There are a few things to remember about using a small loop for DFing on HF.
For "point sources," such as a device in a home being heard from the street,
small loops can work well. I remember years back, when a "wireless modem jack"
product was introducted into the market, operating on frequencies near 3.56
MHz, I had brougth a small receiver and a small 80-m loop I had designed,
using a ferrite rod, on a business trip. I was a hotel and my room had a small
balcony, so I took the receiver and loop onto the balcony, to hear what I could
hear.
As I tuned the band, pleased that this little resonant loop could hear signals,
to my surprise, I heard a T7 carrier on 3.56 MHz, characteristic of the
wireless modem jack. I had a rental car, so I decided to find it. I got a
heading on its general direction by finding the null and knew it was located on
one of two headings. I took a ride and went about a half mile perpendicular to
the heading (more or less) and still heard the signal, so I took a new heading
and now knew which direction it was.
I drove to that area and, with a bit of driving around and taking new headings,
I got to the house.
As I drove past the house, I could keep the null pointed at the house and
watched the heading change as I drove past it. It was clear that the house
wiring was doing most of the radiating, so there was a sharp-enough null. And,
for the record, this was heard from my hotel balcony about a mile from where it
was located, so these devices were WAY above the FCC emissions limits for
carrier-current devices. ARRL worked with the manufacturer to redesign the
product to no longer use the ham bands. AT&T had bought them and installed them
by the thousands in conjunction with its pay-per-view cable television service,
and they ended up doing a system-wide recall of them, so the whole story is
interesting in and of itself.
In another case, though, I used it to try to DF a power-ine noise. It was very
strong, so from afar, I could get a heading, but when I got close, I couldn't
really find a meaningful null. In the first example, the house wiring acted
like a point source, but the power-line noise was radiating from a long length
of overhead wiring, with standing waves and other complicating factors. When I
was close, no matter where I pointed the null, there was signal coming in at
many angles from the null, so there was no null.
The moral of the story is that there are some types of noise sources in which
one cannot find a heading with a loop antenna.
But all is not lost. I remember another case I went out on, loopless. A ham
had a strong 80 meter source, and he had driven around to find out where it was
coming from. I was in the general area on another IEEE meeting trip, but all I
had was my HF mobile set up. I drove to where he said it was coming from, but
it was across a small river and there were no overhead wires on the bridge, so
driving away from his house, the noise disappeared completely and noise was
heard again on the other side of the river. It sounded different, too,
meaningful ONLY if it sounds different. By ear, dissimilar noises can sound the
same.
I went back to his house. I could hear it from his driveway, so I went off in
another direction. There was a standing wave on the line. I got a quarter mile
away and there was a side road to my right, and as I drove past that side road,
the noise started geting weaker, again in peaks and nulls, but clearly
disappearing. Back to the side road and as I drove the road, it got louder and
louder with peaks and nulls, until I got to a section of road whee it screamed
so loud all I could tell is that it somewhere along about 0.2 miles of overhead
line.
I had a step attenuator, so I decided to try a trick. I drove to about where I
thought it was strongest. I listended to the raucous buzz (120 Hz) and cranked
in attentuation until the buzz dissapeared and all I heard was white noise from
the receiver. I cranked back until I could JUST barely hear the buzz over the
white noise of the receiver. My ear was pretty good at that. I again drove
the line and heard it louder as I drove past a certain pole. I stopped at the
pole, cranked in attenuation until it was just audible and sure enugh, it was
heard only driving past that pole.
I looked around and there was a small commercial building to my left, with 240
volt wiring running to the pole transformer. I drove up the driveway and as I
got near the building, it got louder. I laughed as I looked at the building
name -- Acme Welding.
What I realized was a few things. First, one can be misled driving around and
hearing noises. If that noise is strong at your QTH, but disappears when you
drive away, but noise reappears some distance away, it is probably a different
noise you can't hear from your station. Unless it is causing harmful
interference, the power company does NOT need to fix it. If you find a dozen
noises within a few miles and ask the company to fix them all, you WILL wear
out your welcome and they, and the FCC, will think you are on a crusade against
noise. Find and report only noises you hear, and keep in mind how easy it is
to be misled.
