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#11
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Some Guy wrote:
I have no trouble receiving FM radio broadcasts on a small am/fm radio I sometimes listen to while onboard commercial jet airliners (flying at cruise altitude), but I never seem to be able to pick up AM radio stations. It's just static across the AM band. Any explanation for this? Yes, there is. The AM cops have figured you out, since what you are doing is illegal on commercial airliners. The FM cops are a little slower, but they will pull the plug on you also, eventually. Seriously, though, you are inside a metal cigar tube you call an airplane, and you are being shielded by the body of the aircraft. Although windows (portholes, not Gates), and the metal itself, don't block out signals completely, you will see an effect from this (look up "Faraday Cage" on google). AM broadcast is a very long wavelength (hundreds of meters long) whereas FM broadcast is a smaller wavelength (around 3 meters). If you were trying to throw a bunch of marbles through an upstairs window, you would probably be able to do it. But if you were trying to throw a bunch of beachballs through an upstairs window, it wouldn't be as easy, right? The aperture is the important issue. Although the airplane is not a completely shielded RF-proof "screen room", it acts somewhat like one. That is why avionics antennas are on the outside of the plane, not inside. That is also why there is a teeny mesh grid in the door of your microwave oven - they have to be that small to block the microwaves. Using my example befo if you are throwing beachballs (AM broadcast), or marbles (FM broadcast) or a handful of sand (microwaves), how small would you want the window to be in order to block it? OK, getting back to my first paragraph, if you are ever on a plane with me, please let me know, so I can take the next flight. The local oscillator of FM receivers is often on the same frequency as the VOR stations that airplanes use to naviagate with, and can cause interference. There are failsafe solutions that the pilot has, to deal with loss of VOR coverage, but I don't want to depend on them because you are listening to gangster rap at 32,000 feet. Get an iPod or something. All the best, Dave |
#12
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Some Guy wrote:
I have no trouble receiving FM radio broadcasts on a small am/fm radio I sometimes listen to while onboard commercial jet airliners (flying at cruise altitude), but I never seem to be able to pick up AM radio stations. It's just static across the AM band. Any explanation for this? Yes, there is. The AM cops have figured you out, since what you are doing is illegal on commercial airliners. The FM cops are a little slower, but they will pull the plug on you also, eventually. Seriously, though, you are inside a metal cigar tube you call an airplane, and you are being shielded by the body of the aircraft. Although windows (portholes, not Gates), and the metal itself, don't block out signals completely, you will see an effect from this (look up "Faraday Cage" on google). AM broadcast is a very long wavelength (hundreds of meters long) whereas FM broadcast is a smaller wavelength (around 3 meters). If you were trying to throw a bunch of marbles through an upstairs window, you would probably be able to do it. But if you were trying to throw a bunch of beachballs through an upstairs window, it wouldn't be as easy, right? The aperture is the important issue. Although the airplane is not a completely shielded RF-proof "screen room", it acts somewhat like one. That is why avionics antennas are on the outside of the plane, not inside. That is also why there is a teeny mesh grid in the door of your microwave oven - they have to be that small to block the microwaves. Using my example befo if you are throwing beachballs (AM broadcast), or marbles (FM broadcast) or a handful of sand (microwaves), how small would you want the window to be in order to block it? OK, getting back to my first paragraph, if you are ever on a plane with me, please let me know, so I can take the next flight. The local oscillator of FM receivers is often on the same frequency as the VOR stations that airplanes use to naviagate with, and can cause interference. There are failsafe solutions that the pilot has, to deal with loss of VOR coverage, but I don't want to depend on them because you are listening to gangster rap at 32,000 feet. Get an iPod or something. All the best, Dave |
#13
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On Sat, 11 Dec 2004 14:11:44 -0800, "Ed Price"
wrote: You are asking him to allow a potentially dangerous device to be operated just for your convenience and entertainment. Switch roles for just a minute. Hi Ed, This would make sense (to switch roles) if the administration hadn't trumped that call. Reports recently indicate that the FAA may soon allow anyone, anytime, to make cell phone calls while in flight. Anything goes for a price. The FDA has proven that it is no longer the watchdog of medicine, and the FCC is the gateway for spectrum bargains and marketplace sweeps. With these acronyms, one may well wonder what the "F" stands for. 73's Richard Clark, KB7QHC |
#14
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What a load of horse ****.
