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.

Figure 3-1: Radiated power and reach of antennas— omnidirectional and directional

For a given amount of input power, a directional antenna can reach farther with a clearer signal. The antenna must also have much higher sensitivity to radio signals in the dominant direction. When wireless links are used to replace wireline networks, directional antennas are often used. Mobile telephone network operators also use directional antennas when cells are subdivided. 802.11 networks typically use omnidirectional antennas for both ends of the connection.

Antennas are the most likely to be separated from the rest of the electronics. A transmission line (some kind of cable) between the antenna and the transceiver is also necessary. Transmission lines usually have an impedance of 50 ohms. In terms of practical antennas for 802.11 devices in the 2.4 GHz band, the typical wireless PC card has a built-in antenna. The antenna plugs into the card.

Wireless cards all have built-in antennas, but these antennas are, at best, minimally adequate. If you are planning to cover an office or an even larger area, such as a campus, you will almost certainly want to use external antennas for your access points (APs). When considering specialized antennas, you need to pay attention to the following specifications.

Antenna Type The antenna type determines its radiation pattern: omnidirectional, bidirectional, or unidirectional. Omnidirectional antennas are good for covering large areas; bidirectional antennas are particularly good for covering corridors; and unidirectional antennas are the best for setting up point-to-point links between buildings or even different sites. It usually follows that the higher the gain, the narrower the beam.

Factors Affecting Range It's tempting to think that you can put up a high-gain antenna and a power amplifier and cover a huge territory, thus economizing on APs and serving a large number of users at once. This is not a particularly good idea. The larger the area you cover, the more users your APs must serve. A good upper bound to aim for is 20 to 30 users per wireless card per AP. A single AP covering a large territory may look like a good idea, and it may even work well while the number of users remains small. However, if a network is successful, the number of users will grow quickly and the network will soon exceed the AP's capacity.^[1] Once this happens, it is necessary to install more APs and divide the original cell into several smaller ones and lower the power output at all of the cells.

Sensitive Receivers

Besides the antenna, the most critical item in a Wi-Fi system is the receiver. In particular, it is important to look for the receiver sensitivity. The receiver sensitivity is the lowest level signal that can be decoded by the receiver. The lower the receiver sensitivity, the longer the range.

Amplifiers

Amplifiers make signals larger. Signal boost, or gain, is measured in decibels (dB). Amplifiers can be broadly classified into three categories: low noise, high power, and everything else. Low-noise amplifiers (LNAs) are usually connected to an antenna to boost the received signal to a level that is recognizable by the electronics to which the RF system is connected. LNAs are also rated with a noise figure, which is the measure of how much extraneous information the amplifier introduces to the signal-to-noise ratio (SNR). Smaller noise figures enable the receiver to hear smaller signals and thus provide a greater range.

High-power amplifiers (HPAs) are used to boost a signal to the maximum power possible before transmission. Output power is measured in dBm, which are related to watts. Amplifiers are subject to the laws of thermodynamics; they give off heat in addition to amplifying the signal. The transmitter in an 802.11 PC card is necessarily low power because it needs to run off a battery if it is installed in a laptop, but it is possible to install an external amplifier at feed APs. This amplifier can be connected to the power grid where power is more plentiful. This is where things can get tricky with respect to compliance with regulations. 802.11 devices are limited to 1 watt of power output and 4 watts of effective radiated power (ERP). ERP multiplies the transmitter's power output by the gain of the antenna minus the loss in the transmission line. With a 1 watt amplifier, an antenna that provides 8 dB of gain, and 2 dB of transmission line loss, the result is an ERP of 4 watts; the total system gain is 6 dB, which multiplies the transmitter's power by a factor of 4.

^[1]Matthew Gast, 802.11b Wireless Networks: The Definitive Guide (Sebastopol, California: O'Reilly & Associates, 2002), 316–322.