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Q & A: cell phone interference

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Most recent answer: 04/27/2012
Q:
The other day I was at my computer and it started acting a little funny. The screen went wavy and the colors distorted. Seconds later my cell phone,which was right next to my computer, rang...after I answered, the screen went back to normal. I thought perhaps it was a coincidence, but 15 minutes later the screen did the same thing and the phone rang seconds later. I asked the person to hang up and call again...the screen did the same thing. I know thereís some sort of interference going on, but what exactly is happening? Your help is greatly appreciated.
- Vincent (age 16)
Dallas, TX
A:
Vincent- This sounds really weird and interesting. Iíll get you started with an educated guess, but to get a real expert you might look for some electrical engineer.


Assuming that your description is accurate, obviously you are right that thereís some interference. CRT computer monitors (is that the type you have?) are notoriously sensitive to electromagnetic fields, since the electrons on the way from the filament to the screen feel electromagnetic forces.

It is not plausible that the E-M fields are those from some central towers, or satellites, etc., since they would then affect lots of screens, not just the one of the person getting a call. So the fields must be coming from your phone. Probably it sends some strong response when it picks up a signal intended for it, identifying its presence and readiness to receive calls.

Itís interesting that the interference goes away during the call itself. That means that the E-M signal from your phone is much weaker during the call. I suspect (but you should check with someone who actually knows) that the phone ínegotiatesí a signal level with the tower, so that once the connection is made the phone sends signals just strong enough to be easily picked up. That might help a lot in avoiding unnecessary power use and hence battery drain. On the off-chance that the EM radiation might be bad for you, it would also be good to avoid unnecessarily intense levels.

Of course there may be some more boring explanation. Maybe the low-frequency signals to the phoneís ringer/vibrator are radiating some. That would also stop after you answer. You could check this possibility by seeing if the timing of the patterns on the screen matches that of the rings.

Mike W.

The phone may have to negotiate with more than one tower, if it is in an overlap region (or even if it isnít). Itís probably the towers and associated electronics and switching facilities that actually handle the decision of which tower gets to handle the call, but they need a good, clear signal which can be separated from the background to help make this decision. Also, the phone and the tower donít initially know how far apart they are or how good the radio contactís going to be, before the call starts. If the phone is far away from a tower (but still can pick up it signals because the tower puts out a very strong signal or is amplified by some fluke of the geometry of buildings or local topography), the phone has to send its reply signal extra strong to make sure the call doesnít get dropped even before it starts. As Mike notes, the signal level can be stepped down once the proper level is determined. You could go the other way, increasing the power from low levels until the signal gets clear, but that would add extra time at the beginning of the connection, and cell phone users are notoriously impatient.

(published on 10/22/2007)

Follow-Up #1: cell phone interference

Q:
I have the same effect with my computer speakers at my house. Right before my cell rings there is a loud buzzing in the speakers. This also happens periodically when the phone is just laying there. Maybe the phone is checking in with the tower? However the phone does not affect the CRT monitor which is even closer than the speakers.
- Michael (age 19)
Minneapolis, MN
A:

  Yup, I was mystified at some noise coming out of our video-conference setup here in our laboratory, and eventually someone piped up during a meeting that if people switched off their cell phones it would go away.  While the carrier frequency of a cell phone is in the GHz range, the rate at which it switches on and off during the negotiation with the tower apparently is well inside the audio range.  Iím not sure why the monitor wasnít affected.  How much noise affects a particular setup depends critically on solid grounding and shielding.  Computer monitors have additional magnetic shielding because they are designed to sit next to speakers with permanent magnets inside -- this shielding may help at higher frequencies too (but I donít know much of the frequency response of the materials used).

  Tom


(published on 10/22/2007)

Follow-Up #2: cell phone clicking braces?

Q:
Can metal braces cause cell phone interference? I continually here ticking sound (like helicopter rotors) while trying to have a conversation on cell phone. Does not matter where I stand/sit. In vehicle, in house, outside...very frustrating....
- Joy Stefanski (age 24)
Scotts, MI, USA
A:
This sounds like a great question to answer via experiments.

Presumably if the electromagnetic radiation from your cell phone is causing clicks in your braces, the effect would depend on how close the phone is. Have you tried putting it on speaker phone setting and seeing if the clicking continues as you step away from it? Phones may have different properties, including working at different frequencies. Have you tried looking up the frequency your carrier uses and then seeing if you can borrow a phone from a carrier which uses another frequency to see how it acts? Does a bluetooth headset have the same problem?

With a little more data like that, it should be possible to track down what's happening, then figure out some fix.

Mike W.

(published on 05/21/2009)

Follow-Up #3: car acceleration

Q:
Could a cell phone be causing interference with a cars computer? Maybe this is a clue to unintended acceleration of cars.
- James A. Winsor (age 77)
Bloomington, Mn
A:

Interesting thought. I have no idea whether this effect is happening. It should be fairly easy to test, however.

Meanwhile, for people who find their car accelerating out of control, one way to keep full control of the brakes and other functions while stopping the acceleration is to shift the car to neutral.


Mike W.


(published on 03/01/2010)

Follow-Up #4: cell phone interference with monitor

Q:
when my self phone tries to reach its signal or i dont know exactly if it is very close to my pc monitor, it scrolls up and down. And when i see a video it goes back and forth like crazy. Whaaaaaat?
- Unknownx (age 28)
Carolina
A:
This is a common effect, described in some of the answers above.

Mike W.

(published on 04/27/2012)

Follow-up on this answer.