You guys are acting as if the engines and flight control surfaces of an aircraft are intimately tied to the plane's radio receiver, and the slightest odd or out-of-place signal that it receives is enough to send any plane into a tail spin. All this while the air travel industry is considering allowing passengers to use their own cell phones WHILE THE PLANES ARE IN FLIGHT by adding cell-phone relay stations to the planes and allowing any such calls to be completed via satellite. So I guess the feeble radiation by my FM radio (powered by 2 AAA batteries) is enough to cause a plane to dive into the ocean, but the guy next to me putting out 3 watts of near-microwave energy is totally safe. What about my hand-held GPS unit? Any chance me using it (during all phases of a flight, which I do routinely) will result in a one-way ticket to kingdom come? Getting back to the original question (poor to non-existant AM reception), I understand the idea of aperature and long wavelenths of AM radio and the size of airplane windows - but what about the effect of ALL the windows on a plane? Don't they create a much larger effective apperature when you consider all of them? And since the plane isin't grounded, isin't the exterior shell of a plane essentially transparent to all RF (ie it's just a re-radiator) because it's not at ground potential? |
#15
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What a load of horse ****.
You guys are acting as if the engines and flight control surfaces of an aircraft are intimately tied to the plane's radio receiver, and the slightest odd or out-of-place signal that it receives is enough to send any plane into a tail spin. All this while the air travel industry is considering allowing passengers to use their own cell phones WHILE THE PLANES ARE IN FLIGHT by adding cell-phone relay stations to the planes and allowing any such calls to be completed via satellite. So I guess the feeble radiation by my FM radio (powered by 2 AAA batteries) is enough to cause a plane to dive into the ocean, but the guy next to me putting out 3 watts of near-microwave energy is totally safe. What about my hand-held GPS unit? Any chance me using it (during all phases of a flight, which I do routinely) will result in a one-way ticket to kingdom come? Getting back to the original question (poor to non-existant AM reception), I understand the idea of aperature and long wavelenths of AM radio and the size of airplane windows - but what about the effect of ALL the windows on a plane? Don't they create a much larger effective apperature when you consider all of them? And since the plane isin't grounded, isin't the exterior shell of a plane essentially transparent to all RF (ie it's just a re-radiator) because it's not at ground potential? |
#16
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On Sat, 11 Dec 2004 19:10:18 -0500, Some Guy wrote:
Getting back to the original question (poor to non-existant AM reception), I understand the idea of aperature and long wavelenths of AM radio and the size of airplane windows - but what about the effect of ALL the windows on a plane? Don't they create a much larger effective apperature when you consider all of them? This becomes a matter of the distance between them and the phase separation at any wavelength. What you describe is a common technique for coupling power between waveguides (in what are called directional couplers). However, this is not the same thing as accumulating and enlarging an opening because such couplers will add energy in one direction, and subtract it in the other (which makes for their directionality). And since the plane isin't grounded, isin't the exterior shell of a plane essentially transparent to all RF (ie it's just a re-radiator) because it's not at ground potential? Ground does not always mean "at one with the dirt and rocks." At one time it did, when cowboys put up talking wires, and indians pulled them down. Ground has since come to mean "common" (which when you think of it, brings us back to dirt, metaphorically). Common means that everything is at the same potential. If there is no potential difference, then there is no way to measure a voltage based signal. In other words, it's a massive short circuit, and the only way to sense a signal is to inductively couple to the short circuit current. This takes us to the second killer courtesy of physics. High frequency current travels on the surface of smallest, positive radius. AM frequency qualifies here in spades, even though it is conventionally called not HF but MF (even VLF qualifies as High Frequency in this context). The aircraft frame thus presents both curvature and radius such that the current confines itself to the outside of the shell with an inclination for the narrow wings and tail section, rather than the elongated body. You might be tempted to inductively tap into this frame current, but then you are on the negative, inside radius of the current carrier (makes the tube interior self-shielding). Whatever current is flowing, is on the outside of the skin, not the inside - that is, until we consider skin depth and penetration. But then it appears that experience described here suggests that not much of that frame current penetrates inside. 73's Richard Clark, KB7QHC |
#17
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On Sat, 11 Dec 2004 19:10:18 -0500, Some Guy wrote:
