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Wideband interference at regular intervals?

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Chris Russell

Mon Jul 30 2007, 07:45PM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

Okay, here's a mystery that probably won't get solved, but maybe someone here will have some idea of where I might look.

I just put up an active antenna, and it's a great deal more sensitive than other antennas that I've used in the past. Everything is working great, but I am having a problem with some sort of wideband interference. Can anyone point to any suspects? Here's what I know about it:

It occurs at regular intervals. The interference lasts about 24 seconds, then is perfectly quiet for about 52 seconds, making a cycle that lasts 76 seconds.

It fades in, building for a few seconds, but when it stops, it stops abrubtly, with a slight crackle.

It occurs from 200kHz or so, up to 30MHz.

I'm also attaching a screencap, and a recording of a few cycles of the interference. Anyone have any ideas? The only thing I can think is that it's broadband noise radiated by the power lines, but what could be making that noise and would be operating on such a schedule?

]1185824753_1_FT0_interference.mp3[/file]

Dave Marshall

Mon Jul 30 2007, 09:24PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

DFing and/or troubleshooting such wide band noise is tricky, but not impossible.

The first thing you need to do is rule out your equipment. Swap to a battery instead of a mains power supply. Try receiving with your IC-T90A in AM mode, instead of your 857D.

If you still hear it on another radio and/or different antenna, remove power from everything in your apartment, one room at a time. Either by circuit breaker (the best method) or just unplugging stuff.

If you've still got noise, go on a road trip. Take it out in the yard, to the park down the street, etc. Get it out of the environment. This will help you determine if its local, or if you might need to contact one of the regional FCC field agents for some assistance.

Inspect the entire spectrum carefully, from 200Khz all the way to 30Mhz. This might take a while, but you're looking for a noise peak. Even if its a slight peak, it can give you an advantage when trying to DF the noise. If and when you find what looks to be the center of the noise, check it at several different times throughout the day and night. Does the repetition rate change? Does the center freq shift? Is the noise stronger or weaker? Does it change in nature at all? This will help rule out power line equipment and external appliances (furnaces/air conditioners etc) that could be experiencing some kind of thermal cycling. If it changes with the time of day, your hunting efforts will probably need to move outside.

Once you've tried all that, its time to start DFing. This can be done with your IC-T90A fairly effectively, particularly if you can hear it with the stock antenna. If you can't hear it with the ducky antenna, a nice big coil of wire might work to bring it up to an audible level.

The ARRL has some great reading on tracking down noise, and ARRL staffer Ed Hare, W1RFI can be easily reached for some advice. A google for his callsign turns up his address.

I'm going to bet dollars to donuts you're going to find this noise maker in very close proximity. A noise source so wide is going to have a very limited range unless its incredibly powerful. My bet is a cheap/dying SMPS, or some faulty mains powered appliance or line equipment in or very near your apartment building.

Dave

Chris Russell

Mon Jul 30 2007, 10:20PM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

Dave,

Ruled out all my equipment by powering it down and receiving the same exact noise on the T-90A. Much to Noelle's annoyment, I shut off and unplugged everything in the apartment, but the noise continues. The T-90A indicates that the noise is strongest near grounded objects and other appliances. That's probably a pretty big clue; it's likely somewhere in the house, being radiated into the wiring. Of course, that doesn't rule out noise coming down the power lines. I'll take a walk in a bit and see if it's present elsewhere.

Noise seems strongest around the 3/4 mhz range, but that might just be where the bands are the quietest at the moment.

Dave Marshall

Mon Jul 30 2007, 10:43PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I hope it isn't in another apartment in the building. Trying to convince the land lord to throw the breakers in an attempt to locate the noise isn't going to do much for your rapport with him, but then neither will filing an FCC interference complaint that comes back to him. This will call for a light touch to be sure.

Dave

Chris Russell

Mon Jul 30 2007, 11:59PM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

Well, I have access to the breakers. It is tempting to throw them in order when I know nobody is home. I could at least put together a better sniffer and see which circuit the noise appears to be coming from. But first I need to head down the street and see what the interference is like a few houses up and down from mine.

The good news is that even though the noise is overpowering, I find that the noise blanker cuts it back dramatically, at least two S-units (12dB). That still means I'm getting S-5 noise, but at least I can hear something through it.

I still don't see what could be causing it. The noise is so regular, I almost don't see how it could be a normal piece of junk equipment radiating. I keep wondering about the mill down the road... wonder if they could be PWMing a saw or some other inductive load.

Dave Marshall

Tue Jul 31 2007, 12:37AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I would expect that there are several layers of isolation between you and the mill, but its not out of the realm of possibility.

I've seen appliances that radiated with a startling degree of regularity. Thats one of the symptoms alot of hams tend to equate with a problematic SMPS. It would have to be one hell of a power supply to get the entire ground system in the house to radiate though. Maybe they supply on Guitar Guy's amp? On the plus side, if you file a noise complaint against him, maybe you wouldn't have to listen to him any more...

I'd be very curious to hear what Steve Conner thinks about all this. He has quite a bit of experience with power supplies, and might have an idea if in fact an appliance could be at fault here.

Dave

Steve Conner

Tue Jul 31 2007, 08:29AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, from listening to the MP3, it sounds very like SMPS interference. It's got that horrible, rough, buzzing tone you would expect from a poor quality oscillator modulated by 120Hz and its harmonics.

I heard similar sounding nastiness when I was experimenting with powering my HF rig off a SMPS, but by careful filtering I was able to reduce it below the level of all the SMPS in my neighbours' houses

The electronic transformer for the halogen lights in my kitchen still blots out most of the HF bands (again with similar sounding QRM to Chris's, only continuous) so I ended up operating in the dark...

