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Old August 11th 20, 08:11 AM posted to sci.electronics.repair
Jeff Liebermann Jeff Liebermann is offline
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Default cell (mobile) phone detector

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I'm obviously out of date 8-) and more than slightly astonished.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.

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Jeff Liebermann
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