Great 2 part blog on antenna design:
Developing wireless products can be a daunting task. There are many pitfalls, traps, and common mistakes people fall into during RF design and development. It is very common to save the implementation of the antenna design for last. The fact of the matter is that the antenna makes your product wireless. It is the most critical component that launches your signal into space. Often little consideration is given to the antenna location and how the object nearby can affect it. This can be devastating not only to a productís performance, but also to schedule and cost.
An antenna is a function of its environment. Whether itís sitting on a desk, on a manufacturerís development board, or in a product, all three scenarios result in different performance. Unlike most components in an RF design that can be dropped in with an expected effect on the circuit, an antenna is affected by everything around it. The radiated electromagnetic fields from an antenna interact with nearby materials, and can change its frequency of operation. The antenna must be placed in its final environment and impedance matched so that it operates in the desired frequency band. A poorly matched antenna can degrade your link budget by 10-30 dB and severely reduce range. All antennas, whether they are off-the-shelf or designed in a lab may require matching.
Read the rest: Antenna Matching Within an Enclosure Part I: Theory & Principle