[RFI] periodic peaks in the spectrum
Ramakrishnan Muthukrishnan
ram at rkrishnan.org
Wed Jul 21 08:56:06 EDT 2021
Hello Don,
Thanks again, I am learning a lot from this thread. The 1" sniffing loop
trick looks great! I am surely going to build it. I am sure this is not
the last RFI I am going to encounter. I have started collecting parts
for the pre-amp.
The main breaker is outside the house but inside his gate and I have
access to it. The individual breakers are inside. There is a big box
that has fuses to individual houses. I have access to that as well. I
think it will come handy to have a sniffing loop. Thanks again.
73
Ram VU3RDD
On 21/07/2021 14:25, Don Kirk wrote:
> Hi Ram,
>
> I don't know if you can get access to your neighbors circuit breaker box
> or what his circuit breakers even looks like, but I have had great
> success using a simple 1 inch loop to sniff circuit breakers to find
> what circuit the device is located on that's causing RFI (you can
> consider this 1 inch loop an H probe). Typically the RFI from the
> faulty device is conducted onto the electrical house wiring and then
> radiated from the house wiring versus being radiated directly from the
> device.
>
> I recently found my own microwave oven control panel was generating RFI
> using this simple circuit breaker sniffing technique. The sniffing
> method said my microwave oven circuit was causing the RFI and then I
> sniffed around my microwave oven and found its control panel/display
> unit was generating the RFI. I have also used this simple sniffing
> method on circuit breakers at a large apartment complex to find the
> circuit that the device generating RFI was located on without the need
> to shut power down to the entire apartment complex as an example.
>
> Here is the link to a video showing the small 1 inch loop I use for
> sniffing circuit breakers after I had identified the suspect circuit
> https://youtu.be/UqLQ-mmUrVA <https://youtu.be/UqLQ-mmUrVA>. I just
> pass the 1 inch loop over the top of each circuit breaker listening for
> which circuit breaker produces the strongest interference, and this
> typically allows me to identify the suspect circuit.
>
> My 1 inch sniffer loop is just a short piece of coax (like 2 feet long)
> where I expose the last part of the center conductor and make a 1 inch
> diameter loop out of it and connect it's free end to the shield (very
> simple).
>
> Just another trick you might be able to use if you think you have
> identified the correct property generating the RFI, but please be
> careful (remember safety first).
>
> 73,
> Don (wd8dsb)
>
>
> On Wed, Jul 21, 2021 at 3:31 AM Ramakrishnan Muthukrishnan
> <ram at rkrishnan.org <mailto:ram at rkrishnan.org>> wrote:
>
> Hello Don, Dave, Jim and others:
>
> Thanks so much for all the wonderful information. I have started
> reading
> the articles mentioned in order to build a DF loop. I think I am going
> to try Don's design along with a pre-amp.
>
> Apparently the next door neighbour (the houses are densely packed here,
> so next door is "literally" 5-6 ft away) has a 4kVA inverter. Last
> night
> when everyone is asleep, I stepped out with a shortwave receiver
> (sangean radio which I borrowed from another friend) and could zero in
> on the neighbour's house more or less. I initially suspected his fridge
> and politely asked him to turn off the fridge for a few minutes while I
> am looking at the spectrum. The birdies remained (assuming he actually
> turned it off). He did mention that he has a 4kVA inverter powering
> everything from refrigerator to other devices. That is the prime
> suspect
> now because these birdies are present 24x7.
>
> Going to do an on/off test of the inverter (if the neighbour is
> willing)
> this weekend. Here in the India, there is very little one can do about
> flagging violations. So, perhaps installing an x-phase based canceller
> is probably going to be the only workable solution for my shack. :-( I
> feel bad about letting a known source pollute the RF spectrum and I am
> going to try my best to fix it somehow. May be buy toroids and wrap the
> outgoing mains from his inverter or see if the chassis is properly
> grounded on the outside.
>
> 73
> Ram VU3RDD
>
> On 21/07/2021 06:03, Don Kirk wrote:
> > Hi Ram,
> >
> > I recently built and tested side by side numerous direction finding
> > tuned loops, untuned unshielded loops, untuned non shielded
> loops, loops
> > with chokes, loops with transformers, terminated loops, etc.
