Thanks to DigiKEY that published this great article from LINX TECHNOLOGIES about ANTENNAS in 2011 we can understand the fundamental piece of our wireless products.
The antenna is one of the most complicated aspects of RF design, and specifications on an antenna’s data sheet will not necessarily reflect its performance in the final product. Understanding how antennas work in the real world will help to dispel much of the mystery.
The antenna is probably the most overlooked part of an RF design. The range, performance, and legality of an RF link are critically dependent upon the antenna. However, it is often left until the end of the design and expected to fit into whatever space is left, no matter how unfavorable to performance that location may be. Many of these designs will have to ultimately accept degraded performance or go through multiple redesigns.
With so many interdependent variables, antenna design becomes as much art as science. Engineers delving into RF design for the first time can easily confuse or misinterpret the meaning of antenna specifications and how to apply them. For instance, the gain of an antenna is very different from the gain of an amplifier. The most common misconception may be that the radiation pattern on a monopole antenna’s data sheet will be that of the antenna on the final product. In actuality, the radiation pattern for a quarter-wave monopole antenna is so critically dependent on the design and layout of the product, that manufacturers’ gain specifications and radiation pattern plots have little use except to ascertain potential antenna performance.
Since voluminous texts have been written about each of the many antenna styles, it is unnecessary to cover them all here. This article will focus only on those styles which are commonly used in low-power handheld products: dipole and monopole whips. These styles cover a wide range of available antennas and are among the most common to be implemented incorrectly. With that in mind, there are several rules-of-thumb that can be applied to antenna designs. These rules are less “how to design an antenna” and more “how to design with an antenna.”
... the article "Understanding Antenna Specifications and Operations" continue on the attached file below. For SubGHz and BLE-WiFi antennas read below the "TI Antenna Selection Guide".