This first chapter from the book Linux Unwired by Roger Weeks et al. introduces radio waves, antennas, connections without wires, Bluetooth, cellular data and infrared. (O'Reilly Media, ISBN: 0596005830, 2004.)
Although a discussion of the physics of antennas is beyond the scope of this book, antennas are obviously a very important part of any radio. Depending on the type of antenna, radio coverage is narrowly focused or widely distributed, which makes a great deal of difference when building or connecting to 802.11 networks.
Briefly, antennas are transducers that convert radio frequency electric currents to electromagnetic waves that are then radiated into space. Antennas are polarized according to the plane of the electric field radiating from the antenna. A vertically polarized antenna has an electric field that is perpendicular to the Earth’s surface. Likewise, the electric field of a horizontally polarized antenna is parallel with the Earth’s surface.
There are several types of antennas used for Wi-Fi networks. The most common antenna is the integrated antenna, followed by omnidirectional and directional antennas
Integrated antennas
Most PC Card radios have integrated antennas inside the enclosure of the card. A typical integrated antenna design has two very small anten-nas—really just a solder trace or small piece of foil—located at right angles to each other for diversity. Diversity antennas are designed so that one antenna or the other is used to transmit and receive, but never at the same time. The card switches automatically between antennas to choose the stronger signal. The antennas are horizontally polarized, and this layout produces an antenna that has a somewhat omnidirectional pattern in a horizontal beam.
Omnidirectional antennas
If you have a radio card or access point with a single external antenna attached, you are likely looking at an omnidirectional, or omni, antenna. Omnidirectional antennas, as the name implies, are designed to send and receive signals 360 degrees around the antenna. Figure 1-8, which is a sample antenna gain pattern for a commercially produced omnidirectional antenna, shows that the 360-degree pattern is not circular at all. Notice that the antenna has pronounced gain at 0 and 180 degrees, but hardly any gain at 90 and 270 degrees.
While the theoretical beamwidth of an omnidirectional antenna is 360 degrees horizontally, the vertical beamwidth of most omni antennas is less than 8 degrees. See Figure 1-9 for a side view of a typical omni antenna. Notice that if the antenna were mounted high enough, someone directly under the antenna itself would have very poor signal quality.
Most omnidirectional antennas are of the “rubber ducky” type—a rub-ber-covered antenna, which ranges from a few inches long for a low-gain model to several feet for high-gain types.
Directional antennas
Although patch antennas are similar to sector antennas, they are considered directional antennas. Patch antennas generally have horizontal and vertical beamwidths that are similar. An example shown in Figure 1-10 shows the gain patterns for a patch antenna.
Figure 1-8. A Sample omnidirectional antenna gain pattern
Figure 1-9. A Side view of an omnidirectional antenna beamwidth
Yagi antennas are also directional antennas and are designed for highly directional applications. They typically have a beamwidth of less than 30 degrees; most of them look like a PVC pipe or a “Christmas tree” pointed at its target.
Finally, parabolic dish, or grid, antennas are the most highly directional antennas used in the 802.11 world. If you’ve seen a satellite dish, you’ve seen a parabolic dish antenna. These antenna types are suited for sending wireless network signals over several miles. As shown in Figure 1-11, the gain pattern is very tight.
Another antenna type widely used in outdoor applications is a sector antenna. These antennas are generally available with horizontal polarization and antenna patterns from 90 to 180 degrees. They are rectangular with a flat profile.
Figure 1-10.A sample patch antenna gain pattern
Figure 1-11. A sample parabolic dish antenna gain pattern
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