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How To Tell If Your Cell Phone Is Bugged
Lauren Weinstein writes to point us to his essay on the realities of using an idle cell phone as a bug, as a recent story indicated the FBI may have done in a Mafia case. From the essay: "There is no magic in cell phones. From a transmitting standpoint, they are either on or off... It is also true that some phones can be remotely programmed by the carrier to mask or otherwise change their display and other behaviors in ways that could be used to fool the unwary user. However, this level of remote programmability is another feature that is not universal... But remember — no magic! When cell phones are transmitting — even as bugs — certain things are going to happen every time that the alert phone user can often notice."
No, radio does not behave like that.
The GSM radio wavelength is about 30cm which means that in effect all objects which affect the radio path, including the transmitter and LED receiver, are "blurry" in space to the scale of 30cm (this is an order of magnitude, not an exact value). The phone itself, and the distance from the LEDs, are much smaller than that. So the directionality of the radiation is nearly irrelevant to calculating how much is absorbed and transmitted.
In other words, contrary to the parent post, the LEDs attached to the phone will be effectively on the radio path to the base station, no matter where they are attached on the phone.
It's counterintuitive that you can have a radio signal between two small antennae at A and B, and something that's nearly in between but off by say 10cm affecting the signal between A and B, is though attracting the energy towards it (even bending the beam is possible). But that is exactly what happens. Waves are like that.
It's more complicated than that, however, because the LEDs are also in the "near field" - the region where there may be a non-radiating component to the oscillating EM field around the phone transmitter. In this region, the LEDs could, if they are constructed to do so, absorb energy from the near field, and, depending very much on the phone design, potentially do it without affecting the radiated signal.
Also, it is possible that they absorb some of the radiated energy but if they use very little power, not affect it very much.
So we can't easily say what effect the LEDs will have on the transmitted signal, but the parent's argument about having to be "exactly on the path" to the transmitter, as in a straight line, is not correct.
-- Jamie