Cell Phone Interferes With Radio Sometimes

Factual Questions
1

While driving, I keep my cell phone in the drink holder, about 3 inches below the car’s radio receiver.

Sometimes the radio will make a rhythmic series of clicking sounds a few seconds before my cell phone rings. I’ve experimented with this and found that the clicking is louder if I hold the phone closer to the receiver, and fainter if I hold it further away.

The weird thing is, from time to time the clicking sound will take place independently of my phone ringing (although 90% of the time it takes place before my phone rings).

Two questions:

  1. What causes the clicking in the first place? Common sense says it has something to do with cell phone waves interfering with the machinery inside my radio receiver. But since cell phone waves bounce all around us all the time…?

  2. Why does the clicking take place independently of my phone ringing? My theory is that the clicking takes place when a call is being made to a phone whose phone number is a digit or two off of mine (IOW, when the person whose phone number is xxx-xx71 is being called, as opposed to MY number, xxx-xx70). Any thoughts?

2

This happens to me, too, and I always just attributed it to my cell phone giving off large bursts of radio emissions (perhaps in searching for a tower?) and some bands get picked up by the radio.

3

I always figured that my phone was doing a periodic signal check or update. That would explain (I’m guessing) how my phone knows that the local time has changed when I drive or fly through different time zones, despite not making or receiving any calls.

4

You have Nextel, right?

And you’re listening to an AM station at the time, right?

This problem is unique to Nextel users, and should disappear as Nextel is slowly absorbed into Sprint.

What you hear is your phone answering the cell tower. When it transmits it puts out a series of digital bursts that are picked up by the radio receiver in your car. Nextel operates in the 800MHz band, and while I can’t find my RF spur chart, I seem to recall that Nextel throws a very mean spur in the .5-1.8 MHz band (AM Broadcast). And it doesn’t only happen upon ring, once in a while you’ll hear a quick burst as you switch towers and your phone re-registers with the new tower.

Fortunately for you, it is so low power that a Nextel phone must be in very close proximity to a receiver. If you were to use a headset and keep the phone near the receiver while you were talking, you’d hear the digital spur products on your AM radio. When you pick up the phone you remove it from the immediate vicinity of the receiver, so you don’t hear the RF spurs.

Oh yes, and it’s digital so you won’t hear any voice.

5

My phone does the exact same things as in the OP, and I have LG, Cingular service. Also, not only will it do this with an FM station, it does it with my computer speakers both at home and at work.

6

I believe this problem is common on all GSM devices.

7

This annoyance can be easily solved by enclosing either the phone or the radio set (or both) in a grounded mesh box (a Faraday cage). There are trivial side effects to doing this, but don’t worry about them.

8

It’s not limited to Nextel’s iDen, or everyone else’s GSM. My old Cingular phone (think it was CDMA) also put out the occasional “phut phut” noise into my wired desk phone headset.

This afternoon, my GSM phone was on the desk under the monitor, and at the same time my desk phone went “phit phut phut” my monitor image wiggled, so these things evidently affect more than just radios and other phones.

It’s easy to tell if someone in a meeting is using a Nextel as our Polycom conference phones pick up a distinctive rhythmic noise that resembles a helicopter if a Nextel is near. We’ll hear “tukka tukka tukka tukka…” and someone will say “Who’s got the Nextel?”

The culprit, so to speak, is proximity. Just try to move the phone away from whatever it’s interfering with. Other than turning off the phone, that’s really all that you can do about it.

9

My phone does the same thing to my computer’s speakers. I have a Cingular phone. The explanation I’ve gotten is that it is caused by the GSM service. Every 45 minutes or so there will be a short series of clicking sounds, which I’m told is the phone sending data to the tower. Also, immediately preceeding a phone call it clicks and then buzzes. Again, I’ve been told that this is because of data being sent.
Just be glad you don’t have a Treo…

10

IIRC, GSM phones are really bad about interfering with electronic equipment and most cell phone companies are switching to CDMA because of this. I have a friend who works in a hospital and he had to ditch his GSM phone because even when the phone was off it would interfere with equipment.

11

While some phones are worse than others (for reasons previously mentioned, mostly to do with what particular frequencies they use) all cell phones are capable of generating this kind of interference. It shows up on radios, televisions, and computer speakers most often, but could theoretically interfere with just about any electronic device.

The OP is correct that there are cell phone waves bouncing around all the time. Generally, these waves are too low in power to interfere with much of anything. However, when your cell phone transmits something out, you have a lot more radio waves near you than you otherwise do, since your phone has to put out enough of a signal that the tower can pick it up from a fair distance.

