A few days ago, a helicopter flew low over my house. During this time, I heard radio chatter coming through a pair of speakers (which were plugged into the outlet, but not plugged into any input.) Why were these speakers, which contain no internal radio device, picking up this?
Either some part of the circuit in your speakers allowed them to receive the transmission or a harmonic thereof due to inadequate shielding, or spurious emissions were present. Is it safe to assume your speakers are pretty low-rent?
Take your cell phone and put it next to those same speakers (I’m guessing they are little computer-style speakers), and then have somebody call it…right before the cell phone rings, you’ll probably hear a noise that’s kind of like beeping with static. Poor shielding.
Here’s a thread from this board in 2005 about picking up FM radio signals via speakers.
TDMA only (AT&T, T-Mobile). Won’t happen with CDMA (Verizon, Sprint) or WCDMA (AT&T 3G). The joys of radio frequency modulation schemes.
Ahh, very cool – I didn’t know that. When I had desktop computer speakers (I now use a laptop) and experienced this, I had an old AT&T phone. I’ve had 3G AT&T phones for awhile now, but I chalked the lack of noise up to the fact that I don’t have cheap speakers around, not that my phone works differently!
As a flight instructor, I have often heard this noise in my headset, and it got to be quite distracting at times. I would always ask my student if they had a smart phone, and invariably the answer was yes. I always just assumed it was caused by the data connection, and that it didn’t matter which smart phone/carrier they were using. Ignorance fought!
I guess that’s why you have to turn your cell phone off on an airplane.
VHF aircraft radios employ Amplitude Modulation (AM).* AM is rediculously easy to demodulate, even a rusty razor blade will do the job: Foxhole radio. Even loose tooth fillings have been reported to occasionally get the job done.
In fact it is so easy to demodulate, that many electronic circuits can become inadvertent radio receivers. This can be the source of great social friction between ham radio operators and thier neighbors. It is also the reason for the fine print in most electronic device manuals telling you that this might happen and if it does, it is your problem.
- “inferior” amplitude modulation persists in aviation for a couple of reasons:
1-Inertia. All avionics are expensive, so nobody wants to change all the radios.
2-Doubling detection. It leads to heterodynes (“squeeling”) but it is usually possible to understand one pilot while a second is also is transmitting. At a minimum, an air traffic controller will know that a second pilot is transmitting, and can ask him to stand-by. A communication channel is not totally disabled if the microphone button sticks in one airplane. (happens with depressing regularity) With FM only the strongest signal is heard at all (“capture effect”)
3-Historically AM tube radios could use the receiver section audio amplifier to modulate the transmitter. This was a real weight savings in the vacuum tube days…pretty much moot once transistors came along.
I haven’t had one in 6 or 7 years, but NexTels were ridiculous with this. There was often a 2-3 second “warning” if you were near speakers. It was actually kind of handy - you could pick up your handset before the bastard even chirped.
Yep yep, the iDEN protocol is TDMA modulated, luckily (or unluckily, depending on if they actually use DC) this will be a thing of the past when Sprint phases out iDEN and moves its customers to its crappy CDMA PTT solution.
It has more to do with:
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Lithium batteries are actually considered a hazardous material due to their tendency for thermal runaway, and are only allowed onto airliners because the FAA and NTSB realize that it would be impossible to ban cell phones, laptops, portable media players, etc. When your cell phone bursts into flame and sets your crotch on fire during takeoff, the flight attendant won’t be able to get to you to extinguish the fire very quickly because he/she is strapped into a harness in his/her seat. That’s why they say if it has an on/off switch, it must be in the off position for takeoff and landing. ‘Airplane mode’ is ok for the rest of the time. Which leads us to…
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The way cell phones connect to their networks. It is my understanding that a cell phone communicates with any towers from its network that it has line-of-sight with. This allows your conversation to be handed off seamlessly from tower to tower as you drive across the country. In flight, you have line-of-sight to so many towers that it causes serious problems when the system tries to decide how to handle your call. I’m sure IAmNotSpartacus might be able to elaborate or correct me where I’m wrong.
The first rule is an FAA/NTSB rule and the second is an FCC rule.
The reasons for not being able to get a signal in flight are many. First, as StrangerThanFiction notes, when in a plane you have clear line of sight to many towers (cells) and your phone cannot determine which one to latch on to. This is a common problem when I am making a mountaintop service call, phone says I have full bars but no communication back or forth because the reception is “too good.”
Second, the plane moves far too fast and its altitude far too high, and because of this the phone will not be able to hand off from tower to tower as the plane makes its way. Slower moving planes and helicopters can actually get cell service, and my customers that fly news helicopters use their cellphones on a regular basis when they’re up in the air.
Plus, moving at a high rate of speed will cause doppler shift of the RF signals, making them appear off-frequency to any given receiver. Most radio systems have enough tolerance or are otherwise able to compensate for this shift, but as your transmitting frequency gets higher the effects of the shift become more pronounced, to the point that at about 100MHz (not far from the AM airband) and 800km altitude (low earth orbit) the maximum shift of an FM signal is about 1.5KHz, but at 2GHz (near the PCS band used by most carriers) the maximum shift is closer to 50KHz. AM signals are not quite as effected by doppler shift, near 100MHz it tops at about 10Hz, and at 2GHz the shift tops out at a little less than 200Hz. Of course no commercial aircraft flies anywhere near LEO, but the point remains. This is another reason that AM modulation will continue to rule aircraft communications although the major reason is mere inertia and the exorbitant cost of retrofitting every aircraft in the world with a new radio.
Back when I had a 486 that came with battery powered speakers I would note that as the batteries were nearing the end of their charge I would pick up AM radio.