But the moral of the story is that very strong noise sources come from
everywhere, although if I had a loop near that welding shop, although it was
being radiated strongly by the overhead line and there was no deep null, it was
also being radiated by the shop wiring, thee may have been some null. But from
the street, the noise was heard from the overhead line, from the radiation of
the ground wire on the pile to my vertical whip and from the building. They
would have added up to a vector angle that may have "pointed" at some location
other than the building. But I had deciced to try the step attenuator and use
of my trained ear to pinpoint the souce, and, to my surprise, it actually
worked.
I also noted two things. When I was near the source, some of the noise was
coming from the building wiring. This decayed rapidly with distance. But when
it got coupled onto the overhead lines, it was also radiated by a large wire,
so decayed at the well-known inverse square law perpendicular from the line,
but beause the overhead lines were a leaky transmission line, it traveled along
that line for quite some distance.
Okay, those are my experiences. Use them as you will, but note all the caveats
that to really find the souce this way, I had to combine several bits of
knowledge.
Ed Hare, W1RFI
ARRL Lab
________________________________
From: RFI <rfi-bounces+w1rfi=arrl.org@contesting.com> on behalf of David
Eckhardt <davearea51a@gmail.com>
Sent: Tuesday, March 23, 2021 6:29 PM
To: Gary <gary_mayfield@hotmail.com>
Cc: Rfi List <rfi@contesting.com>
Subject: Re: [RFI] New Interference at kk0sd
Another trick I've used in DFing interference is to construct a shortened
helically wound dipole on a wooden dowel stick. One can easily resonate it
for the frequency of interest by extending / trimming the pigtails at the
high-Z end of each helical winding. While the null of the ends is not as
sharp as with the shield loops, it gets the job done and is extremely
narrow in response.
Dave - WØLEV
On Tue, Mar 23, 2021 at 10:20 PM Gary <gary_mayfield@hotmail.com> wrote:
> Don,
>
> I hope to hook the loop to my FT-847 this weekend as it is
> on my list of tests. It is a little more involved to do. I don’t hear much
> in general with antennas in my basement shack…I probably have more faith in
> the loop than my portable shortwave radio. I am glad to hear the loop works
> okay on the shortwave bands. I use a ferrite antenna for the AM band when I
> hear noise there. It still amazes me I do not hear anything on the AM
> broadcast band. The good news is I did hear the noise when connecting my
> portable shortwave to my main antenna…
>
> I have also arranged to borrow an FT-818 for portable
> chasing this weekend. I hear the noise on 6 meters (but not 2 meters) so I
> am hoping with the portable rig and a 6 meter beam I can at least get
> pointed in the right direction.
>
> Thank You Everyone for the Help!
>
> 73,
> Gary “Joe” kk0sd
>
> From: Don Kirk <wd8dsb@gmail.com>
> Sent: Tuesday, March 23, 2021 4:34 PM
> To: Mike Martin <mike@rfiservices.com>
> Cc: Gary <gary_mayfield@hotmail.com>; Kim Elmore <cw_de_n5op@sbcglobal.net>;
> Rfi List <rfi@contesting.com>
> Subject: Re: [RFI] New Interference at kk0sd
>
> Hi Gary and gang,
>
> Let me go back and clarify my previous statement about your shielded loop
> as I just did more testing on higher frequencies. Down in the AM broadcast
> band your loop based on my using RG-58 is deaf compared with a simple 9
> inch single turn loop, but on higher frequencies its sensitivity greatly
> improves (I even thought the sheild on my RG-58 might be inadequate down in
> the AM broadcast band so I added aluminum foil as a secondary shield and
> that had no impact. Up on 10 and 15 MHz I would say the gain of my
> version of your shielded loop is much closer to that of my simple 9 inch
> diameter loop but I did not dig out my test gear to see how close they
> really were, but I did like how the shielded loop was behaving on the
> higher frequencies. I'm not sure what any of the numbers mean because
> there is no attention paid to impedance matching, etc.
>
> I went back and looked at your original video and on 20 meters the RFI is
> very strong, and I can't believe you can't hear it with your shielded loop
> based on some signal strength measurements I did today (maybe your portable
> radio is deaf????). I can just barely hear WWV on 10 MHz using my version
> of your shielded loop when WWV is running about S7 on my HF rig that's
> using an antenna that's resonate on 10 MHz (during the afternoon).
>
> I really am curious what you would hear on 20 meters if you just connect
> your shielded loop to your HF rig. If it's easy to do it would be great if
> you could do this with your shielded loop outside (maybe just connect the
> shielded loop to your coax that's currently going to your vertical.
