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Old August 11th 20, 10:47 PM posted to
Michael Terrell Michael Terrell is offline
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First recorded activity by DIYBanter: Nov 2013
Posts: 137
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

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:
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

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.
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:

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.