Antenna Types
There are three main categories of antennas:
Omnidirectional: Omnidirectional antennas radiate RF in a fashion similar to the way a table or floor lamp radiates light. They are designed to provide general coverage in all directions.
Semidirectional: Semidirectional antennas radiate RF in a fashion similar to the way a wall sconce radiates light away from the wall or the way a street lamp shines light down on a street or a parking lot, providing a directional light across a large area.
Highly directional: Highly directional antennas radiate RF in a fashion similar to the way a spotlight focuses light on a flag or a sign.
Omnidirectional Antennas
Omnidirectional antennas radiate RF signal in all directions. The small, rubber-coated dipole antenna, often referred to as a rubber duck antenna, is the classic example of an omnidirectional antenna and is the default antenna of many access points, although most of the antennas nowadays are encased in plastic instead of rubber. A perfect omnidirectional antenna would radiate RF signal like the theoretical isotropic radiator. The closest thing to an isotropic radiator is the omnidirectional dipole antenna.
An easy way to explain the radiation pattern of a typical omnidirectional antenna is to hold your index finger straight up (this represents the antenna) and place a bagel on it as if it were a ring (this represents the RF signal). If you were to slice the bagel in half horizontally, as if you were planning to spread butter on it, the cut surface of the bagel would represent the azimuth chart, or H-plane, of the omnidirectional antenna. If you took another bagel and sliced it vertically instead, essentially cutting the hole that you are looking through in half, the cut surface of the bagel would now represent the elevation, or E-plane, of the omnidirectional antenna.
Omnidirectional antennas are typically used in point-to-multipoint environments. The omnidirectional antenna is connected to a device (such as an access point) that is placed at the center of a group of client devices, providing central communications capabilities to the surrounding clients. High-gain omnidirectional antennas can also be used outdoors to connect multiple buildings together in a point-to-multipoint confi guration.
Semidirectional Antennas
Unlike omnidirectional antennas that radiate RF signals in all directions, semidirectional antennas are designed to direct a signal in a specific direction. Semidirectional antennas are used for short- to medium-distance communications, with long-distance communications being served by highly directional antennas.
It is common to use semidirectional antennas to provide a network bridge between two buildings in a campus environment or down the street from each other. Longer distances would be served by highly directional antennas.
Three types of antennas fit into the semidirectional category:
■ Patch
■ Panel
■ Yagi (pronounced YAH-gee)
Highly Directional Antennas
Highly directional antennas are strictly used for point-to-point communications, typically to provide network bridging between two buildings. They provide the most focused, narrow beamwidth of any of the antenna types.
There are two types of highly directional antennas: parabolic dish antennas and grid antennas:
Parabolic Dish Antenna
The parabolic dish antenna is similar in appearance to the small digital satellite TV antennas that can be seen on the roofs of many houses.
Grid Antenna
The grid antenna resembles the grill of a barbecue, with the edges slightly curved inward. The spacing of the wires on a grid antenna is determined by the wavelength of the frequencies that the antenna is designed for.
We'll leave the sector antennas (very common in a cellular network) and antenna arrays (for beamforming) to next session.
Hope you all enjoy the theoretical content :)
