[RFI] RFI every 15 KHz on 160 meters, suspect source is a manufacturing facility.

[Cortland Richmond]

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 at 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 at 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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