Your cell phone and the tower talk to each other every now and then just so that they both know that the other is still there and that they can still talk. Then, when you get an incoming call, the tower first makes sure that the cell phone is still in the area, then it goes through the process of setting up the call. When all of that is done (during which you’ll hear lots of clicking on your radio) then the phone finally rings.

Your phone will not produce clicks and such when someone dials a number that is close to yours. Those other clicks are just the tower and phone making sure they can still talk to each other.

You don’t necessarily need Nextel, and you don’t need to be listening to an AM station. This type of interference can happen on AM or FM, or on a tv, or on computer speakers (especially the ones with built in amplifiers), and can happen with any kind of cell phone.

Trivial side effects. *snort *

For those that don’t get the humor, a metal box (aka faraday cage) is going to keep all radio signals inside of it from getting outside, and all radio signals outside from getting inside. As far as radio waves are concerned, there isn’t much difference between a solid sheet of metal and a metal screen, hence a mesh box works as well as a solid one.

The “trivial side effects” are that whichever device you stick in the box (phone or radio) will no longer function.

12

Cell phones periodically communicate with the cellular network, even if you are not talking on the phone. The network does this to keep track of which cell your phone is in, and to tell the phone what channel it should use to communicate with the network.

Clicking or buzzing noises in other equipment, such as radio receivers, hearing aids, and audio equipment are almost always caused by inadequate filtering and shielding in that equipment, not by any defect in the cell phone. For example, an FM broadcast receiver should have an input filter that rejects any signals outside of the FM broadcast band. If it doesn’t have such a filter, or the filter is inadequate, strong signals from outside the FM broadcast band can cause a number of problems that result in audible interference with the desired FM station. GSM cell phones are often implicated because of the way they share the channel (TDMA) with other GSM cell phones. Each phone has an assigned time slot for transmission. This results in the transmitter being rapidly turned on and off as its time slot begins and ends.

13

It is sometimes remarked that the phone is observed to generate this kind of interference immediately before receiving a call; “how does it know someone is about to call me?” is the FAQ; the answer is that it doesn’t; there is a delay between the moment that the caller finishes dialling and the moment the receiving handset starts to ring. The caller may not be aware of this, because the network may sound a ringing tone in his handset before the receiving handset starts to ring.

14

That isn’t my experience.

I used to work for T-Mobile, one of the first American GSM companies. GSM is actually going to take over from what we can tell. To get onto GSM, Cingular bought out AT&T wireless.

GSM offers much better techology, such as 3G, which is operational over in Europe and Japan. The call quality is usually better and you get better battery life. You can have such features as a phone that you take all over the world with you, not just the united states like old digital style bands. Also video phones…

I don’t have time at work to search for a good site to explain GSM technology better, but just hop on Google and do some surfing. It is pretty neat stuff.

Tuckerfan , when you said CDMA you probably were referring to TDMA (modern Sprint and Verizon.) You TDMA is more advanced and allows for the transmission of data and voice at the same time. It isn’t really comprable to GSM however. It has been a while since I have been in the business, but you guys should really check this out.

Eventually though, GSM interference will be a thing of the past when VoIP cellular phones hit the market (5 years if I remember?)

T-Mobile is looking into that, or so I hear through the grapevine.

15

And just so someone doesn’t nitpick me, T-Mobile was one of the first GSM operators in America, they bought out Western Wireless and a bunch of other micro companies, and, yes, T-Mobile is owned by Deuch-Telekom… :slight_smile:

16

Eh? GSM is 2G. 3G is a different animal.

17

Sprint and Verizon are CDMA. AT&T was a missmash of TDMA GSM. It is being converted to GSM and WCDMA (also refered to as UMTS) as part of Cingular. Nextel is IDEN I beleive that sprint is going to convert Nextel to CDMA.

18

You are right. What we ran at T-Mobile (a while ago) was a technology that we called 2 1/2 G. We were in a transistion period between 2g and 3g.

Here was the link that I was looking for earlier:

http://support.t-mobile.com/knowbase/root/public/tm21389.htm

I was told, and I am not exactly sure if it was true, that we couldn’t set up the full 3G network because the FCC hadn’t approved it yet. I dunno about that though.

19

Motorola/Cingular here, and I’ve had the same thing happen. I believe Rico, it’s tower-related. It will happen to me in the same 1/4 mile stretch of road every time I’m there.

20

My phone also creates the chattering when I use the cassette-deck adapter for my iPod in my car stereo.

My land-line phone at work is particularly sensitive, I pick up interference from peoples phones in neighboring cubicles. It gets particularly amusing if we have a bunch of people on teleconference, as someone will inevitably be picking up cell pulses, at times it sounds like digital dueling banjos :slight_smile: