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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