Hi Joe,
The loop you are using is called the "Scoop Loop" which was designed by
W1FB and even without any external capacitor the highest frequency of
resonance for that tuned shielded loop antenna is around 3.6 MHz, and you
really need to use something other than that antenna up on 14 MHz where you
are encountering the RFI.
There are many different simple antennas you can use but considering your
RFI is on 14 MHz and considering the strength of your RFI, I would
recommend what I consider to be the simplest untuned shielded loop you can
build, and I call it the VK loop because it was designed by the Radio
Amateur Society of Australia. My testing uncovered the fact that the VK
loop has very poor performance below 10 MHz (very low gain compared with a
traditional untuned loop of similar size), but on 14 MHz and above it works
well compared with a traditional untuned shielded loop, and it definitely
is the easiest to build, and based on your recent reports you should be
able to hear your RFI with this antenna.
The VK untuned shielded loop is just a piece of coax with a BNC connector
or PL259 connector on each end of the coax, but the center conductor of the
coax is cut off on one end of the coax so it's not connected to the BNC or
PL259 connector. You then remove about 1 inch of shield in the middle of
this piece of coax. You then connect each end of the coax to a Tee
connector to form the loop, and that's the entire antenna. You then
connect a short piece of coax to the center of the Tee connector for your
feedline. The beauty of the shielded loop is that it provides great
balance without the need of any chokes to combat common mode issues that
can destroy the pattern, and the beauty of untuned loops is that they are
broadband. It also has a low impedance so you can get away with just
feeding it into a receiver or preamp with a 50 ohm input without doing any
impedance matching (typically works just fine).
Here is a link to a youtube video showing the VK untuned shielded loop
https://youtu.be/Ivr9VbSgjUU. I really don't recommend it for use below 14
MHz, but for 14 MHz through 30 MHz it works really well for DFing. If you
need something quick and easy the VK shielded untuned loop is definitely
something to consider.
What I call a traditional untuned shielded loop is also very easy to build
and it works much better than the VK untuned shielded loop when you go
below 14 MHz but it's slightly more complicated to build (but still dang
easy and only takes about 30 minutes to build). The traditional untuned
shielded loop is just a piece of coax but one end of the coax has its
center conductor and shield connected together, and they are then
terminated (soldered) to the shield of the coax at the other end of this
piece of coax. Let me know if you want more info on the traditional
untuned shielded loop.
The portable flag with preamp really can't be beat and I bet you would hear
your 14 MHz RFI with the portable flag even without a preamp based on how
strong your RFI is. The beauty of the portable flag is that it's
unidirectional which greatly simplifies radio direction finding as you
instantly know what direction the RFI is coming from.
Just FYI,
Don (wd8dsb)
On Thu, Sep 9, 2021 at 1:06 PM Joe Galicic <galicic@comcast.net> wrote:
> Hi Don,
>
> Thanks for responding.
>
> I am using a 14 inch loop I built from your plans many years ago. I think
> this loop is for lower frequencies though. I used it with success finding
> noises on 160 meters. I've never tracked down noise on higher HF
> frequencies.
>
> I went outside again today and turned the K3 preamp on and I can now hear
> the signal but its pretty weak. However, I can hear all kinds of CW and
> FT8 signals with no problem ?
>
> I will take another drive through the neighborhood and see if I can locate
> the signal now that I can hear it.
>
> Joe
> N3HEE
>
> On 09/09/2021 11:24 AM Don Kirk <wd8dsb@gmail.com> wrote:
>
>
> Hi Joe,
>
> You said you have driven around the neighborhood with DF gear but with no
> luck. The real question is if you can hear this when you step outside your
> house with DF gear (don't jump in a car driving around, first get your DF
> gear working so you can hear the signal when standing outside your house).
> That signal you show is so strong (10 to 20 dB over your noise floor) that
> you should easily hear it with a small DF loop outside your house as long
> as you have enough preamp gain. If you can't hear it then you need more
> preamp gain or the signal likely originates in your house.
>
> K0AV suggested using the portable flag, and that would make finding this
> kind of signal a no brainer based on my experience, but a small (12 to 14
> inch diameter) shielded untuned loop made with a hunk of coax would still
> work wonders as long as you use a preamp (you probably will only need 10 or
> 20 dB of preamp gain but that's easy to achieve). The key is to have
> enough gain so you can hear the signal from right outside your house if at
> all possible before you start walking or driving around. I would love
> having a RFI signal like your currently experiencing to find as it's very
> distinctive.
>
> 73,
> Don (wd8dsb)
>
> On Thu, Sep 9, 2021 at 7:55 AM Joe Galicic < galicic@comcast.net> wrote:
>
> > Greetings All,
> >
> > Here is something for all of you RFI sleuths to have some fun with.
> An interesting "signal" has appeared in the last couple of weeks at my QTH.
> >
> > Clues:
> >
> > I've eliminated my house.
> >
> > I've driven around the neighborhood with direction finding gear (K3
> with small magnetic loop antenna) with no luck so far.
> >
> > A local ham 5 miles away can't hear it.
> >
> > The "signal" is there 24/7.
> >
> > It is also on 40 meters.
> >
> > Any ideas what this could be ?
> > See video here https://youtu.be/sl050wudAm0
> >
> > Joe
> > N3HEE
> >
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