cell (mobile) phone detector

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".

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: Meanwhile, looking for a 13.56MHz tuned circuit might work.

No. Again, the NFC section may not require power to be detectable. However, the other transmitters require power to function. Same with the various "find my phone" programs:

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

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

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

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

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:

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

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.

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

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.

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

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: 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.

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.

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. 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:

Y'er welcome.

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

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

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:

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.

[1] Google Pixel 1 running Android 10.

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

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

No problem. I've worked around high power RF (5MW EIRP UHF)and RADAR (2MW p ulsed). The story just didn't sound right. Also, you would think that he wo uld 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 ser vice, and I suffered a lot of damage. A huge pile of MOVs died, trying to m aintain the side that went high. I discovered that the Dell Optiplex 780 co mputer that was on, will run at 67VAC. I only have a few working lights, an d one good outlet, until the repairs are completed. It went out on June 10t h, and was out for a few days under two months. I had to switch to Hughesne t, to get back on line. Sepectrum refused to restore my service. The open n eutral 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 kill er to lose 40 active outlets at my computer desk. :(

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.

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.