Classification of Antennas
A classification of antennas can be based on:
- Frequency and size. Antennas used for HF are different from antennas used for VHF, which in turn are different from antennas for microwave. The wavelength is different at different frequencies, so the antennas must be different in size to radiate signals at the correct wavelength. We are particularly interested in antennas working in the microwave range, especially in the 2.4 GHz and 5 GHz frequencies. At 2.4 GHz the wavelength is 12.5cm, while at 5 GHz it is 6cm.
- Directivity. Antennas can be omnidirectional, sectorial or directive. Omni-directional antennas radiate roughly the same pattern all around the antenna in a complete 360° pattern. The most popular types of omnidirectional antennas are the dipole and the ground plane. Sectorial antennas radiate primarily in a specific area. The beam can be as wide as 180 degrees, or as narrow as 60 degrees. Directional or directive antennas are antennas in which the beamwidth is much narrower than in sectorial antennas. They have the highest gain and are therefore used for long distance links. Types of directive antennas are the Yagi, the biquad, the horn, the helicoidal, the patch antenna, the parabolic dish, and many others.
- Physical construction. Antennas can be constructed in many different ways, ranging from simple wires, to parabolic dishes, to coffee cans.
When considering antennas suitable for 2.4 GHz WLAN use, another classification can be used:
- Application. Access points tend to make point-to-multipoint networks, while remote links are point-to-point. Each of these suggest different types of antennas for their purpose. Nodes that are used for multipoint access will likely use omni antennas which radiate equally in all directions, or sectorial antennas which focus into a small area. In the point-to-point case, antennas are used to connect two single locations together. Directive antennas are the primary choice for this application.
A brief list of common type of antennas for the 2.4 GHz frequency is presented now, with a short description and basic information about their characteristics.