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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#11
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On Tue, 11 Aug 2020 04:45:31 -0000 (UTC), bob prohaska
wrote: Jeff Liebermann wrote: Well, diodes don't have a Q factor, so that's not relevant. The various resonant circuits and antennas all have a Q. Broadband devices are inherently low Q, so they won't be very efficient for detection and retransmission. The 13.56 MHz loop could have been designed with a fairly high Q, except that body capacitance would ruin the tuning. So, my guess(tm) is that it's also a low Q device. The various RF elements might all be very wide band, but that doesn't offer much if the signal levels and efficiencies are so low as to be useless. I'm obviously out of date 8-) and more than slightly astonished. So, I can walk into a facility posted "no cellphones or cameras" carrying a turned-off cellphone with a camera plus bluetooth and not be found out so long as I don't turn it on? Most surprising! Yep. Better yet, you can have the phone power turned ON, and put the phone in "airplane mode", and not be found. Airplane mode turns off cellular, Wi-Fi, BlueGoof, and possibly NFC. The idea is to prevent any emissions (transmissions) coming from your phone from affected the airplane navigation and communications equipment and causing problems with overloading the local cell towers. Think about 250+ passengers checking into one cell tower upon landing: http://kwc.org/mythbusters/2006/04/episode_49_cellphones_on_plane.html However, there's a catch. Even if airplane mode is turned on, you can turn Wi-Fi and BlueGoof back on. The only part that must be turned off in airplane mode is cellular. Worse, some apps can turn on Wi-Fi or BT when invoked. For example, I recall a BT walkie talkie app that managed to enable BT on startup while in airplane mode. That was years ago, and was presumably fixed by now. There are also apps that want internet access and provide a helpful dialog box asking the users if they want to connect. It's easy enough to do that by mistake. Yep, I just tried it. I turned on airplane mode, which correctly disabled cellular, Wi-Fi, and BT. I started Firefox browser, which immediately complained "Server not found" and offered me the choices of "Enable Wi-Fi" or "Try Again". However, when I clicked "Enable Wi-Fi", it spun merrily for about 5 minutes, but didn't turn on the Wi-Fi. So, I have a phone[1] where one function is trying to turn OFF Wi-Fi, while another is trying to keep it turned OFF. Toss a coin? Chrome browser did it right by simply announcing "No Internet" and only offering "Cancel" as a choice. Edge browser also did it right by providing some useful suggestions and offered only "Download when online". Bottom line is you're probably safe in "airplane mode" but need to be very careful not to be tricked into turning on Wi-Fi or BT, or having some application do it for you. Thanks for writing, bob prohaska [1] Google Pixel 1 running Android 10. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#12
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On Monday, August 10, 2020 at 12:16:56 AM UTC-4, Jeff Liebermann wrote:
On Mon, 10 Aug 2020 03:22:41 -0000 (UTC), bob prohaska wrote: You mentioned 13.56 MHz being used by cellphones. Combined with the 2.4GHz (or maybe 5, on a modern cellphone) would it not be possible to illuminate with both frequencies and then look for harmonics? That still isn't perfect, but it'd help exclude some false positives. The system of illumination you propose relies on three circuit elements being present. There has to be a tuned circuit resonant at the illumination frequency, a non-linear element (diode) to produce the harmonics, and a reasonably efficient radiator of the 2nd (or 3rd) harmonic signal. The tuned circuit is present in the 13.56 MHz loop found in smart phones: http://www.antenna-theory.com/definitions/nfc-antenna.php However, there's no diode or transmit (transponder) antenna in the phone. So, that won't work. (The grid dip meter idea might work because it doesn't need a diode or transmit antenna). For the Wi-Fi/BT frequencies, there's also no tuned circuit, so those also won't work. In the bad old days of analog phones, there were cavity resonators tuned to the cellular operating frequencies, but those haven't been used in smartphones for probably 20 years. Wouldn't that sort of setup have a relatively low Q with fairly uniform response over a wide frequency range? Perhaps I'm suggesting not looking for junctions specifically, but for resonant circuits connected to antennas that must be exposed for the device to function. Obviously no help if