Getting back to the original question (poor to non-existant AM reception), I understand the idea of aperature and long wavelenths of AM radio and the size of airplane windows - but what about the effect of ALL the windows on a plane? Don't they create a much larger effective apperature when you consider all of them? This becomes a matter of the distance between them and the phase separation at any wavelength. What you describe is a common technique for coupling power between waveguides (in what are called directional couplers). However, this is not the same thing as accumulating and enlarging an opening because such couplers will add energy in one direction, and subtract it in the other (which makes for their directionality). And since the plane isin't grounded, isin't the exterior shell of a plane essentially transparent to all RF (ie it's just a re-radiator) because it's not at ground potential? Ground does not always mean "at one with the dirt and rocks." At one time it did, when cowboys put up talking wires, and indians pulled them down. Ground has since come to mean "common" (which when you think of it, brings us back to dirt, metaphorically). Common means that everything is at the same potential. If there is no potential difference, then there is no way to measure a voltage based signal. In other words, it's a massive short circuit, and the only way to sense a signal is to inductively couple to the short circuit current. This takes us to the second killer courtesy of physics. High frequency current travels on the surface of smallest, positive radius. AM frequency qualifies here in spades, even though it is conventionally called not HF but MF (even VLF qualifies as High Frequency in this context). The aircraft frame thus presents both curvature and radius such that the current confines itself to the outside of the shell with an inclination for the narrow wings and tail section, rather than the elongated body. You might be tempted to inductively tap into this frame current, but then you are on the negative, inside radius of the current carrier (makes the tube interior self-shielding). Whatever current is flowing, is on the outside of the skin, not the inside - that is, until we consider skin depth and penetration. But then it appears that experience described here suggests that not much of that frame current penetrates inside. 73's Richard Clark, KB7QHC |
#18
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Some Guy wrote:
What a load of horse ****. You guys are acting as if the engines and flight control surfaces of an aircraft are intimately tied to the plane's radio receiver, and the slightest odd or out-of-place signal that it receives is enough to send any plane into a tail spin. All this while the air travel industry is considering allowing passengers to use their own cell phones WHILE THE PLANES ARE IN FLIGHT by adding cell-phone relay stations to the planes and allowing any such calls to be completed via satellite. So I guess the feeble radiation by my FM radio (powered by 2 AAA batteries) is enough to cause a plane to dive into the ocean, but the guy next to me putting out 3 watts of near-microwave energy is totally safe. What about my hand-held GPS unit? Any chance me using it (during all phases of a flight, which I do routinely) will result in a one-way ticket to kingdom come? Too bad it's not that simple. But if you're really into this kind of argument, do a groups.google.com search of the sci.geo.satellite-nav newsgroup. There you'll find endless argument, speculation, and rationalization ranging from well informed to completely clueless. There's surely more than ample ruminating there to satisfy anyone, regardless of your orientation or clue level; it's surely not necessary to do it all over again here. Getting back to the original question (poor to non-existant AM reception), I understand the idea of aperature and long wavelenths of AM radio and the size of airplane windows - but what about the effect of ALL the windows on a plane? Don't they create a much larger effective apperature when you consider all of them? A bit larger, yes. But the attenuation inside is still very high, since the windows are extremely small and spaced very close, in terms of wavelength. Sort of like the screen of a screen room. And since the plane isin't grounded, isin't the exterior shell of a plane essentially transparent to all RF (ie it's just a re-radiator) because it's not at ground potential? No, being at "ground potential" plays no part in shielding. Currents and fields on the outside aren't magically allowed to violate basic laws of physics and migrate through a good conductor just because a shield isn't at "ground potential". For that matter, a box that is at "ground potential" at the bottom is nowhere near that potential a quarter wavelength up the side. No shield over a small fraction of a wavelength on a side could work if "ground potential" were a requirement. Yet room-sized shielded enclosures are routinely used into the microwave region. Try your own experiment. Turn your portable radio on, turn up the volume, put it into a sealed can, set it on a stool, and see how much you hear. Roy Lewallen, W7EL |
#19
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Some Guy wrote:
