“Radio” on airplanes?

It’s been a long time since I’ve been on an airplane (2004?) and not since I’ve had an iPhone.    Some of you travel on airplanes, and I’m wondering what you’ve bumped into…  I’m also not an expert on radio engineering type topics

My curiosity was aroused by a message post on a radio industry message board where the poster said “And of course, I know I can’t use my AM/FM/CD Walkman in flight”.  

For cell phones, the reason is very clear – turning on the cell phone causes it to seek out the closest cell towers – when you’re at 30,000 feet, potentially any cell phone tower within 200 miles would “hear” your phone.   But this isn’t the 1980s – I’m pretty sure cell towers now are tuned to ignore stray signals of that type. 

Also if you have a GSM phone and have had it near a set of speakers or other unshielded wiring, it generates a very obnoxious buzzing sound when it pings the tower.

So I’m thinking “That doesn’t sound right”…   a walkman has nothing that should be transmitting to interfere with avionics or communications gear.   Why would there be a rule like that?

Obviously, when using a radio you would have to be wearing a headset of some sort, but beyond that… and during takeoff and landing you need to be alert to instructions from the flight attendant and not “zoned out”.

I remember noticing as a kid that radios could interfere with each other (this was in the 1960s – one of the radios was my dad’s shortwave receiver).      The reason turns out to be the way a thing called the “Superheterodyne” works (I’m open here for better or more accurate descriptions – I flunked out of ham radio school).    

The basic issue is that the Radio Frequency (RF) signal coming into your radio is extremely weak.   In order to be useful, the radio has to “tune” to the frequency you  want, and then amplify the signal to make it strong enough to power the speaker part of the radio.

A “heterodyne” circuit demodulates the incoming signal and reverses the process the radio station did.  Let’s say you’re listening to KDKA that has its carrier at 1,020 kHz.   You can’t just feed that into a speaker – you use an oscillator (a crystal (in the old days) that generates a signal at a specific frequency) to “demodulate” the radio signal – when you combine the RF signal with the Oscillator, you recover the original waveform of the audio (it negates the Sine Wave of the carrier).

But there are lots of problems with that – if you make the “beat Oscillator” at 1020 kHz, the oscillator would drown out the RF signal.   Enter Mr Armstrong and his invention of the SuperHeterodyne  (1918).   Insteaad of trying to amplify the RF signal, the tuning side outputs the RF signal you want to hear to an Intermedia Frequency (IF) that is a fixed frequency (455 khz for AM radio).  Then than fixed frequency can be boosted up to the necessary power, and is far enough away in frequency from the 1020kHz that it can be filtered out and not feed back into the RF input.

The IF for FM Radio has been standardized on 10.7 Mhz – that won’t interfere with surrounding FM radios – because they aren’t listening to 10.7 Mhz – which is not in the FM band.    Some FM radios apparently shift the entire FM band up by 10.7 Mhz (which would be from 98.7 to 119.5 Mhz)  – aviation  VOR navigation signals are from 112-118 Mhz if the plane picks up the stray IF signal from an FM radio playing.  Above 118 Mhz is the voice traffic channels between pilots and airports.

So what are airline policies?   It apparently is becoming a moderately frequent issue for flight attendants to be confused by technology and error on the side of caution.   the iPhone is a particular target of confusion.   Most people know that most cell phones have an “Airplane mode” that turns off the telephone part, and Wifi and Bluetooth (but you can turn Wifi and Bluetooth back on without leaving Airplane mode)

A number of airlines are starting to offer in-flight WiFi access (for a price of course).   The plane is equipped with  a WiFi hotspot that is connected to the internet via satellite or ground stations.

So there are “apps” that allow you to use your internet connected cell phone to make and receive phone calls without using the cellular network (Skype, Google Phone).

So if you were on an airplane and you noticed someone else on the plane talking on their cell phone during the flight, what happenss next?

Further complicating things is the iPod Touch – it looks identical to an iPhone, except it has no cell phone – but it can work as a phone over the internet.

“Hey, you can’t use that phone in flight”. 
“Ummm…. it’s not a phone, it’s an iPod Touch”
“How stupid do you think I am – You’re talking on the phone with it?”
“I’m using the internet to talk – it isn’t a phone, honest!”
“Do you want the plane to crash?”
“Umm…  this is using the WiFi service inside the cabin – there is nothing this device is doing that every laptop on this plane isn’t also doing”
“Do you understand that it’s a violation of federal law if you disobey my instructions? – turn off your phone or I’m going to get the Federal Marshall to arrest you for being a domestic terrorist!  You aren’t a right wing Christian, are you?”
“Okay, but I don’t have a phone. Is it okay if I play Angry Birds?”.