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Comment by dchichkov | Hacker News Reader

As far as I understand it [studied electronics engineering and a private pilot] the main problem is with faulty devices. Although it is very unlikely, a faulty device potentially can act as nasty source of interference at _any_ frequency. Effectively blocking this frequency.

And although it is not a problem for a pilot in normal conditions, it could be a factor that complicates things for a pilot. If the visibility on a takeoff/landing is bad, there is traffic congestion, air traffic control is stretched to the limit you'd definitely don't want a faulty cell phone suddenly jamming an ILS approach system ;)

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I'd love to see some real-world examples of this sort of thing. The inverse-square law coupled with the power output of your average iPod makes me seriously question how effectively even a seriously fritzing device could interfere with communications.

I'm sure it happens (the post a bit back about the guy's laptop that was broadcasting mic input due to wacky wiring is a great example), but I'm really dubious that it's significant enough to actually interfere with comms from a system designed to throw radio signals 200 miles.

Take a look at the description of ILS system. Markers - 75MhZ, 3 Watt; Localizer - 108 MhZ, 50 Watt.

These are local nav systems, not FM stations that put kilowatts into the air.

The power density from a 5MW transmitter at 1km and 100 10W cell phones at 10m is about the same. But you have to remember that the navigation antennas are outside the metal body of the plane, so the phones probably don't matter.

Sometimes you hear interference from GSM phones because the TDMA time divisions are in the AF range, but that's really a function of the power envelope rather than the power of the transmitter. I don't even really count this because I've never seen a GSM phone interfere with a non-part-15 device. Radios have to reject nearby signals; noise well outside their frequency range is even easier to reject. (I've never heard anything like GSM interference while operating my HF radio, either, and I have a GSM phone.)

Not any frequency, the radios have physical limitations that only allow them to transmit on a small range of frequencies. They can certainly cause interference outside of the bands they were designed to operate on, though.

Yes, any frequency. Remember we are talking about a faulty piece of electronic. With some pretty weird fault. Not just a transmitter operating in an almost normal mode.

Take an ILS (instrumental approach) frequency 108Mhz. Any piece of wire of around two feet (half dipole) can be a pretty effective antenna at that frequency. Say, headset cable + an output cascade of the amplifier self-excited at this frequency? Doesn't happen in real life, but still...

Remember that your faulty headphones are inside a big faraday cage, and the nav antennas are outside.

I am not sure how much effect this has, but it's something to keep in mind.

Are airplanes really that effective as faraday cages? If they were, it wouldn't be possible to use a cellphone inside one at all, and this wouldn't have been a debate to begin with.

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