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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Old August 11th 20, 08:11 AM posted to sci.electronics.repair
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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I'm obviously out of date 8-) and more than slightly astonished.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.

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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Old August 11th 20, 10:47 PM posted to sci.electronics.repair
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Default cell (mobile) phone detector

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.
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Old August 12th 20, 03:04 AM posted to sci.electronics.repair
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Default cell (mobile) phone detector

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
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Old August 12th 20, 03:33 AM posted to sci.electronics.repair
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Default cell (mobile) phone detector

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'. Unfortunately, I recently lost the neutral to my electrical service, and I suffered a lot of damage. A huge pile of MOVs died, trying to maintain the side that went high. I discovered that the Dell Optiplex 780 computer that was on, will run at 67VAC. I only have a few working lights, and one good outlet, until the repairs are completed. It went out on June 10th, and was out for a few days under two months. I had to switch to Hughesnet, to get back on line. Sepectrum refused to restore my service. The open neutral fried the shield on the cable drop, since it was bonded on both ends.. I wouldn't let them into the house with no lights, and a lot of boxes in the way of where they wanted to go. The previous owner used particle board instead of plywood for the floor in that room, and hid it with cheap carpet.. Spectrum's answer? "We don't do emergency repairs!"

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.
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Old December 31st 20, 04:01 PM posted to sci.electronics.repair
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Default cell (mobile) phone detector

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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