this is a follow up question to Question ID: 205841
URL: http://answers.google.com/answers/main?cmd=threadview&id=205841

Is there a way to detect the presence of idle cell phones even if they
are not making an outbound call, or otherwise communicating with a
cell base station?

jheller-ga,

Thanks for the opportunity to answer this question.

You have several options with regards to detecting the presence of
idle cellular telephones.  Methods of detecting the presence of idle
cellular telephones vary based on the type of technology used.  In the
US, GSM, CDMA, TDMA, analog, and IDEN (a TDMA/GSM variant) are used. 
Netline appears to market a device which detects cell phones:

Generally, the best method for detecting cellular phones would be an
RF or radiation detector, which is available from several companies
and can in many cases be locked to specific cellular frequencies or
cellular bands.

However, the researcher in question 205841, leader-ga, as well as you,
raise an interesting question.  If a cell phone is not making an
outbound call or otherwise communicating with a cellular base station,
it is unlikely much RF will be emitted by the cellular phone.  If this
is the case,  you may have to have extremely sensitive equipment.  If
you are discussing a potential case where a cellular phone is off
during the detection period, there is no other method of locating the
device.

Best of luck to you and if I can be of any further assistance, please
let me know!

Arimathea-ga,
Researcher

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Request for Answer Clarification by jheller-ga on 20 May 2003 16:38 PDT

I am aware of the netline stuff and, in the original thread, got my
answer for how to detect phones when they DO signal base stations.  My
question however was how to detect phones when they DO NOT transmit or
receive base station signals.  simply a 'very sensitive' reciever is
quite vague.  Is there an actual product or technology or method that
can detect an idle phone that is on but not transmitting?

---

Clarification of Answer by arimathea-ga on 20 May 2003 18:21 PDT

If a phone is not transmitting or receiving base station signals (e.g.
is off), there is no way to detect it.  Most phones when not receiving
base station signals will search for a new network, e.g., emit RF. 
This RF - including signal strength, frequency, -- varies based on
network type.  If the phone is a) not searching for base station
signals or b) not in contact with a base station (cell) there is no
way to detect it other than the standard physical means (pat downs,
metal detector, etc).

Very sensitive detector means something that can detect equal to or
less than 0.5mW (transmit power of most phones).  Many amateur radio
tools can do this.

May I prove of any further assistance or would you prefer I withdraw
my answer?

did not read the question and specifically answer it

Here's an excerpt from an interesting article that seems to imply the
existence of an electronic bug-sweeping device that can detect the
very presence of cell phones:

"Stenbit has complained to colleagues about classified meetings being
interrupted when electronic bug-sweepers in specially designed
conference rooms detect the presence of cell phones and handheld
computers."

http://www.usatoday.com/tech/news/computersecurity/2002-07-31-defense-wireless_x.htm

Many, many types of electronic equipment emit some sort of
electromagnetic energy when they're on. Put a sensitive radio close to
a computer, for example, and you'll notice interference.  That's the
reason airlines are pretty sensitive about electronic equipment that
operates at key points of the flight, why you should turn off some
electronic equipment when you're near blast sites, and why cell phones
should be turned off around some types of hospital equipment.  I don't
know about what detection equipment is available, but certainly in
theory it would be possible to detect cell phones, although I assume
the power radiated by an idle cell phone is quite low.

By the way, a cell phone transmits at much more power than half a
milliwatt! Several hundred milliwatts is more like it.

Cellphones transmit at several hundred milliwatts, but the question
was how to detect them when they're *not* transmitting.

Considering that cellphone manufacturers put so much effort into
reducing battery consumption, and hence stray radiation, this problem
could be quite tricky. Perhaps some kind of active detector, tuned to
detect signal disturbances caused by the phone's antenna?? (pure
speculation)