>
> Sorry I have probably gotten you way off track.
> Don (wd8dsb)
>
> On Tue, Mar 23, 2021 at 3:11 PM Don Kirk <wd8dsb@gmail.com<mailto:
> wd8dsb@gmail.com>> wrote:
> Hi Mike, Gary, and gang,
>
> Well after seeing Garys shielded un-tuned loop, I watched the video on how
> to build that loop. I just built one almost identical to his loop but used
> RG-58 coax since that's what I had on hand. I also spent 10 seconds
> building a simple 1 turn 9 inch loop (no form, just free standing) for
> comparisons which is even smaller than Garys loop, and I can tell you there
> is no comparison. Without doing any impedance matching, the 9 inch
> un-tuned single turn loop I built has a gain that is approximately 23 dB
> greater than Garys loop when tested in the AM broadcast band and it still
> exhibits nice nulls which comes as no surprise. My single turn non
> shielded loop just has about 6 inches of twisted wire (the same wire that
> forms the loop) to form the feedline to it, and then I just used a 3 foot
> piece of 50 ohm coax to connect it to my receiver. I used the same 3 foot
> piece of coax to connect the shielded un-tuned loop to the receiver. While
> my un-tuned 9 inch loop might not be as well balanced (one null might be
> deeper than the opposite null), it makes one fine DF antenna in a pinch.
> Also if Gary needs more gain he can just make the loop I made bigger in
> diameter. It really just takes seconds to make this antenna (much less
> than a minute).
>
> I now fully understand why Gary can't hear his noise, as his un-tuned
> shielded loop has very poor sensitivity based on my trying to copy the
> design. While I normally use preamps with my DF loops, Garys RFI is so
> strong that I'm pretty darn sure he can make a simple one turn loop to
> figure out what possible directions the signal is coming from. Something I
> have done in the past is to use a cardboard box as the antenna form as it
> does not have to be round. Need to figure out what directions the signal
> is coming from before leaving his property if at all possible.
>
> Just my opinion, and I will try and post some comparison videos but they
> will not be pretty as I'm buried in other tasks.
>
> 73,
> Don (wd8dsb)
>
>
>
> On Tue, Mar 23, 2021 at 8:33 AM Mike Martin <mike@rfiservices.com<mailto:
> mike@rfiservices.com>> wrote:
> Remember in his shack he has a very good radio and a very good antenna
> which together increased sensitivity and allow him to hear much further
> away. when he goes outside with a portable and a smaller antenna he doesn't
> have the sensitivity he had inside there for he may have to travel a lot
> further before he's able to receive it.
> Be safe,
> Mike
> Get BlueMail for Android<http://www.bluemail.me/r?b=16696>
> On Mar 22, 2021, at 11:37 PM, Gary <gary_mayfield@hotmail.com<mailto:
> gary_mayfield@hotmail.com>> wrote:
>
> Kim,
>
> I hear it on the portable when connected to the vertical antenna, but
> will undertake an antenna substitution experiment this weekend.
>
> Keep the suggestions coming!
>
> 73 and Thanks,
> Gary "Joe", kk0sd
>
>
>
> -----Original Message-----
> From: RFI <rfi-bounces+gary_mayfield=hotmail.com<http://hotmail.com>@
> contesting.com<http://contesting.com>> On Behalf Of Kim Elmore
> Sent: Monday, March 22, 2021 4:44 PM
> To: Don Kirk <wd8dsb@gmail.com<mailto:wd8dsb@gmail.com>>
> Cc: Rfi List <rfi@contesting.com<mailto:rfi@contesting.com>>
> Subject: Re: [RFI] New Interference at kk0sd
>
> That’s a great idea, too! I once ran into an interference problem that was
> inherent to the antenna, but only under certain conditions. It didn’t sound
> like Gary’s but I pulled a lot of hair out fixing that one. I’m simply
> suspicious that he hears NOTHING with his portable rx so I’m after
> eliminating anything inherent to his station. Standing by his vertical with
> his portable rx is a great idea.
