How do you track down RFI of unknown origin? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: September 29, 2020, 09:00:47 pm
content: I have a lot of RFI in my vicinity and it doesn't help that I am also trying to track it down with cheap equipment. Once Ive verified that its present on multiple receivers, its not as easy as one would hope to find it. Even when I try to proceed methodically. How do you find sources of RFI in your homes (easier but still hard as I only have one SDR enabled desktop.) or neighborhoods? (very hard!)There has to be a better way.

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2020, 09:17:06 pm
content: you can power off all electronic equipment and listen noise on the receiver. If it's still present, try to switch off mains from your home. If it still there, it means this noise source is not from your home. When the noise is missing on the receiver, you can put your electronic equipment into mains one by one and check which one produce noise.

author: geggi1
author_rank: Frequent Contributor
timestamp: September 30, 2020, 12:39:20 pm
content: If ther e ate RFI all frequencies an old HAM trick is to use a AM broadcast reciever. There is a signal null at the end of the AM antenna rod.http://www.arrl.org/files/file/RFI/Thompson%20Noise.pdf

author: Kalvin
author_rank: Super Contributor
timestamp: September 30, 2020, 01:17:04 pm
content: Quote from: geggi1 on September 30, 2020, 12:39:20 pmProbably cdev could build a simple ferrite core antenna or a small, receive-only magloop for his receiver/dongle/equipment.

author: cdev
author_rank: Super Contributor
timestamp: September 30, 2020, 07:21:53 pm
content: I've tried that but for two reasonsits not been up to it, the worst interfercence I am dealing with appears to be frequency modulated, so on AM it just sounds like s very quiet swishing sound that appears in a carrier dozens of times across the bands, and the distance between the instances of that carrier changes.Also, the only receiver I have that does even a moderately decent job with those lower frequencies (where I think it originates) is an RTLSDR on direct sampling, and my direct sampling receiver is delicate, What I should try since loss is low on low frequencies is put my antenna on the end of a iece of coax long enough to go all around my tiny home, even unpstairs and downstairs from my computer and have somebody else look at the screen while I hunt around. That might work.I have some ferrite rods, any way to make a broadband LF antenna? I dont know the fundamental frequency.I wonder about my mini-whip.. thats another option.Hmm, another option is my sound card, sampling at 192 khz, I suppose, Its worth trying.Quote from: geggi1 on September 30, 2020, 12:39:20 pm

author: cdev
author_rank: Super Contributor
timestamp: September 30, 2020, 07:40:16 pm
content: I've only made mag loops for HF, but there is no reason why a lenth of coas could not be pressed into service to make a temporary one..Also I have a free long ferrite that would be ideal for a MF or LF antenna.Then decoupling it from the receiver so the coax didnt pick up signal would make the null sharper.I think that - does anybody know whether hunting down noise would work better on the fundamental, or the harmonics.It may be a nearby AM station connecting up with some rusty metal and rectification? What would that do? Fundamentals being in the AM BC bands..We have orange street lights, high presure sodium. I have not measured the QRM before and after the interference, I have not compared if all of the birdies are also on during the daytime. It seems like the problem is maybe even worse in the daytime than at night.Quote from: Kalvin on September 30, 2020, 01:17:04 pm

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2020, 01:36:06 pm
content: I had a friend who lived in a low populated area that was into amateur radio. He would drive around with the radio in the car and find which power lines were leaking and call them in. I guess they took care of them.Most of what I see are TV, radio stations, bluetooth, ethernet, microwave oven and various lights. Normally is doesn't cause me any problems. If I need to track it down, I had made some probes that can detect which trace on a PCB is the problem. I also designed one of my own that works really well. I've posted the link for the article here someone. Maybe from EDN in the 80s. Maybe under my home made probes.

author: A.Z.
author_rank: Super Contributor
timestamp: October 01, 2020, 03:47:31 pm
content: Quote from: cdev on September 29, 2020, 09:00:47 pmYou may try building this simple devicehttp://www.techlib.com/electronics/allband.htmor one of the variants shown on the same page (including the really amazing "sinitsa" one), it's basically a "no selectivity" receiver, you may then walk around with the device in your hand and pinpoint (ok, almost) the noise sourcethis circuit may possibly work even betterhttp://www.techlib.com/area_50/bugduster.htm

author: bob91343
author_rank: Super Contributor
timestamp: October 01, 2020, 11:31:10 pm
content: You might try a nontechnical solution. Complain to the power company. I did that and they sent out people with sniffing gear. They wandered around the neighborhood and even connected to my antennas.They never found the source. In time, the noise abated somewhat and now at least I can use 40 meters.

