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

Fri Feb 2 22:36:07 EST 2018

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 at 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 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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