>
> 73,
>
> Kim N5OP
>
> "People that make music together cannot be enemies, at least as long as
> the music lasts." -- Paul Hindemith
>
> On Mar 22, 2021, at 2:24 PM, Don Kirk <wd8dsb@gmail.com<mailto:
> wd8dsb@gmail.com>> wrote:
>
>
> Hi Dave,
>
> I still recommend tuned and untuned loops to folks on an almost daily
> basis but they definitely don’t shed as much light on the situation as a
> unidirectional antenna does. It would be nice to see what your shielded
> untuned loop design is.
>
> The beauty of the portable flag which is unidirectional is that you
> typically can walk right to the source (no guessing which direction the
> signal is coming from). I still pack 7 antennas when I go on a RFI case,
> and this includes 3 tuned HF loops, but I have found that I typically don’t
> use bidirectional loops anymore (unless I’m trying to be more stealth) as
> the portable flag saves me an enormous amount of time. Also in heavily
> congested areas (like downtown Indianapolis) having the unidirectional HF
> antenna is priceless as it unmasks noise that’s mixing from other sources
> that are in the opposite direction, etc. Yesterday I worked on a case
> (more of a country setting) where I had 2 different bad poles that were
> only 0.14 miles apart plus a 3rd RFI source generating spikes every 1 KHz
> within the same area and having the unidirectional HF antenna helped me
> locate all 3 sources in a matter of minutes (way under an hour).
>
> You can never have enough antennas in your direction finding bag of
> tricks but after using a unidirectional HF antenna like the portable flag
> for direction finding I can’t imagine not using one. It typically takes me
> seconds to tell which direction the signal is without the need of
> triangulation, etc. Traditional triangulation has almost become a thing of
> the past for me.
>
> I also have been helping a ham that has the MFJ unit (the one with a beam
> and the one with a dipole) and while it has helped him, it has an odd
> characteristic that makes the meter jump which causes confusion to him and
> after looking at the schematic I question how they implemented AGC in those
> units. I’m starting to think AGC in a radio direction finding system is
> typically not desirable. Manual attenuation is priceless but I don’t want
> anything changing gain automatically.
>
> Just my opinion from tracking down RFI on an almost continuous basis in
> addition to my day job.
>
> Please share details on your shielded untuned loop and thanks.
>
> 73,
> Don wd8dsb
>
>
>
>
> On Mon, Mar 22, 2021 at 1:22 PM David Eckhardt <davearea51a@gmail.com
> <mailto:davearea51a@gmail.com>> wrote:
> We recently found a faulty lightning 'arrestor' on a pole by using
> both the MFJ device and a small shielded loop (easy home brew
> project) into a battery-powered receiver. We had two of us in the
> field. One wielded the MFJ and I had the small (0.5-meter) shielded
> loop fed into the Yaesu VR-500 on 1.830 MHz (160-meters). We both
> independently found the same source / pole in the field. The null on
> the shielded loop is extremely sharp and this piece of hardware
> should be in the bag of tricks of anyone sleuthing RFI. It is a
> powerful tool when coupled to a battery-powered receiver with its
> extremely sharp null (perpendicular to the plane of the loop). One
> doesn't need something as large as the 'Flag" that was recently published
> in QST and referred to in a previous post in this thread. Another plus
> for the shielded loop is that in the field, one can tune the receiver
> to the worst-case frequency of the RFI as I did in the related story,
> above.
> The shielded loop is untuned. The MFJ relies only on VHF noise. In some
> cases, an ultrasonic microphone at the focus of a small parabolic or
> spherical reflector is good at finding coronal discharges as well as
> active arcs and discharges. MFJ also makes that at a reasonable price.
>
> Our power provider was out the next day to fix the problem. We even
> gave them the pole number which is a great help to any power provider.
>
> Dave - WØLEV
>
> On Mon, Mar 22, 2021 at 2:28 AM Kim Elmore <cw_de_n5op@sbcglobal.net
> <mailto:cw_de_n5op@sbcglobal.net>> wrote:
>
>
> It's odd that you can't hear it on your portable rx. That made me
> curious: What if this is something internal to your '847? Have you
> tried pulling your '847 and listening on a different receiver using
> the same antenna? If you have, do you still hear the noise? It's a long
> shot, but...
>
> Kim N5OP
>
> On 3/21/2021 7:28 PM, Gary wrote:
>
> Mike,
>
> Thanks for the info. I understand locating the
> noise is
>
> more important than identifying the noise. I am just try to play
> all the angles as I was hoping to work the WPX contest nest
> weekend. I have been walking the neighborhood a good chunk of the
> afternoon, and have not been able to hear the noise on my portable
> shortwave and loop antenna. It is as strong as ever on my home station.
