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  1. First, what determines the strength of an RF signal?
  2. Why is it said that a cellphones radiates more energy when the reception from signal tower is lower in strength.
  3. Why is it said that RF gets transmitted along wires such as in wired headsets making them act as antennae?
  4. Is it then safer to use bluetooth headsets over wired headsets or direct usage?

Note: Do excuse if this is not purely in the realm of physics.

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Your question assume that this is harm, a matter which is not a well established as some popular accounts would have you believe. (reposted to fix typo.) – dmckee Feb 2 '11 at 22:18
Yes, low power rf has not been determined to be causative for health risks. There have been weak correlations only and disputed studies on both sides. – Gordon Feb 2 '11 at 23:55
  • Phone's radio stack is programmed to both conserve battery and maintain the connection with base station as long as possible. So, it will increase its sending power when it receives weak signal and reduce it when the received signal is strong to save battery power.
  • It is just that changing EM field induces current in all conductors, so the signal from phone will induce some current in headphone cables (and vice-versa, current induced in cable by base station signal may be delivered to phone). It is not effective at all, but may help in very bad reception conditions or when phone's antenna is dumped or shielded by some material around it.
  • Health is a difficult issue since human body is just a too complex and too inseparable system to predict its behavior with good confidence. Yet the picture emerging from current studies suggest that there is no significant influence.
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The transmission power of the handset is controlled by the BTS, so that the signals from all the mobile devices in it's range arrive with similar power density at the BTS. – datenwolf Mar 5 '11 at 20:37
@datenwolf Are you sure? It makes little sense to me; any citations? – mbq Mar 7 '11 at 11:04
The GSM standard (6000 pages!), and the various talks around the OpenBTS , OpenBSC and OsmoCom projects. What they do is implement a fully functional, open source cellular communication infrastructure. Also Harald Welte gave numerous talks on these topics, e.g. – datenwolf Mar 7 '11 at 11:17
@datenwolf Thanks, I'll take a look. – mbq Mar 7 '11 at 11:30
Let me explain a bit: GSM uses time multiplex. Since the location of a single mobile equipment device (ME) is only slowly changing the RX and TX power there needs only little adjustments over time. However since a base transciever station (BTS) is serving up to 128 MEs, and those MEs are usually scattered over a large area, the different signal path conditions change significantly for every time slot. Since no TX and RX amplifier of reasonable price can adjust it's gain as fast as the time multiplex required, the adjustments are laid of to the MEs, but the BTS controls them. – datenwolf Mar 7 '11 at 11:40

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