The antenna is fed from a 50 ohm coaxial line. At the feed, the external conductor of the coax line is soldered to the case of the phone. The case forms an integral part of the antenna system. Under normal circumstances, a monopole antenna has infinite ground-plane. In the case of the cellphone, such a ground plane does not exist.
Currents flow over the case, even if the transmitter system and the antenna are perfectly matched.
This fact is quite often neglected in practice. Only half of the power will be delivered to the receiver system. The problem is in fact identical to that of a generator and a matched load. Consider the following two drawings. The antenna can be represented by a generator in series with the antenna radiation impedance Z A .