Chapter 4 How Antennas Work

RF propagation

10 feet (3 meters) away from the transmitter, LMR-100 cable will introduce a loss of 3.9 dB (39 dB/100 0.39 dB per foot 10 3.9 dB), which means that significantly more than half of the energy produced at the transmitter output is lost, and that is before adding the connector losses! Using LMR-400, the loss introduced by the cable will be only about 0.7 dB. To keep loss at a minimum, you may have to use a cable that is too thick for the connector type used in your transmitter, antenna, or both. The first consideration when deciding to change the manufacturer-provided or equipment-mounted antennas should be the locations of the transmitter and antenna(s). In addition, you should be aware that LMR cable is significantly more expensive than regular coaxial cable, easily costing over 10 times the price. LMR cable is also difficult to add connectors to and difficult to bend, often making its installation a challenge.

parabolic dish antenna

A highly directional antenna that uses a passive dish shaped like a parabola to focus an RF signal into a tight beam.

Monopole Antenna

A monopole antenna is basically a straight piece of wire or metal, usually a quarter of the wavelength, with no reflecting or ground element.

omnidirectional antenna

A type of antenna that issues and receives wireless signals with equal strength and clarity in all directions. This type of antenna is used when many different receivers must be able to pick up the signal.

Active Antenna

Active antennas are essentially passive antennas with built-in amplifiers. The amplifier is connected directly to the piece of metal that forms the antenna itself. Most active antennas have only one electrical connection. The RF signal and the power for the amplifier are supplied on the same conductor. This is intended to reduce the cost and make active antennas easier to install. Active antennas are not common, due to their much higher cost.

Fresnel Zone

Although the transmission path for point-to-point links is usually represented by a straight line, recall that RF waves have a tendency to spread out. This means that the space between the two antennas would be more accurately represented by something similar to an ellipse (see Figure 4-23). This elliptical region is called the Fresnel zone , and its shape is an important consideration in wireless links. When planning a wireless link, at least 60 percent of the Fresnel zone must be kept clear of obstructions, which may affect the height of the antenna tower. If any obstruction, such as a tree or building, blocks more than 40 percent of the RF signal, it blocks the radio signals completely. The Fresnel zone is sometimes referred to as RF line of sight, and it is far more critical than visual line of sight, meaning that although a person standing next to an antenna may be able to see the other antenna, the transceivers at either end of a point-to-point wireless link may not be able to communicate.

Antenna Sizes and Shapes

Antennas come in many sizes and shapes, depending on the following three characteristics: The frequency that the antenna is designed to transmit and receive The direction of the radiated electromagnetic wave The power with which the antenna must transmit or how sensitive it needs to be to receive very weak signals

two-dimensional antennas

Antennas organized in a two-dimensional pattern, with both height and width, are known as two-dimensional antennas . Examples include patch and satellite dish antennas. A satellite dish works like a signal collector, scooping up any signal that comes in a straight line with the center axis of the antenna. A patch antenna is usually a flat piece of metal, with different heights and widths, depending on the desired vertical and horizontal radiation angles. Another type of two-dimensional directional antenna is a horn antenna , such as the one shown in Figure 4-14, which resembles a large horn with the wide end bent to one side, in a parabolic curve. These antennas are common in telephone networks and are used to transmit high-power microwave signals between two very distant towers, usually over 18 miles (30 kilometers).

Antenna Polarization

Antennas radiate RF signals with the amplitude of the waves fluctuating either vertically or horizontally. The orientation of the antenna is referred to as either vertically or horizontally polarized.

Decibel

Decibel (dB) is a ratio between two signal levels a relative measurement (i.e., one in which a value is dependent on another value) that makes it much simpler to express and calculate power gain or loss. Here is how it s done.

Point-to-Multipoint Links

In most wireless communications applications, one transmitter communicates with several mobile clients. This is called a point-to-multipoint wireless link . If the receiver is installed in a fixed location, as in the case of a central building in a campus with wireless links to other buildings, it is possible to maximize the performance of an RF link by using an omnidirectional antenna at a central location and directional, higher-gain antennas at the remote locations. Figure 4-21 illustrates this type of application.

