It's a question of signal strength. Interference has been observed only very close to transmitting antennas where there is strong signal strength. Interference has been observed in some medical electronics due to hand-held cell phones. Roof-top antennas are some distance away from any susceptible electronics. In addition to proximity, the directionality of the antennas is important. Most rooftop antennas have strong gain in the forward direction—towards the horizon—and very low gain in the vertical direction. Finally, there are layers of building materials shielding the interior areas from the rooftop antennas. Proximity, directionality, shielding—all go towards reducing the signal strength inside a building due to the antennas on the roof so that interference is not generally an issue. This does not mean that you cannot receive the signals from the roof top antennas inside the building; it simply means that the received signals are not strong enough to cause interference. Gary H. Zeman, ScD, CHP
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HOSPITALS DO NOT ALLOW CELL PHONE USE DUE TO POTENTIAL INTERFERENCE. LAST WEEKEND I WAS IN A HOSPITAL WHERE THE TOP OF BUILDING STUDDED WITH TOWER ANTENNAS. WHY IT THAT PHONES WOULD CAUSE INTERFERENCE, BUT ANTENNAS NOT?