fed to several speed controllers on a large
irrigation system (20-housepower AC motors). The proper and required decoupling
components were not bought and installed due to cost. We experienced switching
noises some 55 dB over baseline on a portable R&S spectrum analyzer. Our
emissions peaked right in the middle of the 40-meter band but were detectable to
low VHF. The controllers were from China with a storefront on the west coast -
what else is new these days with no FCC enforcement.
Dave - WØLEV
On Sat, Feb 3, 2018 at 3:36 AM, Cortland Richmond <ka5s@earthlink.net>
wrote:
Great Don.
On 2/2/18 8:28 PM, Don Kirk wrote:
Hi Cortland,
Thanks for the info. I just constructed a 3" diameter loop (not
tuned) using some pretty rigid RG58U, and will try it along with a 6"
diameter tuned loop (adjustable between 20 and 30 MHz). I have
basically tracked the RFI down to a room, but the room is packed full
of motor controls (numerous VFDs, etc.). The 3" diameter loop
appears to be working well with my SDR receiver based on some simple bench top
testing I just did.
Thanks,
Don (wd8dsb)
On Fri, Feb 2, 2018 at 4:00 PM, Cortland Richmond
<ka5s@earthlink.net>
wrote:
Hi, gents. Cortland here, KA5S.
I had about a 30 year career in EMC engineering, and often had to
locate specific sources of EMI.
A quite small loop will do very well if one is in close proximity to
the magnetic field of an emitter. That is, there is a good deal of
utility in entering the near field very closely.
You may remember the famous 3520 kHz television modems of some years ago.
For those I tried using three antennas; a surplus EMC loop antenna
about
1
m in diameter, ad un-tuned whip antenna, and about a 3 inch loop
antenna made by bending a piece of semi-rigid cable back on itself
and soldering the exposed center conductor at the non-connectorized
end to the outside of the shield near the connector.
This last was unbalanced, but its small wavelength aperture made it
good almost through VHF.
At 80M, on a handheld receiver, it could accurately lead me to the
exact room one of those modems was radiating from.
Much smaller loops can be constructed on the open end of a single
piece of coax, and can follow individual traces on a PWB – but that
is beyond the scope of this discussion.
Good luck!
Cortland
ka5s
On 2/2/18 2:32 PM, Don Kirk wrote:
Hi Dave and gang,
I totally agree on the smaller loop, but I believe what is also
important is to factor in the implications of being in the near
field. That's why I think going to a much higher frequency would
be very beneficial (get out of the near field as much as possible,
but we will see).
Stay tuned (no pun intended).
Don (wd8dsb)
On Fri, Feb 2, 2018 at 1:51 PM, Dave Cole (NK7Z) <dave@nk7z.net> wrote:
I would try a a much smaller mag loop, like a foot in diameter. I
have
used a 6 inch loop in some cases, they are still sort of directional.
Add a
ferrite on the coax at the feed point. I have one loop that is
about an inch in diameter, I use it seldom, but it is handy when
needed.
I also have a small probe I built out of a piece of coax, I just
cut the shield back 3 inches, used dollop of liquid electrical
tape on the end of the center conductor for insulation, and then I
probe around...
73s and thanks,
Dave
NK7Z
http://www.nk7z.net
On 02/02/2018 10:20 AM, Don Kirk wrote:
Quick follow up on my locating the source of the repeating 15
KHz
signal
(actually 15.6 KHz).
Today I met with the suspect facilities machine controls engineer
that just happens to be an inactive but still licensed ham (turns
out we also have mutual friends in the machine controls world).
I first walked around the entire perimeter of the building
(outside in very cold temperatures) using my portable SDR system,
and was able to determine that the source of the interference is
located near the west central part of the building where there
are several water cooling towers as well as material storage
silos. The signal is indeed cyclic but still need to see how
repeatable the cycle is (typically on for about 20 minutes and
then off for about 20 minutes). Inside the west wall there are a
bunch of facilities related equipment (pumps, variable speed
drives, etc.).
Unfortunately my 160 meter tuned loop is useless when in so close
to the source, and we were not able to determine the actual
source in the allotted time we had today. With the SDR receiver
gain set at 0, the signal was still near full scale when in close
to the source (it appears we have the source nailed down to about
a 50 foot by 25 foot area, but there is a ton of motor controls
equipment in this small area as well as metal building structure
that likely is confounding our DFing).
We are now going provide my vehicle description, name, etc. to
the security department that patrols the grounds of this facility
so they know I have permission to be on the property 24 hours a
day 7 days a week. I'm going to obtain more data on the cyclic
nature of the interference (to see if it provides any clues), and
also evaluate capturing the signal at much higher frequencies
(determine what the highest frequency is that I can still capture
the signal), and then build a DF antenna for the higher frequency
to help pinpoint the source when in close. After I obtain more
data from outside the building and develop a better antenna for
in close DFing, I will then go back into the building to continue
our in close DFing (as time permits).
My portable SDR system consists of a Dell laptop, NooElec SDR
dongle, and ham it up converter (stuck to the back of the laptop
screen using double stick tape). The SDR dongle and ham it up
converter are powered via the laptop USB ports which makes it
very convenient. I just plug my DF antenna into the ham it up
converter and adjust the SDR dongle gain as needed (via the
SDRSharp software I'm using).
Just FYI,
Don (wd8dsb)
On Mon, Jan 22, 2018 at 9:15 AM, Don Kirk <wd8dsb@gmail.com> wrote:
Yesterday I tracked down RFI that has been bothering me on 160
meters for
the past year. The source was a little hard to find because it's
not
always on. I finally had to use my SDR dongle with Ham it up
converter in the car with my small tuned loop to track down the
source of the interference (in order to make sure what I was
seeing at home was what I was actually tracking). The suspect
source is 0.75 miles from my house.
Here is a link to a video showing my efforts to date.
http://www.youtube.com/watch?v=LKf1EWTV8xs
Today I will contact the manufacturing facility that I tracked
the RFI down to in order to take the next step with them.
I will report the actual source of the RFI as soon as it's
determined which might take some time (usually takes time to
establish a good working relationship with the suspect property
owner). Based on past experience it sure looks like a variable
speed drive, but in order to keep an open mind I try not to
guess ahead of time.
Just FYI,
Don (wd8dsb)
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