RF guys would appreciate this - Page 1
Forum section: electronics / general

author: Wartex
author_rank: Frequent Contributor
timestamp: July 29, 2013, 10:09:25 pm
content: http://www.npr.org/blogs/thetwo-way/2013/06/04/188713366/beer-fridge-blamed-for-cellphone-network-blackout

author: smashedProton
author_rank: Frequent Contributor
timestamp: July 29, 2013, 10:38:08 pm
content: That would have to be a really really bad brushed motor, or a really rf transmission.

author: John Coloccia
author_rank: Super Contributor
timestamp: July 29, 2013, 11:27:02 pm
content: Small price to pay.

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: July 30, 2013, 01:41:12 am
content: The internet urban legend machine is at it again.

author: AlfBaz
author_rank: Super Contributor
timestamp: July 30, 2013, 01:52:03 am
content: Seem like a marketable fridge feature... Who wants to be disturbed when in your man cave having a beer

author: ejeffrey
author_rank: Super Contributor
timestamp: July 30, 2013, 07:01:42 am
content: That seems like bullshit. As far as I know, fridges normally use AC induction motors, not brushed DC motors. Brushed motors are really only used for very small motors, motors that need to run on low voltage DC sources, or motors that need to have simple, crude speed control (PWM or just variable voltage). Even then, BLDC motors have mostly replaced them in all but the smallest, cheapest applications.Even if it were a really bad brushed motor it might interfere with AM radio but it isn't going to cause substantial interference with a cell phone operating at 800 MHz. A hypothetical brushed motor would have to be emitting megawatts of broad spectrum RF power to interfere with a cell phone signal.In principle, the low frequency (like a few MHz) could be picked up directly by the audio frequency circuits in your cell phone and cause interference. However, that is pretty unlikely due to the small size. A cell phone doesn't have any long cables connected that could act an antenna for ~1 MHz EMI. If you hooked up a charging cable or headphones, that would be a different story.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 30, 2013, 07:27:09 am
content: I wouldn't be too quick to rule out this actually happened. First, spark gap transmitters can be made to work at quite high frequencies, and secondly, it doesn't have to be the commutator - if it has one - which is faulty. This could be something as simple as a loose, internal connection in the AC mains wiring. An induction motor would fight back against any loose connections, which might perhaps even be caused by the motor's own vibration.A spark gap is just a very fast voltage controlled switch, and it can make practically any resonant circuit 'ring'. Generating a sustained oscillation with a spark gap is much harder, but that isn't required for a noise generator.

author: amyk
author_rank: Super Contributor
timestamp: July 30, 2013, 07:58:42 am
content: Quote from: ElectroIrradiator on July 30, 2013, 07:27:09 amThe issue is the amount of radiated power needed to cause that much interference.Then again, the condenser coils might've formed some sort of crude tuned circuit and had some weird effects on the emissions...

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 30, 2013, 08:31:29 am
content: Quote from: amyk on July 30, 2013, 07:58:42 amThe catch with the power is that cell phones simultaneously operate at low (received) power levels, and they have the ability to jump frequency. So if they encounter local interference at a particular frequency, they can move out of the way. This would normally allow them to operate in noisy environments, including the interference caused by other nearby cell phones.Unfortunately a spark gap transmitter generates extremely wide band emissions, allowing a sufficiently unlucky construction to blanket a whole band of frequencies. So it is not unreasonable to think a spark gap transmitter could crush cell phone reception in a whole neighborhood. It is possibly to build a cell phone jammer, which nukes all reception in an area. It doesn't take much power if the design is intentionally or accidentally evil.Secondly, 850 MHz RF has a wavelength of just 35 cm. Random pieces of ordinary AC mains wiring of about half this length could make excellent resonators for this frequency. No need for any exotic, mechanical structures. Once generated, then the RF could be conducted to the outside via the AC mains wiring. An induction motor usually wouldn't need a mains EMI filter, and an old fridge is unlikely to have one in any case.All it takes is just a few random variables stacking up in the wrong way. Obviously not something you can easily create by accident, or we would hear about this much more frequently. I cannot see any reason why it couldn't work, though.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 30, 2013, 08:56:53 am
content: Quote from: ElectroIrradiator on July 30, 2013, 08:31:29 amOr it could be the cell tower's receivers getting blocked, which effectively would block a lot more phones.Not sure how good the coverage in that area is though, if they can access multiple towers the scenario is not so likely.

