Archived posting to the Leica Users Group, 2001/10/05
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]> > Trust me, a cell phone on a plane could conceivably trigger every cell > > tower within several tens of miles. > > When the cell phone is inside a completely enclosed metallic structure? I > can't get service in Home Depot! The wavelength of an 850 MHz signal is about 14 inches. Such a signal could go out the plane windows easily. It would probably also cut in and out as the plane moved relative to the cell towers and the person moved relative to the windows. However, someone else has pointed out that cell towers are highly directional, and would drastically attentuate signals from above. > > 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? > They weren't cell phones from what I understand, but the on-board plane > phones... Do you know they were in fact cell phones? No, I don't know this for a fact. I'm relying on the news reports which said so and so called on "his" or "her" cell phone. So I can't confirm this. It would be interesting to find out. > > 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 (snip) > It's the energy of the harmonics that matters, and the energy is very low. Right you are. Note that I did not say it "would" interfere, only that it "could." But a runaway local oscillator in a radio receiver that got coupled to a piece of metal inside could easily radiate a few milliwatts, and if a harmonic of that happened to be on a navigation frequency, it could conceivably interfere with a navigation signal. That's a lot of "ifs," certainly, but it could happen. > > 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. > > What frequencies are you talking about? I have a SW radio right next to my > computer, two scanners, two printers etc. I don't get any interference. > Yes, you can get SOME interference with some devices, but it is VERY > localized (ie, very close proximity). My old P-166 puts out several spurious signals over the shortwave spectrum, up through VHF. My *keyboard* has a nice "spur" near 144.2 MHz. They are weak, yes, but enough to interfere with my reception of very weak VHF terrestrial signals or amateur satellites, using coax-fed via antennas on my roof. Many amateur operators have reported such problems in QST and such publications, and in amateur radio and shortwave newsgroups. Also, newer, faster computers generate less spurs in the HF range and more in the VHF and UHF range, as you would expect. Interestingly, current name-brand laptops are often the "cleanest" computers around. > I believe you are exaggerating the magnitude of the issue substantially. > This is VERY low energy we are talking about here, and as such, I do not > believe it is possible for it to cause the problems you so envision. Perhaps, but it is of some concern to the airlines. Remember, a device that has been treated roughly or mishandled can operate out of specs. Airplane electronics are routinely checked, but not Billy's CD player or Suzie's ten year-old transistor radio that's been dropped umpteen times. > > 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. > > WRONG. Notebooks and ANY consumer electronics are subject to the same > regulatory requirements, it has nothing to do with cost. Legally, yes, but I strongly suspect that such certification is more strongly enforced and complied with when it comes to laptops, which are often used on airplanes by business travelers. Frankly, the lack of shielding and bypassing on many consumer electronics is appalling in this increasingly RF-saturated world. Over the years, I have personally encountered a couple of dozen FM radio receivers that would pick up nearly police cars or airplanes overhead, on frequencies well away from the receiver's normal operating band. I've also encountered stereo amplifiers that received radio signals while playing records or CDs. All of them had UL and FCC part 15 stickers on them. > > 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. > > Do you KNOW there is a law that prohibits the use of (specifically) cell > phones on airplanes? I go on a ferry boat a lot, and there is no such > restriction for cell phones. I can only guess that this restriction you > believe does not pertain to cell phones, but if you have a URL for an exact > wording of this "restriction", I'd be interested. I do not know of any *laws* regarding cell phones on planes. I do know, as I said, that U.S. amateur regulations require the permission of the captain or master of a plane or ship to operate on board. I have read that similar regulations exist for other radio services. Cell phones, due to the strength of their companies' lobbying forces, are treated (legally) very differently than other radio transmitting devices. > Irrespective of this, the claim was made that it interferes with the > avionics. From doing FCC/EMI work for over 20 years as well designing MANY > pieces of digital equipment, a lot of what you say above has absolutely NOT > been my experience at all, and goes against practical engineering. I do > agree with your comments about wireless distances though...but I am > skeptical about how effective that is inside a completely enclosed metallic > structure. I am also skeptical of how *likely* it is that they would interfere. I was just discussing possible ways that electronic devices *could* interfere. My point was that airline rules usually take worst case scenarios and make prohibitions based on them, because of the stakes involved. The real world is often considerably dirtier than an engineering lab. And given millions of consumer electronic devices carried on hundreds of thousands of flights, the odds are that eventually one of them going to encounter a problem. As I said, airline *rules* and procedures (as distinct from laws) are formulated based on unlikely worst-case scenarios, because that's how accidents happen. I base this on discussions I've had with friends who work at Boeing. This is not to say that the airlines don't also take full advantage of these conditions to charge premium prices for the "airphones" on the plane. Years ago, I was on a Canadian ferry between the mainland and Victoria, B.C. I asked a ship's officer if I could operate my 2-watt 144 MHz transceiver to call a friend in Victoria. "Oh, no," he said, "it will interfere with our navigation equipment." I knew this was pretty unlikely, but I complied--on the ship, his word was law. This was in the early 80s. In recent times, I've never been refused, in fact, the ship's officers have been amused that I even asked. The only thing that's changed is that now there are so many cell phones out there. I think I've contributed all I can to this OT subject, and will now drop it. Fortunately, I know of nothing in my Leica cameras or lenses that could interfere with airplane navigation. - --Peter Klein Seattle, WA - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html