Can you identify the AM radio interference?  EDIT: standing-wave video - Page 1
Forum section: electronics / rf-microwave

author: Whales
author_rank: Super Contributor
timestamp: May 27, 2021, 02:57:51 am
content: My AM radio reception went bad a few years ago. Currently my alarm clock can barely hear even the local AM stations without having a loud "fast steam train chugging" noise on top, making things impossible to understand every few seconds. My other AM radios are affected by different amounts, some I can only hear this interference on when tuned off a station.Attached is a short recording. Notable interference events start around 4s, 30s and 34s. Does anyone find the patterns familiar or have any clues as to the source?* Definitely digital. It's broken into short repetitive units and each is a wall of noise.* Sometimes I can hear what sounds like a very faint communications partner talking back (or perhaps just a distant echo)* Only exists reliably in my bedroom. I'm having a hard time picking it up in other places. It's not emissions from my equipment -- I've traced around with a handheld AM radio to check with everything on & everything off.* Biggest suspicion: the new mobile tower that went up a few years ago that's <100m from my room and neatly faces my windows. This might explain why it only really affects my room (skinny building, I have the best direct view of it).I am going to see if my RTL-SDR still works and have a poke around at 900MHz, but IIRC the bandwidth used by mobile phones is too wide to easily spot most of. I'll give it a try.Any random thoughts or ideas appreciated

author: Whales
author_rank: Super Contributor
timestamp: May 27, 2021, 03:23:27 am
content: EDIT: Ignore this message, something very wrong with my captures. Trying again...EDIT2: Doh, none of the RTL-SDR variants can tune as low as 1MHz. The axis labels below are completely incorrect, making the waterfall unknown/useless data.Ooh yes this looks like intentional radio between two parties! My ears were not deceiving me.Waterfall graph: vertical axis is time (about 20sec), horiz axis is frequency and colour is intensity (blue/black=weak, yellow/red=strong)Note the missing central freq, looks like a suppressed carrier. These are really wide transmissions for the 1MHz band! Unless my RTL is deceiving me and folding it down into here.

author: Whales
author_rank: Super Contributor
timestamp: May 27, 2021, 04:03:04 am
content: It doesn't seem to matter what freq range I'm tuned to, the RTL-SDR picks up the interference.I've confirmed this is my culprit by listening to an AM radio and watching the waterfalls in Cubic-SDR at the same time. The waterfalls go solid green whenever the "rapid steam-train chugging" sound comes over the radio, looking approximately like the waterfall screenshot above. It's night and day covering the whole-spectrum at 700Mhz, 900Mhz 1700Mhz, etc. The only place it's hard to see seems to be where there are existing signals to drown it out (eg 1400Mhz, 400Mhz).I presume this means the frontend of my RTL-SDR is being overloaded by the strength of the interfering signal.

author: BradC
author_rank: Super Contributor
timestamp: May 27, 2021, 04:23:49 am
content: Quote from: Whales on May 27, 2021, 02:57:51 amI can't help with the data you've provided, but I can say this. A "few" years ago I built local pager systems. These used the 148.3375 MHz paging frequency. We had an "issue" whereby someone did something dumb in a regional town resulting in one of the transmitters getting jammed on. We received a call from the ACMA asking if we knew what might be going on, which triggered a quick trip to site and immediate "rectification".Someone had complained about "something" destroying their reception, ACMA had investigated (went to site with a Spec-an) and done some digging. This made its way back to us and we fixed it.It *might* be worth at least a call to the ACMA because they tend to (or at least used to) take this sort of thing pretty seriously. As the "responsible party" I had expected the ACMA to come down on us like a ton of bricks, but they were more than happy to work with us and get it sorted with a good outcome all round. I get the impression they only get nasty when they have to.

author: Whales
author_rank: Super Contributor
timestamp: May 27, 2021, 04:43:07 am
content: It's nice to hear that ACMA does that. I guess it makes sense financially too, working with people is probably cheaper than threatening them all.Sadly this problem still seems to only exist in my room (so I'm no-where near calling ACMA just yet!). I've just tried turning everything off at the wall again & removing my sleeping laptop from the room, but the problem persists.Outside in the garden I can't pickup this interferenceand elsewhere in the house is fine.I think I'm going to go for a walk around the neighbourhood with my AM radio tuned off-station now. First stop is nearer the phone tower facing my window, so hopefully I can rule it out. Worse case scenario is my room turns out just to be a particularly bad spot for AM reception, so whatever this interference is it wins over in my room and only my room (even if it comes from elsewhere).

author: bob91343
author_rank: Super Contributor
timestamp: May 27, 2021, 04:53:28 am
content: Looking at the frequencies of the interference, don't you mean kHz rather than MHz?

