Whether you see it or not, every wireless device uses an antenna to send and receive radio frequencies. The size and shape of an antenna depends on the size of the signal's wavelength. High radio frequencies, like those used in cell phones, have short wavelengths, so the antennas can be very small. Low-frequency radio signals have long wavelengths, so longer antennas are needed. Antennas can be made from almost any metal or alloy. The part that receives or sends a signal is called an element. Other parts you may see protruding from an antenna are called reflectors or directors, which help capture the signal on the element.
Base-Station and Directional Antennas
Radio-station towers and cell-phone towers are known as base-station antennas. These are omnidirectional on the horizontal plane, meaning they can send and receive signals in any direction parallel to the horizon. Directional antennas focus on one direction only. Antennas mounted on parabolic dishes are directional. Other examples include corner reflectors, which are two plates at 90 degrees of each other to focus the signals, and Yagi antennas. Yagi antennas consist of several straight elements, each a half-wavelength long, with straight rods parallel to each element called reflectors or directors. The most common examples of Yagi antennas include VHF and UHF TV antennas and high-frequency amateur-radio antennas.
The antennas you see on vehicles, like trucks and emergency service vehicles, are called monopoles, or quarter-wave whip antennas. These antennas consist of a single vertical element, one-quarter wavelength long. When mounted on the metal roof of a vehicle, the roof becomes a ground plane reflector, making the vehicle and antenna a dipole antenna. The lower the frequency, the longer the monopole needs to be. For example, a 50 MHz low band VHF antenna should be 5 feet tall. A 850 MHz monopole is just 3.5 inches tall.
The antennas on older portable radios use a wire that is wound helically around a rod inside the radio. This keeps the antenna small enough that the radio can be worn on a belt or clipped to a lapel, but makes it easy to lose a signal. Because the wavelengths used by cell phones and laptop computers are so high (2.4GHz and 5.8GHz for Wi-Fi, for example), even the smallest case can accommodate several antennas inside. Until 1993, cell phones had external pull-out antennas. This feature was required because cell phone frequencies were in the 900 MHz range, close to that of cordless phones, requiring a much longer antenna than today.
Smart antenna arrays include several sets of elements and computerized controls to move the antennas. Some of the elements are moved in order to more accurately pick up intended signals. Other elements are used to identify radio signals from unwanted sources that would otherwise cause interference. The array can then adjust itself to focus on the intended signals and remove the interfering signals. Smart antenna arrays are expensive, but generally present a cost-savings because they are able to receive and process many signals simultaneously from different senders.
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