RF Components

 
RF Components
RF systems complement wired networks by extending them. Different components may be used
depending on the frequency and the distance that signals are required to reach, but all systems are
fundamentally the same and made from a relatively small number of components. Three RF components
of particular interest to 802.11 users are antennas, sensitive receivers, and amplifiers. Antennas are of
general interest since they are the most tangible feature of an RF system.
Antennas
Antennas are the most critical component of any RF system as they convert electrical signals on wires
into radio waves, and vice versa. To function at all, an antenna must be made of conducting material.
Radio waves hitting an antenna cause electrons to flow in the conductor and create a current. Similarly,
applying a current to an antenna creates an electric field around the antenna. The electric field changes
as the current to the antenna changes. A changing electric field causes a magnetic field, and the wave is
off.
The size of the antenna used depends on the frequency; the higher the frequency, the smaller the
antenna. The shortest simple antenna possible at any frequency is one-half wavelength long. This rule of
thumb accounts for the huge size of radio broadcast antennas and the small size of mobile phones. An
AM station broadcasting at 830 kHz at a wavelength of about 360 meters has a correspondingly large
antenna, but an 802.11b network interface operating in the 2.4 GHz band has a wavelength of just 12.5
centimeters. With some engineering tricks, an antenna can be incorporated into a PC card or the top of a
laptop computer.
Antennas can also be designed with directional preference. Many antennas are omnidirectional, which
means they send and receive signals from any direction. Some applications may benefit from directional
antennas, which radiate and receive on a narrower portion of the field. Figure 3-1 compares the radiated
power of omnidirectional and directional antennas.