author: Whales
author_rank: Super Contributor
timestamp: May 27, 2021, 04:56:19 am
content: Quote from: bob91343 on May 27, 2021, 04:53:28 amMhz is correct. Not KHz.I suspect that the strength of the signal is heavily overloading the frontend (RF amp & mixer) of my RTL-SDR, which leads to all sorts of non-linear stuff and garbage appearing at all frequencies regardless of where you try to tune to.EDIT: Also note I can't tune my RTL-SDR to anything below a few MHz, thehardware physically doesn't support itwithout mods.

author: Whales
author_rank: Super Contributor
timestamp: May 27, 2021, 05:02:46 am
content: I'mreasonablycertain this is coming from my friendly local mobile phone tower now.I just turned off power to my whole house at the switchboard & removed my laptop (emits notable RF when sleeping) from my room. AM radio reception was then fine everywhere on my property until you enter my room OR a particular spot in the front garden, where it suddenly goes to garbage with the same problem interference.In each of these locations the key seems to be getting the mobile phone tower to suddenly enter line of sight.I'm going to do some walks to various spots in my area to test. My AM radios are standard jobbies with bar ferrites, but with enough spot testing as I enter & exit line of sight I might be able to become more (or less) certain.Fun side theory: the transitions between interference & no-interference occur suddenly over less than a meter of travel and are really well defined to line-of-sight only areas. This suggests to me that the interference is not occuring at AM frequencies (very long wave) but probably instead at much higher freqs and the shear power of the signal is enough for it to be picked up in my radios even though they are not tuned to it. This also explains why some of my radios are more immune than others: the main tuned antenna circuit probably matters less than usual, with other shielding & design choices affecting susceptibility too.

author: Whales
author_rank: Super Contributor
timestamp: May 27, 2021, 06:54:02 am
content: This is hilarious, I'm measuring standing waves with my AM radio, a sheet of metal and a phone tower. You don't need expensive signal analysers to determine RF frequency!http://halestrom.net/misc/eevblog/amradiointerference_crf30.mp4Those distances are in the realm of 30cm, 45cm, etc; which immediately makes me think 900MHz or something close to a multiple. I'll do some more accurate measurements, but it's a little bit fiddly to find the exact positions due to the AGC in the radio. EDIT: also need to check what sort of standing wave pattern this should be.(@EEVBlog, we need <video> embeds on SMF)

author: A.Z.
author_rank: Super Contributor
timestamp: May 27, 2021, 10:35:24 am
content: Now that's "crazy", you've a pretty darn strong EM field there, it would be interesting using an EM probe and a spectrum analyzer to check it, also, since you have an SDR, you may try placing a variable attenuator at the antenna input to gather a better spectral view; in any case I believe that reporting the issue to the ACMA as suggested, also since the EMI is obliteranting your (and possibly others) ability to receive "public services" (AM broadcasts...) and because a so high field intensity may have negative health effect

author: Whales
author_rank: Super Contributor
timestamp: May 27, 2021, 10:47:02 am
content: I've got to be careful about jumping to conclusions too early.All I know is that the interference is strongrelativeto the intended AM stations. I don't know if the interference is strong in absolute numbers. For example: I might be in an area with really bad AM reception to begin with. Alternatively this sort of AM radio interference might just be considered normal near high-wattage mobile phone towers.I doubt it's going to have health effects; the rest of my equipment works fine and AM radios are quite sensitive.QuoteI think I need to run the SDR and the attached computer/laptop from _outside_ my room and any other affected areas, then run the leads & antenna into somewhere affected. Ie make sure the signals/interference only enter my SDR via the antenna, not via other means that might bypass the mixer (keep in mind the RTL-SDR is unshielded, just like my AM radios). I'll then try attenuators as you suggest.EDIT for those trying to follow along: the interference pulses seen in my SDR waterfalls "match" the pulses I hear over the radio (exact same timing); but I can't determine exactly what frequency these pulses are actually on until I fix how I'm using my SDR. Currently these pulses appear "everywhere" at any frequency tuned to.

author: A.Z.
author_rank: Super Contributor
timestamp: May 27, 2021, 10:56:11 am
content: Quote from: Whales on May 27, 2021, 10:47:02 amA metal box (to host the SDR) with good grounding and a number of chokes on the wires going inside the box will help, I think; that said, you may have a look at these two documents describing devices and procedures which could (and possibly will) allow you to pinpoint the noise source with good precisionhttp://www.arrl.org/files/file/RFI/Thompson%20Noise.pdfhttps://www.arrl.org/files/file/Technology/tis/info/pdf/031112qex048.pdfHTH

author: EEVblog
author_rank: Unknown
timestamp: May 27, 2021, 01:11:46 pm
content: I have an old somewhat related video:

author: A.Z.
author_rank: Super Contributor
timestamp: May 27, 2021, 01:19:34 pm
content: Quote from: EEVblog on May 27, 2021, 01:11:46 pmOMG, we've got the attention of the admin hereNow, I think I'll step back, since you are much more knowledgeable than me and I'm not the "competitive" kind of guy