cell (mobile) phone detector

[fynn...@gmail.com]

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.

[Jeff Liebermann]

On Sun, 9 Aug 2020 07:06:57 -0700 (PDT), fynn...@gmail.com 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.

No. When the phone is turned off, all of the various transmitters
(cellular, Wi-Fi, BlueGoof, and NFC) are turned off. It might be
possible to detect the NFC (near field communications) resonant
circuit at 13.56MHz with something like a "grid dip meter".

>2. When the phone is on

Yes, as long as "airplane mode" is turned off. There are
transmissions from the cellular section to the tower to allow the
phone to check into the network and establish its presence. If
enabled, Wi-Fi and Bluetooth also transmit keep alive signals.

At this time, power-on NFC services (such as Google Pay) intentionally
do NOT work on a smarphone unless the phone is turned on and NFC is
enabled. However, that might change as NFC begins to encroach on RFID
territory:
<https://www.xda-developers.com/future-android-smartphones-nfc-to-work-disabled/>
Meanwhile, looking for a 13.56MHz tuned circuit might work.

>3. when the battery has been removed

No. Again, the NFC section may not require power to be detectable.
<https://en.wikipedia.org/wiki/Near-field_communication>
<https://nfc-forum.org/what-is-nfc/about-the-technology/>
However, the other transmitters require power to function. Same with
the various "find my phone" programs:
<https://www.google.com/android/find>

>thank you.

Good luck. The big problem is not detecting whether there is a cell
phone present. It's how to identify a particular phone and how to
deal with the clutter caused by a multitude of phones in the same
area. It might be possible to detect a phone if it was the only phone
in the area, but not in a crowd of phones.


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

[fynn...@gmail.com]

Thanks to you all. My next problem is after detection, can the phone be zeroed in on? (located)

[bob prohaska]

This may be bending the OP's intent a little, but (many) years ago I
was led to believe that it was possible to detect semiconductor devices
by "illuminating" them with RF and looking for second harmonic scatter.
It's merely a "there or not there" test, supposedly used to test secure
spaces for for eavesdropping devices. No firsthand experience, alas.

Have I been misled?

Thanks for reading,

bob prohaska

[Jeff Urban]

>but (many) years ago I was led to believe that it was possible to >detect semiconductor devices...

I can see that working under certain circumstances, but the limitations would make it not all that useful.

[Jeff Liebermann]

On Mon, 10 Aug 2020 01:44:30 -0000 (UTC), bob prohaska
<b...@www.zefox.net> wrote:

>This may be bending the OP's intent a little, but (many) years ago I
>was led to believe that it was possible to detect semiconductor devices
>by "illuminating" them with RF and looking for second harmonic scatter.
>It's merely a "there or not there" test, supposedly used to test secure
>spaces for for eavesdropping devices. No firsthand experience, alas.
>
>Have I been misled?

It's quite real. The principle is still being used in some retail
theft prevention tags. The tag has a 900 MHz antenna, diode, and 1800
MHz antenna. When the customer leaves the store, the tag is
illuminated by 900 MHz. If the tag is intact, the diode produces a
2nd harmonic, which is re-radiated by the 1800 MHz antenna. A
receiver near the 900 MHz transmitter listens for 1800 MHz and sounds
an alarm. When the customer pays for merchandise, the checker waves
the tag over some kind of induction device, that fries the diode so
that it doesn't trigger the alarm as the customer leaves the store.

You could do something like that to detect cell phones. The problem
is that if the sensitivity of the receiver was sufficient for such a
system to work with most any semiconductor, it would also false
trigger anything with electronics inside. It has no way to
distinguish between a smartphone and car alarm key fob. It might also
trigger on harmonic generated by diodes created by dissimilar metals
and corrosion. Such false triggering would probably make it useless.
Note that the concept has been extended to passive RFID tags:
<https://www.researchgate.net/figure/Harmonic-RFID-concept_fig1_234682635>
<https://www.google.com/search?q=harmonic+rfid>

It wouldn't take much to shield a wireless bug from being detected by
such a system. The best place to hid a bug is inside something
stuffed full of electronics, such as a phone, TV, monitor, keyboard,
mouse, etc. If you "sweep" a room looking for diodes, it would be
triggered by all these devices, which would then need to be
individually disassembled and inspected for tampering.

Drivel: I wonder how such a system would respond to an OLED display
in a smartphone, which has at least one diode per pixel.

[Jeff Liebermann]

On Sun, 9 Aug 2020 16:07:57 -0700 (PDT), fynn...@gmail.com wrote:

> My next problem is after detection, can the phone be zeroed in on? (located)

No, if you're asking if the GPS in the phone can be interrogated.

No, if you're trying to do direction finding. The likely problem is
that with the power turned off, the only thing that might be usable is
to direction find on the RF energy being absorbed by the 13.56 MHz NFC
loop antenna. I can think of few ways that might be possible, but the
range will be very small, and due to the low frequency, accuracy very
poor.

Yes, if the phone is transmitting on cellular, wi-fi, or Bluetooth
frequencies. Unfortunately, there is no idea system. The problem is
that there are other users also transmitting on cellular and Wi-Fi
frequencies. You not know which frequency is being used by the phone
being located and you don't know when it will be transmitting. Instead
of a nice clean direction indication, you'll see a jumbled mess of
users, frequency hoppers, spread spectrum junk, collisions, and
interference. You could build something that would work under
idealized conditions, but in a crowded RF polluted environment, it
will be very difficult to use.

Offhand, I think the best approach is also the simplest. Build a
rotating directional antenna on the frequencies of interest. A polar
display will show both the bearing and a radial line for the signal
strength. In other words, an AM (amplitude modulated) receiver. Use
an SDR spectrum analyzer for finding the signals coming from the
phone, and then switch the SDR receiver to the frequency of interest
for direction finding. Such an arrangement is far from perfect and
there will be situations where it doesn't work, but it's the closest
and easiest DF system to build.

[bob prohaska]

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.

> It wouldn't take much to shield a wireless bug from being detected by
> such a system. The best place to hid a bug is inside something
> stuffed full of electronics, such as a phone, TV, monitor, keyboard,
> mouse, etc. If you "sweep" a room looking for diodes, it would be
> triggered by all these devices, which would then need to be
> individually disassembled and inspected for tampering.
>
> Drivel: I wonder how such a system would respond to an OLED display
> in a smartphone, which has at least one diode per pixel.
>
>

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-).

One question is power levels; if the interrogation signal starts melting
chocolate bars to get a recognizable return it's likely a bad idea....

Thanks for replying!

bob prohaska

[Jeff Liebermann]

On Mon, 10 Aug 2020 03:22:41 -0000 (UTC), bob prohaska
<b...@www.zefox.net> 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.

>bob prohaska

[bob prohaska]

Jeff Liebermann <je...@cruzio.com> 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!

Thanks for writing,

bob prohaska

[Jeff Liebermann]

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.

[Michael Terrell]

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.

[Jeff Liebermann]

On Tue, 11 Aug 2020 13:47:08 -0700 (PDT), Michael Terrell
<terrell....@gmail.com> 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.

[Michael Terrell]

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. :(

[bilou]

I don't see why current steal protecting devices would not work with phones.
AFAIK they are based on the grid dip principle.
RFID devices don't need battery either and can provide much more
information.
Both solutions do not use destroying levels of RF energy.
Wrapping the target with aluminium foil is a good way to
render them useless.

[bruce bowser]

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.