author: G7PSK
author_rank: Super Contributor
timestamp: July 30, 2013, 10:56:42 am
content: The real catch is domestic fridge compressors tend to be of the diaphragm type the motor consists of a coil only. The whole thing is also a hermetically sealed unit that is earthed so how did any RF from the motor sparking get out, the fridge case also tends to be earthed metal as well. Any arcing had to take place out side the fridge as I cant see much HF or higher getting out via the lead.

author: rsjsouza
author_rank: Super Contributor
timestamp: July 30, 2013, 11:34:42 am
content: My guess is the guy works for the telecom company, needed to keep a hidden stash of beer for the work hours and unplugged one of the multiplexers on his neighbourhood's cell tower to connect a fridge...

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 30, 2013, 11:35:00 am
content: It doesn't have to be the motor, it could be the thermostat or the light switch... I once found a lightbulb (incandescent) that produced a good amount of noise between 2 and 8 Mhz.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 30, 2013, 12:04:42 pm
content: Quote from: G7PSK on July 30, 2013, 10:56:42 am(Emphasis added.)How do you think the RF energy comes out of the shielding enclosure around a high power RF amplifier?Unless a wire leaving a shielded enclosure passes through a feedthrough capacitor, then you can pump very nearly unlimited amounts of RF energy out through a tiny hole. At some point the conductor will start to melt due to the current, but that is about where the practical limit is.Even a single FT cap may not give enough isolation for some sensitive applications. Some RF gear employ double, sealed shielded enclosures, with separate FT caps in each wall, and with serious dampening in the form of resistors, inductors and ferrite material between them.I would suggest a bit of reading up on the fundamentals here, if people find it difficult to imagine what is happening in this scenario.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: July 30, 2013, 12:27:25 pm
content: QuoteI wonder if the motor created a spark once every 1.176 s.

author: G7PSK
author_rank: Super Contributor
timestamp: July 30, 2013, 12:50:05 pm
content: Quote from: nitro2k01 on July 30, 2013, 12:27:25 pmBut we are not talking about high power RF amps, in my experience rf that is leaking out through power leads is attenuated or even stopped fairly quickly by bends and twists in the lead, and the power lead exits the compressor and then has to travel through the rest of the fridge before emerging into the world outside. If you think that mains power leads are good conductors of RF why are we spending all that money on cro probes and co ax.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: July 30, 2013, 01:02:12 pm
content: Read my comment again. I was merely satirizing the fact that they wrote MHz as mHz.

author: AlfBaz
author_rank: Super Contributor
timestamp: July 30, 2013, 01:15:23 pm
content: It is quite possible the fridge wasn't earthed. We don't know the circumstances in enough detail. What do they actually mean by saying it brought the network down?As for the cable being the antenna, what about the radiator?Here's a picture of one

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 30, 2013, 01:29:36 pm
content: Quote from: G7PSK on July 30, 2013, 12:50:05 pmA quarter of a wavelength at 850 MHz is about 9 cm, so if you twist a piece of wire, carrying this frequency in free air, once every handsbreadth or so, you will very quickly have crafted a nice antenna array.Many fridges doesn't even have metallic backs, as this just hinders their cooling ability. Even if they did, then the steel box will guaranteed not have the corners welded hermetically shut, which means the RF can still get out. Check out 'slot antennas'.Additionally, then there is such a thing as a single wire transmission line. They are only very rarely used though, as they have to be built to rather exacting symmetry over a ground plane, or you will loose the signal to radiation / suffer interference from received signals. Which is one of the major reasons why we mostly use coax in cramped conditions.Quote from: AlfBaz on July 30, 2013, 01:15:23 pmEarthing the metallic chassis would have made absolutely zero difference at 850 Mhz. Zero. Nil. Nada. Zilch.Your intuition about grounding can be thrown straight out the window, once we start to talk about UHF signals. There are no 'grounds' at those frequencies. Mains PE is pretty useless as a RF ground, even at the lower amateur radio frequencies, down about 3 MHz. Yes, I know a ton of dumb books claim it can be used that way. It cannot, unless it is significantly shorter than a quarter of a wavelength.