You use weasel words like 'to some degree' to avoid talking about the truth. Radio waves don't go thru a sheet of metal.
No, not thru a metal wall. I saw the radar go thru the wooden walls of the bldg when I was in radar repair school in the army. But that was wood. Your so-called metal quonset hut was probably wood or fiberglass.
If you saw anything, it was probably your own reflection off the metal walls, IF it didn't fry you like a porkchop in a microwave oven!
I do not think your objections concerning the floor or the bonding of the panels are too relevant. The ends are metal and not relevant either. The radar was pointing right at the wall (no windows nearby); any presumed leakage via remote holes that you assumed might allow the transmitted signal to leak, but would then not be focused on the bird(s) and the path lengths would vary. But the reflected signal from the bird or birds would be rather weak and could not possibly be received via the same wild path(s) to a very directional antenna.
My point is that a Farady shield is a good attenuator, but not "perfect" as ASSuMEd. And it sure is not "flat" in attenuation characteristic as a function of frequency.
Those weren't objections, they were speculations on my part as to how you boys could have been finessing the generally applicable laws of physics.
But truly, the story stinks. So you and your army buddies are in this metal hut, with a fairly high-power radar, and somebody comes up with the bright idea to turn the thing on. Apparently no thought about RF personnel hazards and no concern about strong reflections cooking your detector. Did you test your M16's in a Quonset hut too?
Next point. "The radar was pointing right at the wall..." Now tell me, in a semi-circular Quonset hut, how do you point anything "right at the wall"? Maybe straight up?
Now, a bird doesn't have a very big radar cross section, maybe only about
0.01 square meters, so the return loss is really big. And to resolve a single bird, I'm gonna guess that you had an X or K band radar. So let's run some numbers. Let's say you had a 100kW radar, with a 30 dBi antenna of 1 square meter aperture. At 1500 meters, your detector power would be about 1 picowatt, or -60 dBm. Well hey, that's pretty decent, I'll bet you could see a bird at one mile.
But that's assuming no loss at all due to the metal hut skin. Let's see what happens if we say that the metal hut walls give us only 40 dB of shielding (by absorption or reflection, it doesn't matter). That bites 80 dB out of your path budget, putting your detector signal down to -140 dBm. I think your story just ran out of luck.
Now you can argue about the 40 dB shielding effectiveness of the metal wall, but I'll say that I was being very generous about that. At 10 GHz, I know (How? Easy, I do it everyday. Just 3 days ago, I was keeping some 1.3 GHz from radiating off of some cables, and it was common old Reynolds Wrap to the rescue.), I can get >100 dB out of a sheet of aluminum foil. The SE is so damn high from the material that the only significant factor is when the energy finds a path around the shield.
Don't try to argue that a Faraday cage leaks; you appear to be trying to build a general case based on your experience of always having observed leaky structures. Sure, I know that shielding varies with lots of factors, conductivity, permeability, thickness, frequency, angle of incidence, distance from source, and then there's the problem of apertures. But your hut, with plain old galvanized steel about 1/16" thick, would make a great shielded enclosure, as long as the joints didn't leak.
BTW, I don't like using the term "Faraday cage". Despite all due respect to Mr. Faraday, calling it a shielded enclosure is a clearer description.
I was not alluding to leakage; a more accurate term would be re-radiation. Take an ordinary transformer; it radiates a magnetic field, despite the fact that the core is a closed loop. In fact, one could get nasty and say the same thing about a toroid transformer. Now add a shorted copper turn around the outside of the ordinary transformer's core (i have seen this on many TV power transformers and others as well). What happens? That magnetic field induces a current in that shorted turn, making an opposing magnetic field - thereby reducing the net radiated magnetic field greatly - but not to zero. Now, instead of using that closely wrapped copper shoted turn, put that transformer inside that shielded room you love. Results: great reduction, but not to zero. Increase the frequency to something one might consider RF. Now one has an RF transmitter inside that shielded room, inducing currents in the wall(s). Those currents create opposing fields, and greatly attenuate the signal outside the walls. But they are not zero. BW, radar is usually pulsed, and in the megawatt to multi-megawatt region for the pulse. Also, the quonset huts i saw had relatively vertical walls; the rounded curvature was more so near the top. And it might help to ask the bird(s); they even dislike those pesky jets getting in their way.
FAR more likely that the antenna for the radar was outside the hut. There may have been one slaved to it, inactive, inside the hut to demonstrate what's going on upstairs.
There are plenty of good reasons why such a demo wouldn't work as described, from killing the detector with strong reflections, to massive re-re-reflections inside the building, to the fact that radar relies on a "pencil" beam that wouldn't survive through those walls in any rational way, and so on.
My shielded enclosure only asks that I respect it; I don't think it would provide better SE even if I told it that I loved it.
Nothing ever goes to zero; I'll usually settle for "great" reductions.
