[drwu]

The requirements and constraints depend on the product. For example, the geometry of your product to accommodate the antenna is not always the same; the (internal and external) environment of the case is also different; there may be requirement of combining various frequency bands into one antenna; etc.

[buescher]

Right. Everything in the near field of the antenna is part of the antenna. Handheld products are an extreme example of this: remember "just avoid holding it in that way"? (after that, Apple built big test chambers and brought journalists in to see them)

Where are the antennas on your phone today?