performance. The following key terms are important to understand: All FCC rules and all antennas are measured against what is known as an isotropic antenna, which is a theoretical antenna. This is the basis for all other antennas. The coverage of an isotropic antenna can be thought of as a balloon. It transmits equally in all directions. When an omnidirectional antenna is designed to have gain, it results in loss of coverage in certain areas. Imagine the radiation pattern of an isotropic antenna as a balloon that extends from the antenna equally in all directions. Now imagine pressing in on the top and bottom of the balloon. This causes the balloon to expand in an outward direction, covering more area in the horizontal pattern, but reducing the coverage area above and below the antenna. This yields a higher gain, because the antenna appears to extend to a larger coverage area. The greater the gain of an antenna, the more horizontal and/or vertical beamwidth is reduced. The 2-dBi Rubber Duck dipole antenna for a 2.4-GHz frequency band is an example of an omnidirectional antenna. Figure shows the vertical radiation pattern. A directional antenna redirects the energy in a single direction. Consider an adjustable-beam flashlight in which the intensity and width of the light beam can be changed. The adjustment is controlled by moving the back reflector and directing the light in tighter or wider angles. As the beam gets wider, the intensity in the center decreases, and it travels a shorter distance. The same is true of a directional antenna. The same power is reaching the antenna, but by modifying the construction, the RF energy can be directed in tighter and stronger waves, or wider and less intense waves. The 6.5-dBi Diversity Patch Wall Mount Antenna for the 2.4-GHz frequency band is an example of a directional and diversity antenna. Figure shows the vertical radiation pattern. External antennae require special radio frequency connectors that help to match transmitted RF energy to the radiating element. The RP-TNC connector is an excellent connector (both physically as well as electrically) and, therefore, is the Cisco connector of choice for WLAN applications. Cisco connectorized 5-GHz (802.11a) radios use the same RP-TNC radio connector as 2.4 GHz (802.11b/g) radios. It is possible that someone might connect the wrong antenna to the unit, so Cisco is now using the color blue to denote 5 GHz to minimize this possibility from occurring. Accidentally connecting the wrong antenna does not damage the unit but results in reduced performance. Cisco also offers multiband antennas for the 2.4-GHz and 5-GHz frequency bands, which have a yellow dot. Figure lists some of the 2.4-GHz and 5-GHz antennas. All antennas have RP-TNC connectors. Sector, integrated, and omnidirectional antennas are vertically polarized. This is only a subset of the available antennas. Additionally, Cisco offers multiband antennas for the 2.4-GHz and 5-GHz frequency bands.
Content 6.5 Implementing Wireless LANs 6.5.8 Multipath Distortion Multipath interference occurs when an RF signal has more than one path between a receiver and a transmitter. Just as light and sound bounce off objects, so do RF waves. RF waves can take more than one path when going from a transmitting (Tx) to a receiving (Rx) antenna. These multiple signals arrive at the Rx antenna at different times and phases, which causes distortion of the signal. Multipath interference can cause high signal strength yet low signal quality, causing the data to be unreadable. One indication that you are getting multipath interference is drastically fluctuating signal strength and quality when you move the client only very minor amounts (inches). When an antenna transmits, it radiates RF energy in more then one definite direction. This transmission causes RF to move between the transmitting and receiving antenna in the most direct (desired) path, as well as take other routes that include reflecting or bouncing off metallic and other RF reflective surfaces. The process of reflecting the RF waves causes several things to occur: When these reflected signals are combined at the receiver, the data is unrecoverable because the non-direct signals effectively become noise to the receiver, even though the RF energy (signal strength) may be high. In the end, the desired wave, along with many reflective waves, are combined in the receiver. As these different waveforms combine, they cause distortion to the desired waveform, which can affect the decoding capability of the receiver and result in poor performance. It is also quite common for the radio signals to cancel each other out, causing what is known as a radio null or dead spot. Changing the location of the antenna can change these reflections and diminish the chance of multipath interference and nulls. Diversity systems use two antennas. The access point samples each of the antennas and chooses the antenna with the best performance. The pattern in which signals reflect is greatly affected by the physical wavelength of the signal. Because the wavelength is inversely proportional to the frequency, each frequency has differing multipath effects (fading). For instance, in a location, one frequency has a large multipath interference issue, and another close frequency does not. Since orthogonal frequency-division multiplexing (OFDM) is based on many different frequencies all operating in parallel, the odds are good that some of the information in at least some of the frequencies will be communicated successfully. This approach provides much greater performance in multipath environments.
Content 6.5 Implementing Wireless LANs 6.5.9 Definition of a Decibel Antennas and RF power measurements use units based on decibels. A decibel (dB) is the ratio between two signal levels. This measurement is named after Alexander Graham Bell. The different types of decibel measurements are as follows: These values are all estimated using 0 dBm = 1 mW as a starting point: Add 3 dB to any number = double power
Subtract 3 dB = ½ power
Add 10 dB = 10x power
Subtract 10 dB = divide power by 10 Example: 0 dBm = 1 mW, and 14 dBm = 25 mW
0 dBm = 1 mW,
therefore 10 dBm = 10 mW,
therefore 20 dBm = 100 mW,
subtracting 3