What a load of horse ****. You guys are acting as if the engines and flight control surfaces of an aircraft are intimately tied to the plane's radio receiver, and the slightest odd or out-of-place signal that it receives is enough to send any plane into a tail spin. All this while the air travel industry is considering allowing passengers to use their own cell phones WHILE THE PLANES ARE IN FLIGHT by adding cell-phone relay stations to the planes and allowing any such calls to be completed via satellite. So I guess the feeble radiation by my FM radio (powered by 2 AAA batteries) is enough to cause a plane to dive into the ocean, but the guy next to me putting out 3 watts of near-microwave energy is totally safe. What about my hand-held GPS unit? Any chance me using it (during all phases of a flight, which I do routinely) will result in a one-way ticket to kingdom come? Too bad it's not that simple. But if you're really into this kind of argument, do a groups.google.com search of the sci.geo.satellite-nav newsgroup. There you'll find endless argument, speculation, and rationalization ranging from well informed to completely clueless. There's surely more than ample ruminating there to satisfy anyone, regardless of your orientation or clue level; it's surely not necessary to do it all over again here. Getting back to the original question (poor to non-existant AM reception), I understand the idea of aperature and long wavelenths of AM radio and the size of airplane windows - but what about the effect of ALL the windows on a plane? Don't they create a much larger effective apperature when you consider all of them? A bit larger, yes. But the attenuation inside is still very high, since the windows are extremely small and spaced very close, in terms of wavelength. Sort of like the screen of a screen room. And since the plane isin't grounded, isin't the exterior shell of a plane essentially transparent to all RF (ie it's just a re-radiator) because it's not at ground potential? No, being at "ground potential" plays no part in shielding. Currents and fields on the outside aren't magically allowed to violate basic laws of physics and migrate through a good conductor just because a shield isn't at "ground potential". For that matter, a box that is at "ground potential" at the bottom is nowhere near that potential a quarter wavelength up the side. No shield over a small fraction of a wavelength on a side could work if "ground potential" were a requirement. Yet room-sized shielded enclosures are routinely used into the microwave region. Try your own experiment. Turn your portable radio on, turn up the volume, put it into a sealed can, set it on a stool, and see how much you hear. Roy Lewallen, W7EL |
#20
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Well you have been referred to the FAA Regs and the Airline policies and
ignored them. http://www.fordyce.org/scanning/scan.../scan_fly.html http://www.airweb.faa.gov/Regulatory...e?OpenFrameSet Section 91.21 -------------------------------------------- So here is another URL we hope you read about GPS http://gpsinformation.net/airgps/gpsrfi.htm Just a snip: There are documented cases of AM/FM radios causing interference with Avionic systems and as a result, AM/FM radio receivers are generally prohibited. You will be happy to learn of this quote; "By design, (or happy accident), the "spurs" generated by a GPS generally fall outside the communications frequencies used by Aircraft and so have not been a problem even though a few "spurs" exist. But SOME airlines do not permit the use of GPS receivers. Why is that if they are "safe"?" You will be unhappy with the answers. Hope you go to the URL for the answers Here is one: If a GPS is safe, why can't I use it on an airplane anyway, even if the pilot says NO? Answer: This would be a) unwise, b) illegal and c) dangerous. Never presume that you have more authority than the Captain of a ship! He is responsible for the lives of his passengers and likely has knowledge and experience about his aircraft and/or equipment and/or this particular flight that no one else has.. The use of a GPS by a passenger is NOT worth a confrontation and a possible visitation from the police or FBI when you land.. READ THE LAST SENTENCE AGAIN Be safe, obey the law, stop guessing -- get educated and read these URL's -- RF Gotta Go SomeWhere "Some Guy" wrote in message ... What a load of horse ****. You guys are acting as if the engines and flight control surfaces of an aircraft are intimately tied to the plane's radio receiver, and the slightest odd or out-of-place signal that it receives is enough to send any plane into a tail spin. All this while the air travel industry is considering allowing passengers to use their own cell phones WHILE THE PLANES ARE IN FLIGHT by adding cell-phone relay stations to the planes and allowing any such calls to be completed via satellite. So I guess the feeble radiation by my FM radio (powered by 2 AAA batteries) is enough to cause a plane to dive into the ocean, but the guy next to me putting out 3 watts of near-microwave energy is totally safe. What about my hand-held GPS unit? Any chance me using it (during all phases of a flight, which I do routinely) will result in a one-way ticket to kingdom come? Getting back to the original question (poor to non-existant AM reception), I understand the idea of aperature and long wavelenths of AM radio and the size of airplane windows - but what about the effect of ALL the windows on a plane? Don't they create a much larger effective apperature when you consider all of them? And since the plane isin't grounded, isin't the exterior shell of a plane essentially transparent to all RF (ie it's just a re-radiator) because it's not at ground potential? |
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