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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