> >
> > Based on all my testing the simplest but still reliable
> bi-directional
> > antenna for MF and HF portable direction finding that you can go
> with
> > would be a simple single loop of wire similar in size that Dave
> (W0LEV)
> > mentioned (12 to 17 inches in diameter) fed directly with coax,
> but you
> > need to use a choke right at the antenna feedpoint consisting of 14
> > turns of coax (RG58 or RG174, etc.) wound on an FT240-31 toroid core
> > (this choke is based on the K9YC choke design). I would use
> enough coax
> > to get the 14 turns of coax wound on the toroid core plus another 36
> > inches for running to your preamp or receiver input. The output
> > impedance of this single turn loop is reasonably low, and you can
> > therefore connect it direct to the 50 ohm input on your preamp or
> > receiver, etc (no transformer required). If you omit the choke, the
> > antenna balance is not reliable depending on what frequency you
> use the
> > antenna on (the feedline becomes part of the antenna which then
> alters
> > the pattern and you will find big differences in the depth of the
> nulls
> > and even experience only one null depending on how high in
> frequency you
> > go). This antenna will be bi-directional if built properly.
> >
> > The other simple option that I prefer is a untuned shielded loop
> like
> > Dave mentioned made with a piece of coax. I tested numerous
> designs,
> > and the most reliable and easy to build untuned shielded loop for
> full
> > MF and HF coverage is what I call a traditional untuned shielded
> loop in
> > which the shield of the coax that makes up the antenna has its
> shield
> > exposed (jacket removed) near the antennas connector and the far
> end of
> > the coax (coax that makes up the antenna) shield and center
> conductor
> > connect to each other and they are then soldered to the shield
> that was
> > exposed near the antennas coax connector. Then about 1 inch of
> shield
> > on the coax that makes up the antenna is removed at the center of
> the
> > loop which was formed by the coax. You can then feed this simple
> > untuned shielded loop directly into your preamp or receiver since
> once
> > again the impedance of this untuned shielded loop is reasonably
> low (no
> > transformer needed). A choke is not needed because the shielded
> loop
> > has a built in balun based on the geometry of the design. This
> antenna
> > is really my favorite portable untuned bi-directional loop. Once
> again
> > I would make it about 12 to 17 inches in diameter (the larger it
> is, the
> > more sensitive it will be but you need to stay 0.1 wavelengths in
> > circumference or smaller).
> >
> > The signal strength of the interference will dictate if you need a
> > preamp or not regardless of what portable MF/HF direction finding
> > antenna you use. A 20 dB preamp will normally be adequate in all
> but
> > the weakest levels of RFI. My favorite preamp is the DX Engineering
> > preamp that was designed for use with my portable flag, but I
> also like
> > the W7IUV preamp which you can power with a 9 volt battery for
> portable use.
> >
> > My favorite antenna for direction finding is the portable flag I
> > designed for radio direction finding that Jim (K9YC) mentioned since
> > it's unidirectional and this antenna saves me an enormous amount
> of time
> > since I instantly know what direction the RFI is coming from (no
> need to
> > triangulate like you have to do with a bi-directional antenna in
> order
> > to determine what direction the signal is coming from). Here is my
> > simple website that describes the portable flag, and it really is an
> > easy antenna to build, but once again you will likely need a preamp.
> > https://sites.google.com/site/portableflagantenna/
> <https://sites.google.com/site/portableflagantenna/>
> > <https://sites.google.com/site/portableflagantenna/
> <https://sites.google.com/site/portableflagantenna/>>
> >
> > You could also just go walking around with a portable shortwave
> receiver
> > and whip antenna looking for the strongest signal, but that's
> really a
> > process that can cause a lot of indecision and guessing.
> >
> > P.S. it does you no good guessing what device is generating the
> RFI as
> > long as you understand its behavior (example: on 24 hours a day, not
> > drifting, what frequencies you hear it on, etc). You first must
> locate
> > the property where the RFI is coming from using radio direction
> finding
> > gear, and then you can figure out what the device is.
> >
> > 73, and let me know if you need more details on the antennas I
> mentioned
> > above. I have lots of pictures and diagrams I can share with
> you, and
> > they really are simple to build.
> >
> > Don (wd8dsb)
> >
>
>
> --
> Ramakrishnan
>
--
Ramakrishnan
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