So if you assume it's a SMPS, whose? where? and how to get it to stop? The fact that it pulses on and off regularly suggests that it's an appliance in standby, in some sort of energy saving mode. Maybe your neighbour just got a new TV or DVD player or whatever. Or it could be one of those cheap wall warts plugged into the wall but unloaded.

Also, SMPS interference should occur over and over as you tune the band, in broad lumps at a spacing equal to the switching frequency. Mine cropped up roughly every 100kHz and it did in fact switch at 100kHz. The lumps are far wider than your rig's IF filter, so they look broadband when you're in the middle of one.

Tip: If the offender is a SMPS, and it's nearby, the tone of the noise will often change when you throw a heavy load on the electrical supply. That might help you figure out if it's in your house or not.

BTW, I am my block's Guitar Guy. Most guitar amps seem to use a regular transformer and rectifier, but some new ones are Class-D with a SMPS :( Even then, I've never seen one with a low-power standby mode...

Chris Russell

Tue Jul 31 2007, 04:59PM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

Steve,

It's all across the band, there are no nulls or lumps. It sort of gets stronger around 60m/80m, but there's definitely not a single break in it. Your suggestion of an appliance in standby makes sense, though. My upstairs neighbor has wireless internet, so I assume he probably has a laptop as well. I wonder if it could be a laptop sitting in standby. It would still have to be a terrible SMPS though, as my laptop only causes minimal noise, and Noelle's causes none at all. This noise is present everywhere in my apartment, strongest near appliances and wiring, and is readable outdoors across the parking area.

When I go to work today (about 300 yards down the road), I will bring along my T-90A and see if the noise is present there, or at any points along the route. That way I'll at least know if it's a local problem or not.

Chris Russell

Fri Aug 03 2007, 03:04AM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

Update:

Found it! I know exactly where the noise is coming from, and who to contact to get it fixed.

The story:

I did some more digging around the house, and noticed that the noise seemed strongest near grounded appliances, and especially near the TV. Upon closer inspection, it seemed as if most of the noise I was picking up in the house was being carried in by the cable. Not good news... if it takes Time Warner a week or two to install cable, how long is it going to take them to show up for some nutter complaining about interference? Disconnecting both my TV sets and the cable modem from the cable helped with the indoor noise, but my active antenna is 25 feet from the house, and was still receiving the noise just as strongly.

I mentioned this to Dave, and he suspected it might be a malfunctioning cable repeater. That seemed like a very likely possibility, so I decided I would have to at least figure out *which* cable box it was, so that I could at least point Time Warner in the right direction.

It's been very very hot here lately, and extremely humid, so I've been hesitant to go trudging around the neighborhood with my T-90A, sniffing for noise, eyeballing cable boxes, and swatting at mosquitoes. Well, tonight it finally got cool enough for a walk, so I decided to try to track it down. Boy am I glad that I'm lazy and decided to go at night. That turned out to make all the difference.

I grabbed the T-90A, and started north up main street, after getting a baseline reading on the noise from my porch. The noise was still coming about once every minute, so I got about 200 feet down the road before the next noise burst was heard. This time the signal was weaker, but still very audible. That would seem to rule out something in my building, unless it was arcing internally and draining hundreds of watts of power. I walked further north: weaker. Further still: even weaker. I realized that I was headed the wrong way, but wherever it was coming from, it was damn strong.

So, I started heading south, and it got louder each time. Louder, louder, and then I passed my house and it started getting quieter, quieter, quieter. Not good news. The noise source is definitely strong if it can be heard a quarter of a mile down the road in each direction, but the origin appeared to be my house. Damn!

So, still walking south, I caught a side street, and started walking back to my house from the back street. The scenic route, I call it. Sure enough, louder, louder, louder... the closer I got to my house, the louder the noise got. I was almost home, just about to round the last corner when... the street light in front of me turned on at EXACTLY the same time the buzzing stopped. Intrigued, I stood under the light and waited. Sure enough, after 30 seconds or so, it shut back off, and the buzzing started up again after another 20 seconds. Buzzing, buzzing... until the light came back on with a little flickering crackle. That's what it's been doing, 24/7, for God knows how long. I know I saw it cycling on and off a few months ago. Why the people on that street haven't called to have it fixed, I'll never know. Having a bright street light turning on and off every minute would sure annoy the hell out of me.

I suspect that the reason I found the noise present on my cable lines is because on my street, the cable lines are suspended on the power poles, below the power lines. The light is physically close to the cable lines and the power lines, so noise should be coupled into both, and probably is. However, the electric wiring in my house has a massively inductive transformer to protect it, one that is also presumably built to maximize insulation (and therefore minimize capacitance) between the primary and secondary windings. The cable lines just come straight in.

Anyway, I'm going to give Bangor Hydro a call first thing in the morning. I'll mention the interference thing in the hopes that it will get them to move this to the top of their priority list, but I suspect it will take a week or so to get it fixed.

Whew. I'm glad I found it. I'm also glad that the problem belongs to someone that I can count on to fix it, without having to invoke the FCC. I hope. Stay tuned for the exciting conclusion.

Dave Marshall

Fri Aug 03 2007, 03:29AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Ah excellent! The starters on those big sodium lamps are well known noise makers. I just wrote them off because of the radiation originating in the house.

When they go bad, they arc. Typically this isn't noticed by the average hams because it only happens right before the light turns on in the evening. However, if the lamp is also bad, and thus isn't staying on once its powered up, it starts cycling. The starter fires up, the lamp eventually illuminates, the lamp then dies, the sensor tells the starter its supposed to still be on, so it tries to restart the lamp.

I had thought that most street lights had arc protection built in, to keep them from consuming huge amounts of power in just such a situation. Apparently this one is either old enough that it doesn't have such a system, or the breaker isn't working for some reason.

Dave

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