Having traveled similar tracks as Cortland (EMC/RFI Engineer), I can not
stress the usefulness of the 3" loop. Just make one and use it. Take our
word (writing) for its utility!
Dave - WØLEV
EMC Design & Test, LLC
On Fri, Feb 2, 2018 at 9:00 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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--
*Dave - WØLEV*
*Just Let Darwin Work*
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