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The performance of the RF electronics used in modern communication radios largely determines the level of achievable performance. Fortunately, most RF electronics are integrated into commercially available wireless cards, which means that by selecting a quality high-performance wireless card, you can safely assume the components in the RF signal path have been carefully selected to deliver excellent performance. There are a few RF components, external to the wireless card, that are also of great importance in ensuring superior performance. This section discusses these components and provides the relevant information to allow the reader to better understand their operation.
Quite simply stated, the antenna is what puts the signal into the air and gets it back out again. It does this by converting the electrical energy being delivered to the antenna into electromagnetic waves that are then able capable of traveling over long distances. Antennas are reciprocal in nature, which means that they are capable of both transmitting and receiving a signal equally well. This allows the same antenna to be used for both transmission and reception of RF signals.
When discussing different types of antennas, there are several characteristics that are used to describe an antenna's top-level behavior and performance. These characteristics are gain, radiation pattern, resonant frequency, polarization, and efficiency. The combination of these characteristics determines the types of applications in which an antenna can be effectively used. Conversely, if the desired application for an antenna is already known, these characteristics can be used to help select an appropriate antenna.
Antenna gain is used to describe how well an antenna is able to focus RF energy in a particular direction and varies depending on the direction at which the antenna is being viewed. Antenna gain is expressed as a ratio (usually in decibels) that compares the antenna's performance to that of a known reference antenna. The most common reference antenna is an isotropic radiator, which is a purely theoretical antenna that radiates energy equally well in all directions. If an isotropic radiator is used as the reference antenna, the antenna gain is expressed in decibels using the dBi unit (the i stands for isotropic). Another very common reference antenna is a half-wave dipole antenna. If a half-wave dipole is used as the reference, the gain is expressed in decibels using the dBd unit (in this case, the d stands for dipole). These two methods for expressing antenna gain are related to each other and 0 dBd is equal to 2.15 dBi. As an example, consider an antenna that radiates four times as much power in a given direction than an isotropic radiator. The gain of this antenna would be equal to 6 dBi in that direction.
It is important to note that the total amount of RF energy radiated by the antenna cannot be greater than the amount of RF energy being delivered to the antenna. This means that if an antenna is able to focus more RF energy in one direction, then it must radiate less energy in other directions.
The manner in which a practical antenna's gain varies as a function of direction defines its radiation pattern. The radiation pattern of an antenna is commonly depicted graphically by plotting the antenna's gain as a function of angle on a polar plot. In order to accurately describe the radiation pattern of an antenna, two plots are required: azimuth and elevation. The azimuth radiation pattern describes how the gain of an antenna varies when it is viewed from different points on the horizon. An example of an azimuth radiation pattern is shown in Figure 2-17 for a simple directional antenna. From the plot, you can see that there is more gain (roughly 10 dBi in this case) in the boresight direction than in all other directions. Figure 2-18 shows the elevation radiation pattern for the same antenna as in Figure 2-17. The elevation radiation pattern describes the variation in gain when viewed from different angles (or elevations) above the antenna.
The directivity of an antenna refers to how well it is able to focus energy in one specific direction while preventing energy from being radiated in other directions. The greater an antenna's directivity, the higher its gain will be in its primary direction.
A term that is commonly used when working with antennas is Effective Isotropic Radiated Power (EIRP). EIRP is used to describe how much power would have to be radiated by an isotropic radiator (a source that emits energy equally well in all directions) in order to achieve the same amount of power radiated in a particular direction by a transmitting antenna. As convoluted as that may sound, it is an easy quantity to calculate. Just take the amount of RF power being delivered to the antenna (in dBm) and add the gain (in dB) of the antenna, as shown in Equation 11:
As with most RF components, the range of frequencies over which any one particular antenna will exhibit acceptable performance characteristics is limited. In the case of antennas, this is because the electrical size of the antenna components is critical to their performance. Electrical size refers to how large a physical dimension is as compared to a signal's wavelength. If you remember from Equation 2, a signal's wavelength is calculated based on a signal's frequency. This means that the electrical size of an object also changes as a function of frequency. As it turns out, an electrical size (or length) of λ /4 (one-fourth the wavelength) has a tendency to crop up repeatedly in the world of RF. In the case of antennas, it is common for many of the elements in an antenna to have an electrical size that is mathematically related to this λ /4 value (either directly or by some multiplicative factor). Since the wavelength of a signal changes as a function of frequency, while the physical size of an antenna remains constant, the elements of an antenna can only be properly sized at certain frequencies. When the elements are properly sized as compared to a signal's wavelength, the antenna is said to resonate, and the frequency at which this happens is referred to as the resonant frequency.