the phone is under a tinfoil hat, 8-). Well, diodes don't have a Q factor, so that's not relevant. The various resonant circuits and antennas all have a Q. Broadband devices are inherently low Q, so they won't be very efficient for detection and retransmission. The 13.56 MHz loop could have been designed with a fairly high Q, except that body capacitance would ruin the tuning. So, my guess(tm) is that it's also a low Q device. The various RF elements might all be very wide band, but that doesn't offer much if the signal levels and efficiencies are so low as to be useless. One question is power levels; if the interrogation signal starts melting chocolate bars to get a recognizable return it's likely a bad idea.... That was Percy Spencer, inventor of the microwave oven, who noticed that a chocolate bar melted in his pocket while working on a radar set for Raytheon. https://en.wikipedia.org/wiki/Microwave_oven#Discovery At the present state of the art, illuminating a smartphone with that level of RF will likely destroy the phone before it melts the chocolate. FCC 15.247(b)(2) limits tag readers to 1 watt RF output: https://www.law.cornell.edu/cfr/text/47/15.247 Thanks for replying! Y'er welcome. What kind of chocolate bar doesn't melt from body heat? Back then, they were wrapped in thin aluminum foil which would reflect most of the Microwave RF, as well. |
#13
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On Tue, 11 Aug 2020 13:47:08 -0700 (PDT), Michael Terrell
wrote: What kind of chocolate bar doesn't melt from body heat? Back then, they were wrapped in thin aluminum foil which would reflect most of the Microwave RF, as well. Good point. It appears that it was actually a peanut cluster bar, not chocolate: https://www.popularmechanics.com/technology/gadgets/a19567/how-the-microwave-was-invented-by-accident/ "He loved nature (due to his childhood in Maine)... especially his little friends the squirrels and the chipmunks," the younger Spencer says of his grandfather, "so he would always carry a peanut cluster bar in his pocket to break up and feed them during lunch." This is an important distinction, and not just for the sake of accurate storytelling. Chocolate melts at a much lower temperature (about 80 degrees Fahrenheit) which means melting a peanut cluster bar with microwaves was much more remarkable. Sorry for the recycled misinformation. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#14
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On Tuesday, August 11, 2020 at 9:04:58 PM UTC-4, Jeff Liebermann wrote:
On Tue, 11 Aug 2020 13:47:08 -0700 (PDT), Michael Terrell wrote: What kind of chocolate bar doesn't melt from body heat? Back then, they were wrapped in thin aluminum foil which would reflect most of the Microwave RF, as well. Good point. It appears that it was actually a peanut cluster bar, not chocolate: https://www.popularmechanics.com/technology/gadgets/a19567/how-the-microwave-was-invented-by-accident/ "He loved nature (due to his childhood in Maine)... especially his little friends the squirrels and the chipmunks," the younger Spencer says of his grandfather, "so he would always carry a peanut cluster bar in his pocket to break up and feed them during lunch." This is an important distinction, and not just for the sake of accurate storytelling. Chocolate melts at a much lower temperature (about 80 degrees Fahrenheit) which means melting a peanut cluster bar with microwaves was much more remarkable. Sorry for the recycled misinformation. No problem. I've worked around high power RF (5MW EIRP UHF)and RADAR (2MW pulsed). The story just didn't sound right. Also, you would think that he would have felt the heat from his body adsorbing that much RF. I might joke with you, nut I wouldn't try to insult you. Life is too short to waste on spreading anger. Like your 1200 sq foot house. My garage is 30' by 40'. ![]() I still have no phone service. I can't get power to the Magic Jack. I have to go outside to get cell service, and sometimes a mile away. It was a killer to lose 40 active outlets at my computer desk. ![]() |
#15
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#16
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On Sunday, August 9, 2020 at 10:07:01 AM UTC-4, wrote:
Please is there any gadget/circuit/App that can detect a mobile phone when ; 1. even when the phone is off but the battery is inside. 2. When the phone is on 3. when the battery has been removed thank you. I saw something like that at the start of a 1983 season #2 episode #3 titled: "Red Hot Steel" from the 1980s Remington Steele drama series starring Stephanie Zimbalist. Pretty scary. |
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