Why don't cellphones use unidirectional antennas instead of omnidirectional (single dipole) antennas?

This could be accomplished by using a unidirectional array of dipole antennas (specifically, a broadside array) placed at different locations in the phone, where each antenna's phase could be changed in such a way to minimize radiation directed into the user's head and maximize radiation sent in the direction of the cell tower, producing a radiation pattern as is shown in the picture below.

But, with digital electronics, arbitrary phase-delays could be introduced to change the orientation of the major lobe.

Cellphone batteries would last longer and there would be less adverse health effects due to EMF exposure. Unidirectional Array

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    $\begingroup$ What if you be making a call and you suddenly turn around? $\endgroup$ – fibonatic Jul 30 '16 at 19:17
  • $\begingroup$ @fibonatic Many cellphones can already tell what direction they're oriented. They have built-in compasses and accelerometers. Changing direction would simply change the relative phases of the antennas such that the maximum power is always directed toward the cell tower. $\endgroup$ – Geremia Jul 30 '16 at 20:06
  • $\begingroup$ And how does a cell phone "know" where the "best" tower is located? Is it going to save power to have a cell phone have a massive database of all tower locations, powers, and gain patterns and to combine that with an always on GPS for specific location? At any given time a cell phone could be in range of multiple towers. $\endgroup$ – hazzey Jul 31 '16 at 1:28
  • $\begingroup$ how does a cell phone "know" where the "best" tower is located? How they normally determine this. One dipole alone of the array suffices for that. Once connected to the tower, it could switch into unidirectional mode (i.e., use all the dipoles of the array). $\endgroup$ – Geremia Jul 31 '16 at 1:46
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    $\begingroup$ Do you understand how physically large such an array would have to be in order to be able to provide useful gain (directionality) in any arbitrary direction? I don't think you'd want to put such a thing in your pocket! And there would be no power savings -- the circuits needed to provide electronically-variable delays to such a large number of elements would consume far more power than the "excess" transmit power used with omni antennas. $\endgroup$ – Dave Tweed Jul 31 '16 at 15:43

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