author: cdev
author_rank: Super Contributor
timestamp: October 02, 2020, 12:38:21 am
content: Quote from: A.Z. on October 01, 2020, 03:47:31 pmAZ, thats a great idea. I definitely will give that a shot. Its cheaper than buying another radio just to track down RFI.I also have a signal tracer and in the past its been helpful for finding some RFI.I also realized I should put chokes on more of the long wires around here.. There is a rusty fence also not far away that may be a source of RF rectification.If so, it might be possible to find where the nonlinear juction(?) is?--------Quote from: bob91343 on October 01, 2020, 11:31:10 pmI'm not sure if its the power company.There is a surprising amount of junk - it seems to be getting worse fairly rapidly. But maybe thats just me, maybe I'm just forgetting how bad it was in the past..I'm sorry, I am bering very unscientific.If it is the power company, they will fix it, they are very good around here.A couple of years ago I did have an obvious problem (you could hear it) with arcing on a nearby pole, a big problem with a nearby pole and they did fix it.

author: srb1954
author_rank: Super Contributor
timestamp: October 02, 2020, 12:39:56 am
content: Quote from: bob91343 on October 01, 2020, 11:31:10 pmEarly in my engineering training I went along on job with a radio tech who was trying to locate RFI from bad connections or insulators on overhead power lines.He located the approximate location using a wide-band receiver and a small Yagi antenna. My job was to then hit the side of the suspect power pole with a large hammer. If we had the right pole the vibration dramatically increased the RFI produced by the bad joint/insulator.

author: cdev
author_rank: Super Contributor
timestamp: October 02, 2020, 12:42:54 am
content: It was quite possibly a log periodic antenna. If it was VHF/UHF)or a loop antenna for lower bands.Quote from: srb1954 on October 02, 2020, 12:39:56 am

author: srb1954
author_rank: Super Contributor
timestamp: October 02, 2020, 01:07:18 am
content: Quote from: cdev on October 02, 2020, 12:42:54 amWe were only concerned about interference to the local VHF land-mobile radio system so we only needed to look at a signal bandwidth of a few MHz.We were working out of a small radio depot a long way from head office and were not given the budget for a log-periodic antenna. We had to manage with a 3 element Yagi.

author: cdev
author_rank: Super Contributor
timestamp: October 02, 2020, 01:34:24 am
content: If those are the only frequencies you use, then thats all you need!

author: A.Z.
author_rank: Super Contributor
timestamp: October 12, 2020, 10:43:15 am
content: Quote from: cdev on October 02, 2020, 12:38:21 amAny update on the RFI issue ?

author: El Rubio
author_rank: Regular Contributor
timestamp: October 13, 2020, 01:23:56 am
content: I’m a little confused. What os being interfered with? Was it just the am radio? Have you seen the interference with your RTL rig? If so, what does it look like, wide band, pulsing, frequency range? Electrical pulse interference from arcing connections on utility poles is very wideband and pulses at 60hz. I’m not that familiar with the RTL and you may not be able adjust sweep time to see the full pulse bandwidth. I doubt it is a harmonic from your am station. Those dissimilar metal junctions create intermod distortion and usually mix two or more different signals. Those signals are usually close to their respective sources also. You could have harmonic interference from the station transmitter though. If harmonic or IMD involving audio transmission, you should be able to demodulate and possibly recognize it. The bandwidth will increase by the same factor as the harmonic or IMD order. 3rd harmonic will have 3 times the bandwidth. IMD bandwidth is the sum of all components freq’s. This might help ID the source.I haven’t messed with an RTL in several years, but It seems like there was an app for android that you could run with one of those RTL USB dongles.Another problem with the RTL will likely be saturation when you get close to any transmission source. You might have some luck adding some attenuation inline or filters, but then you are getting away from anything practical.

author: cdev
author_rank: Super Contributor
timestamp: October 14, 2020, 03:12:11 am
content: Well, actually I have a lot of cheap SDRs. several RTLs , a Soft Rock and more recently I got a Mirics dongle from China. And since the price has come down a lot I am thinking about getting a Hackrf bare bones board.One kind that I have a lot of are the birdies that wiggle around. They are probably from switching power supplies. A fair amount of the worst interfecece recently resolved when I replaced my keyboard. Ive also been buying a lot of ferrite RFI suppressor beads, and putting them on both suspected noise sources and the anetnnas, as an aid to decoupling them from the feedline. Ive gotten fairly good at that.My QTH is still noisy but recently Ive gotten the noise down enough to hear hams on the bands that were previously mostly noise. Which is good. Replacing my PS2 keyboard with a USB KB has reduced the noise noticably.I get very bad interference from local FM and UHF TV stations, (I suspect) in any case the measured signal strength is very high. There are several transmitter towers within sight of my house, one is just huge.This is the same situation I had to live with in another big city I livewd in not so far ago. It was so strong that when it was foggy, it would cause all sorts of RF driven interference in houses around it. When the fog rolled in and I was walking my dog on some of those streets, I would see the flourescent lights in all the houses up and down the street flickering along with the fog. You could see the house lights flickering it was that bad. It was eerie..I have a fairly good FM band stop filter I made myself and when I have the forethought to use it it helps a lot. For some other interference, I have a high pass filter tuned to around 1.7 MHz. That also cleans up a lot of junk.Quote from: El Rubio on October 13, 2020, 01:23:56 amI'll take some screen shots or maybe short videos of them with sound.