The structure of the iPhone 4's main antenna used for mobile networks is presumed to be as follows.
(1) The lateral side of the lower part of the phone and (2) the resin part placed on the speaker module seemingly works as antennas.
When a slit located on the lower part of the chassis is covered by a hand, connection becomes unstable (See related article). Near the slit, the radiation electrode on (1)'s open-end side is exposed, and (1) and (2) are connected to each other. In other words, when there is a resistive element like a human hand on this part, it is likely to affect antenna gain.
"Seeing this structure, now I understand why the reported problem occurs," said an engineer who is specialized in antennas.
The reception problem can allegedly be prevented by attaching a cover to the phone probably because it electrically insulates the part.
At first, we projected that only (2) works as an antenna. But its electrode pattern is too small for an antenna that supports five bands, an antenna engineer said. And, the sensitivity of (2) is too low to use (1) only as a ground plane. So, our original prediction seems to be wrong.
Apple used two antennas probably to avoid the risk of employing an innovative design in which part of a chassis is used as an antenna. This design is one of the sales points of the iPhone 4. And Apple CEO Steve Jobs said that the design enhances the sensitivity of antenna.
However, with only (1), when it is necessary to fine-tune, for example, frequency characteristics, even a metal mold has to be made from scratch, an antenna engineer said. That probably is the reason why there is another antenna inside the chassis. (2) is equipped with a coil and a capacitor, which seem to be used for fine-tuning antenna characteristics.