[RFI] periodic peaks in the spectrum

[Don Kirk]

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.  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> 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/>
> >
> > 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
>