That's great, Don.
The big advantage with getting up close with a magnetic loop is that the
H field increases quite rapidly as you approach the conductor, so you
won't need as much sensitivity and you can turn the gain down or insert
attenuators as needed to pinpoint the exact source.
I once found a problem with a computer returned to the manufacturer
for not meeting FCC part 15, and in frustration I turned it over and
used one of my little loops to sweep along the bottom. I could actually
follow the 33 MHz clock through the improperly painted shield. Whoops.
(That of course ended up with my having to change the spray application
and devising a test jig to assure sufficient attenuation once that had
been done. Fun times.)
Good luck!
Cortland
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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