Smart antennas

Multiple antennas and changing the phase of waves coming from different antennas an access point can focus signals toward individual hosts instead of broadcasting them.

Passive Antenna

Passive antennas are the most common type and are constructed of a piece of metal, wire, or similar conductive material. A passive antenna does not amplify the signal in any way; it can only radiate a signal with the same amount of energy that appears at the antenna connector minus any losses introduced by the antenna cable and connector. can produce directional gain

free space loss

The result of the gradual dispersion of the RF energy as it travels through space.

Directional Antennas

The shape of an antenna also affects the intended direction of the radiated RF waves. A directional antenna is used to transmit a signal in one direction only. Although this may sound obvious, it represents an important difference between omnidirectional and directional antennas. Directional antennas, by focusing the RF waves mostly in one direction, concentrate the energy in one direction at the transmitter, or receive more energy, in the case of a receiver antenna, from one direction. This means that they exhibit more gain than an omnidirectional antenna.

Dipole Antenna

The standard straight-wire antenna that provides most omnidirectional function.

absolute power

Therefore, if you want to know the absolute power level of a particular signal that is supplied by a transmitter and you know from the specifications for this particular unit that the strength of the output signal is 36 dBm, you can calculate the absolute power by breaking down this number like this:

Low-loss LMR cables

To calculate the total cable loss, divide the loss per 100 feet by 100 and multiply by the required length of your cable. For example, if you needed to install the antenna about 10 feet (3 meters) away from the transmitter, LMR-100 cable will introduce a loss of 3.9 dB (39 dB/100 0.39 dB per foot 10 3.9 dB), which means that significantly more than half of the energy produced at the transmitter output is lost, and that is before adding the connector losses! Using LMR-400, the loss introduced by the cable will be only about 0.7 dB. To keep loss at a minimum, you may have to use a cable that is too thick for the connector type used in your transmitter, antenna, or both. The first consideration when deciding to change the manufacturer-provided or equipment-mounted antennas should be the locations of the transmitter and antenna(s). In addition, you should be aware that LMR cable is significantly more expensive than regular coaxial cable, easily costing over 10 times the price. LMR cable is also difficult to add connectors to and difficult to bend, often making its installation a challenge.

Point-to-Point Links

Two computer networks in different buildings can be connected by a point-to-point wireless link. In this case, directional antennas provide the most reliable method of transmitting RF waves. Their narrow beams and high gain ensure that most of the energy of the RF wave will be used between the two antennas. The cost is often much lower and the performance comparable to or higher than that of a digital telephone company line. Telephone companies make extensive use of point-to-point microwave links, instead of cables, for long-distance voice and data communications. Although repeater towers are required, the cost of maintaining a wireless link is usually much lower than the cost of installing and maintaining cables, which can be easily damaged and are harder to troubleshoot. Figure 4-22 shows an example of a point-to-point link.

size

When antennas with a higher gain are required, you can increase the size of the antenna to the next bigger fraction. A larger antenna exhibits a higher gain than a shorter antenna. Almost any metallic object or any object that conducts electricity can act as an antenna, but if you use an antenna that is much shorter than the wavelength of a particular frequency, it will not radiate any significant amount of RF. Alternatively, if the antenna is much longer than the wavelength, it will send out some RF energy, just not very efficiently, and this may affect the reliability of the transmitter circuits as well.

decibel to miliwats

You can add or subtract any value represented in dB (dBm, etc.) using the tens and threes rule; you can also convert a dBm value directly to milliwatts of power. Let s look at some examples:

Patch antennas

emit an RF energy beam that is horizontally narrow but vertically wider, or taller, than that of a yagi antenna. Considered a semidirectional antenna, it is often used to send RF energy down a long, narrow corridor, although some varieties are designed for installation on the walls of buildings for example, to send an RF signal in one direction away from the structure where they are mounted. Cellular telephony antennas are also designed to emit signals away from the tower or side of a building, where they are usually mounted. Figure 4-7 illustrates an example of antennas used in a cellular telephone tower, and Figure 4-8 shows a small patch antenna for use indoors.

coaxial cable

insulated copper wire; used to carry high-speed data traffic and television signals

Yagi antenna

unidirectional antenna; works well transmitting and receiving signals in some directions but not in others