>
> I have an MFJ 852 Noise Finder and it seems to
> go crazy
>
> when broadside to any house. The neighbors on each side of me drive
> the meter full scale.
>
> Yep I only want to knock on one door, as I know
> that
>
> can get dicey. I would rather place a call to a utility, but can’t
> really do either until I know.
>
> Keep the suggestions coming!
>
> Thanks and 73,
> Gary “Joe” kk0sd
>
> From: Mike Martin <mike@rfiservices.com<mailto:mike@rfiservices.com>>
> Sent: Sunday, March 21, 2021 6:53 PM
> To: Gary <gary_mayfield@hotmail.com<mailto:gary_mayfield@hotmail.com>>
> Cc: rfi@contesting.com<mailto:rfi@contesting.com>
> Subject: Re: [RFI] New Interference at kk0sd
>
> Joe I haven't had time to read all the other input but I can tell
> you it
>
> is not an arcing source or a sparking source. either one of the two
> would be much broader than what you're reporting. if someone were
> to tell you exactly what it is you really wouldn't be any further
> ahead. You would still have to find which residents or business
> it's in. My suggestion would be to either go mobile or on foot,
> preferably on foot listening to the frequency that you know the
> problem affects. once you locate the residents then go through the
> breaker scenario and locate what the source is. Be 100% certain of
> the structure or home that the problem is in before knocking on the
> door. The last thing you want to do is start guessing. Guessing we'll
> ruin the relationship with your neighbors.
>
> Be safe,
> Mike k3RFI
> www.rfiservices.com<http://www.rfiservices.com><
> http://www.rfiservices.com>
> Get BlueMail for Android<http://www.bluemail.me/r?b=16696>
> On Mar 21, 2021, at 2:01 PM, Gary <gary_mayfield@hotmail.com<mailto:
> gary_mayfield@hotmail.com><mailto:
>
> gary_mayfield@hotmail.com<mailto:gary_mayfield@hotmail.com>>> wrote:
>
> My last problem was tracked down to a neon sign in a neighbors
> garage. I
>
> had several months of blissful relatively low noise operation, but
> now have a new RFI issue.
>
> You can see a recording of it here:
> https://www.youtube.com/watch?v=GKf4P44FXMo
>
> I have turned all of the breakers in my home off and run the
> radio from
>
> a battery and the noise remains.
>
> My neighborhood utilities are all underground.
>
> There are no solar installations in my neighborhood, and the
> noise is
>
> there at night anyway...
>
> The noise is not detectable on the AM broadcast band, but starts
> around
>
> 2 MHz and runs to at least 6 meters.
>
> It is there 7 days a week 24 hours a day.
>
> It is not detectable from my pickup when I drive the streets of
> my
>
> neighborhood (checking both AM broadcast and Shortwave).
>
> If any one has heard something like this, I would love to hear
> what you
>
> found.
>
> Thank You for your input and help!
>
> 73,
> Gary "Joe", kk0sd
>
>
> ________________________________
>
>
> RFI mailing list
> RFI@contesting.com<mailto:RFI@contesting.com><mailto:RFI@contesting.com
> <mailto:RFI@contesting.com>>
> http://lists.contesting.com/mailman/listinfo/rfi
>
> ________________________________
>
> RFI mailing list
> RFI@contesting.com<mailto:RFI@contesting.com>
> http://lists.contesting.com/mailman/listinfo/rfi
>
> --
>
> Kim Elmore, Ph.D. (Adj. Assoc. Prof., OU School of Meteorology,
> CCM, PP SEL/MEL/Glider, N5OP, 2nd Class Radiotelegraph, GROL)
>
> /"A great second violinist plays second fiddle to no one." //– Robert C.
> Marsh, Chicago Sun-Times./
>
> ________________________________
>
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> http://lists.contesting.com/mailman/listinfo/rfi
>
>
>
> --
> *Dave - WØLEV*
> *Just Let Darwin Work*
>
> ________________________________
>
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>
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>
> ________________________________
>
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> http://lists.contesting.com/mailman/listinfo/rfi
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>
--
*Dave - WØLEV*
*Just Let Darwin Work*
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