You're getting a little fuzzy here. The propagating wave induces surface currents on the metal barrier. The currents "sink" into the metal, decreasing to about 37% (1/e) in what's called a "skin depth". At 10 GHz, a "skin depth" in steel is really thin. After even 10 skin depths, the current is down to only about 1/100,000 of what was on the surface. And there's a whole lot of more skin depths to go before the current is presented to the far surface of the steel barrier. And only then does the surface current on the far side of the barrier get to launch a propagating wave.
Note that the "opposing fields" you mentioned are on the INSIDE, the near surface, of the barrier. The reflected field is 180 degrees out of phase with the incident field, so, real close to the metal surface, the E-field nulls. OTOH, that reflected wave now goes marching back at you, creating lots of fun with out-of-phase energy pumped back into the original radiating element. Everybody sees bad, bad VSWR. And, since you guys were inside a metal hut, there's even more fun in store for you. Not all that energy goes back into the originating antenna. A lot of it just keeps bouncing around inside the hut, creating 3-D variations in power density. Think of yourself as a potato, slowly cooking.
So, to keep this straight, the current that survives Ohmic losses to make it to the far side of the barrier doesn't "greatly attenuate the signal outside the walls". It actually creates the signal (the propagating wave) on the far side of the barrier.
We can talk about aperture leakage and re-radiation from barrier impedance discontinuities some other time.
Multi-megawatt? Hmm, 10 MW? OK, and maybe a duty cycle of 0.01%? Isn't that
1 kW average? I own a 250 kW X-band radar that will do up to 0.1% duty cycle. I sure wouldn't sit in a metal box with that thing running! I wouldn't even want to be in the boresight of the antenna within a few hundred feet. I was trying to be charitable in assuming that nobody would be so dumb as to fire up a multi-megawatt radar INSIDE a metal hut. Looks like you guys proved me wrong!
No, I didn't write that. However, "Julia", rest assured that, by posting your question, your address will soon be harvested for use by the Hipcrime bot in the DOS attack. Don't reply, don't post about it, don't help the bot.
Why is an AM/FM radio receiver potentially more dangerous than laptop PCs, gameboys, DVD players, and other electronic devices that are used quite routinely on airplanes?
I'd say it's more a case of people tending to think that various metal structures such as cars, airplnes, metal boxes, etc. are close to ideal 'Faraday shields' when, in actuality, they might only be a poor approximation. (It's this line of reasoning that usually flummuxes people when they try to shield a monitor that has a wavvy display from some extneral field with a steel box and find it's not very effective.)
Hmm... any idea if the folks inside weren't being exposed to far more radiation that what we'd typically consider safe? :-)
There's a Simpsons episode where Homer and Marge check out the house for sale next door, and find that it has a very high end, contemporary kitchen including a microwave oven big enough to walk in... something that Homer immediately does, of course.
The other devices may have circuits that incidentally radiate a little noise in the aircraft VHF band. A broadcast FM receiver almost certainly has an oscillator running by design, in the band. Where it lands in the aircraft band, is determined by where it's tuned to.
For one thing, they have an antenna which can transmit as well as receive, although poorly compared to a *real* antenna. How much radiation depends on the design of the radio. A cheapo one could be pretty bad.
Ah... you're thinking... FM broadcast range is 88-108MHz... with a 10.7MHz IF... a high side LO is at ~98-118MHz, easily landing within the aircraft band (which is... 108-??? MHz, right?).
The original poster is long gone -- refused any info and advice we gave him including a list of airlines that prohibit AM/FM radios and other devices And the FAA stance on the matter Must have been 50+ responses So I guess we can put this to bed
In fact the Avalon Peninsuala in the most eastern part of the island portion of the province of Newfoundland and Labrador, Canada, is a 'Way Point' for many transatlantic flights headed to/from Europe. Clear days Transatlantic flight con-trails, at 30,000 feet etc. can be seen almost continuously. That is why so many of the flights that were prevented from entering US air space 9/11 had to land in eastern Canada. Many US/Canada friendships were founded between grounded travellers that day and eastern and western Canadians who voluntarily accommodated them during the delay. Cape Spear near St John's is the most easterly point in North America. Marconi received the first transatlantic wireless telegraph signal near St John's in Dec. 1901. French is one of the 'Official Languages' in Canada. A significant percentage of the population, mainly in Quebec, New Brunswick, but also elsewhere in Canada, is French speaking. Many/most are bilingual. Same way Spanish is significant in the USA? The word 'Cajun' in southern US comes from the French word "Acadian"; originally inhabitants of Acadia or what is now the eastern Canadian Province of Nova Scotia. Terry. PS. Staff at the National Historic Park at Signal Hill, St. John's, which incorporates the memorial and events which celebrate Marconi's first wireless telegraph reception say that visitors unaware of the approximately
1800 miles across the Atlantic, (4.3 hours by jet to London-Heathrow) will sometimes ask "Can you see across to England/Ireland etc.". The answer is; "No, but sometimes you can see "Whales"! :-) And sometimes icebergs as well.
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