Archived posting to the Leica Users Group, 2001/10/04
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]I'm a long-time ham radio operator, so I've got some experience in this area, The "range" of any radio device is meaningless unless we talk about the terrain, and the height above it that the transmitter and receiver are located. Put even a very weak transmitter high up, and it can get much farther than its specs or actual performance over "average terrain" would indicate. Case in point: Several ham radio operators in Latin America and Hawaii (where there was no competition from higher powered stations) routinely talked to the Mir space station while standing in a parking lot, using a handheld transceiver with a couple of watts of power. Over flat land, the range of the hanheld would probably be 10-15 miles, tops. I don't remember the exact height of Mir, but it was at least 125 miles and probably more. I myself have had several conversations with Shannon Lucid and John Blaha aboard Mir from my home using power and antenna gain that could not travel nearly that far over normal terrain. I have also stood on a 5300 foot mountaintop and spoken with another ham over 100 miles away using 2 watts of power. Given a clear line-of-sight, VHF and UHF transmissions can go a lot farther than experience on the ground would indicate. Cellular phones are weaker than the examples cited above, but from a plane you're talking about clear line-of-sight once the signal gets out of the plane. A plane at 30,000 feet has a radio horizon of over 200 miles. Trust me, a cell phone on a plane could conceivably trigger every cell tower within several tens of miles. In rural areas, this probably wouldn't matter. But near an urban area, with the position of the plane changing rapidly, it could confuse hell out of the cellular system. One other point--if a cell phone shouldn't work from a plane, how do we explain the numerous cell phone conversations from the doomed airliners on Sept. 11? I don't think all of them were using the plane's own radiophones, and several of the people who talked to the survivors said that so-and-so called on "his" or "her" cell phone. As far as interference goes, the issues become very complex. In the presence of any two imperfectly connected conductors, which act as a rectifier, any radio signal can mix with any other radio signal to create sum and difference frequencies, or radiate multiples (harmonics) of the initial frequency. Cell phones don't operate on the same frequencies as navigation equipment. But mixing effects within equipment or nearby objects can create other signals which could conceivably interfere with navigation. On a plane, where one "oops" can get you killed, it's probably best to avoid random and uncontrolled radio signals as much as possible. As for computers, CD players, etc: Any digital device has clocks which create square waves--waves that are turning on and off instantaneously rather than oscillating smoothly. Square waves are by their nature loaded with harmonics. And there are many such square waves in digital devices, because clock frequencies are routinely multiplied and divided to get various timing pulses. Shielding and bypassing can eliminate the radiation of such radio "hash" for all practical purposes. But given the marketplace, devices that have been made cheaper by not including such refinements are going to be around. For a real eye-opener, try putting a portable shortwave radio next to your computer and tune around. Laptops are more tightly controlled and tested than the cheap consumer CD player you picked up at K-Mart for $19.95, which is why the former may be allowed and the latter not. Finally, U.S. FCC Amateur regulations state that no amateur radio transmitter may be operated on board a ship or plane without the permission of the captain or master. I believe there are similar restrictions in most other radio services. So whether there's a good reason or not, the pilot has discretion, and he's bound by the policies of his company. As stated above, there are good reasons. - --Peter Klein Seattle, WA > > My understanding--I'm sure I've read this in more than one place--is > > that at an airliner's altitude, cells over a large fraction of the > > country would pick up transmissions from the cell phone. This would > > really screw up the system. I also read that the prohibition is an > > FCC, not an FAA, regulation. Austin says: >I don't believe that's right. Cell phones typically only have a 1 mile >range. I use one on my boat quite frequently, but more than a mile or so >off shore...it doesn't work. - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html