Dipole Antennas

Dipole Antennas

Dipole antennas are, by far, the most common antennas used for access points (APs). Many vendors manufacture them, and they come in various form factors. Some are hinged so that they can be positioned straight or at right angles, whereas others are rigid with no adjustments at all. At 2.4 GHz, a dipole antenna will vary from 2.5 inches to as much at 6 inches, depending on manufacturing methods. A dipole antenna exhibits a gain of 2.14 dBi, but many vendors actually round up to 2.2 dBi. (As explained in Chapter 2, dBi is a reference of gain in dB, when compared to an isotropic antenna.) Typically, they are attached directly to the AP, but in some cases they might come with a weighted (or even magnetic) base and a short cable.

Table B-1 lists the specifications of a typical dipole antenna. The frequency range defines the maximum and minimum frequencies that the antenna was designed to operate at, and the voltage standing wave ratio (VSWR) indicates the maximum reflected power over the frequency range. The polarization of the antenna is important for proper installation, and therefore it is a requirement of any specification sheet. The Azimuth and Elevation ratings define the beam width (BW), or radiation-pattern angle, that is contained between the two points where the radiated power is reduced by 3 dB, when compared to the peak power point. See Chapter 2 for a detailed description of these parameters.

Figure B-3 illustrates a typical 2.4-GHz dipole antenna along with the vertical radiation pattern (polar plot). As with most omnidirectional antennas, the horizontal radiation pattern is assumed to be a 360-degree coverage pattern with only minor variations around the complete circle pattern.