Typically, the resonant frequency of an antenna is where it is going to exhibit the best performance characteristics. As the frequency increases or decreases, antenna performance begins to change, usually for the worse. The frequencies at which antenna performance drops below acceptable limits define its bandwidth. The amount of bandwidth associated with an antenna depends on its design and construction.
When an antenna is excited with an electric current, it generates electric and magnetic fields in a specific pattern around its radiating elements. The pattern in which these fields are generated depends on the design of the antenna and the environment in which the antenna is located. The orientation of these fields is what determines the polarization of the antenna.
In order for an antenna to be able to convert electromagnetic waves back into electric currents, the polarization of the electromagnetic waves must match the polarization of the antenna. If there is a partial mismatch between the polarizations, then only some of the electromagnetic energy will be converted into electric current. If the polarizations are perpendicular to each other, then theoretically speaking, none of the electromagnetic energy will be converted to electric currents. This means that in order for an antenna to effectively receive the signal transmitted by another antenna, the polarizations of both antennas must be the same.
Antenna polarizations are typically defined by the orientation of the electric (or E) field. If the electric field is oriented vertically, then the antenna is said to have vertical polarization. Conversely, if the electric field is oriented horizontally, the antenna is said to have horizontal polarization. There is another type of polarization called elliptical polarization, where the electric field rotates as the signal propagates through space. For any given type of wireless communication, typically only one type of polarization is used, so that all the antennas in the system have the same orientation and can effectively communicate with each other. Most commercial wireless applications utilize vertical polarization. This is why most WLAN devices always have their antennas pointed upward.
One last comment about polarization is that simply because a signal was transmitted with vertical polarization doesn't mean it will arrive at the receiver with that same polarization. As microwave signals reflect off of surfaces and travel through and/or around obstacles, the electric and magnetic fields can rotate. It is hard to predict when this sort of thing will occur, but it is important to keep in mind that it is possible. Because of this, adjusting the orientation of an antenna can sometimes improve signal strength. But as a general rule, it's a good idea to keep it oriented either vertically or horizontally.
Radiation efficiency is a metric for describing how "well" an antenna works. You could also refer to this as an antenna's "suck factor." All antennas are not created equal. Even if two antennas have similar radiation patterns, one will more than likely perform better than the other. There are many factors that contribute to an antenna's performance but common ones include the type of metal used for construction, dimensional accuracy in antenna components, and the antenna design itself. Technically speaking though, an antenna's efficiency is defined as the amount of electromagnetic energy generated by the antenna divided by the amount of RF energy delivered to the antenna.
It should not come as a surprise that most home-brew antennas have lower radiation efficiency than commercially available antennas of similar design. Determining an antenna's radiation efficiency is often a difficult task, even for the pros. For the average person it is probably better to think of this in terms of a general "suck factor" than a mathematical quantity. If one antenna works better than the other, it probably sucks less.
Let's now talk about some of the most common antenna designs that you are likely to encounter when working with wireless communications.
Omnidirectional Designs The dipole is probably the most widely used stock antenna in wireless hardware. The basic dipole design consists of a metal element one-half wavelength long that is symmetrically fed from the center. There are many methods for constructing this antenna depending on its application. The flexible rubber antennas found on wireless access points are dipole antennas. Also, the integrated antennas used in some USB 802.11 adapters are dipole antennas fabricated using planar PCB technology. As another example, the driven element on a Yagi antenna is, in fact, a dipole antenna.
Dipole antennas have a relatively omnidirectional azimuthal radiation pattern when the dipole element is oriented vertically (hence giving it vertical polarization); this means that the amount of signal radiated by the antenna is constant as you look at it from different points on the ground. The dipole antenna does not radiate much power in the upward or downward directions, which is why it is able to exhibit gain in the azimuth. The gain of an ideal dipole antenna is on the order of 2.15 dBi.
Monopole antennas are another type of omnidirectional antenna. They are constructed out of a vertically oriented element that is typically 1/4 th or 5/8 ths of a wavelength long. For a monopole antenna to be effective, the surface underneath the radiating element should be a flat conductive surface that is parallel to the ground. Monopole antennas have a constant gain when viewed from the azimuth that is higher than a dipole antenna. This is because a monopole antenna does not radiate any energy toward the ground. Because of this characteristic, a monopole antenna's radiation pattern is generally considered to be desirable as compared to that of a dipole. Unfortunately, practical monopole antennas are difficult to realize because of the ground plane required.
Directional Antennas The Yagi-Uda antenna is named after the engineers who first developed it, although it is commonly referred to only as a "Yagi." The Yagi antenna is an extremely prevalent directional antenna due to its relatively simple design and high gain. Yagi antennas are an array of metal rods (or elements) that are all oriented parallel to each other and lie in the same plane. The length and spacing of these elements determines the frequency band at which the antenna will operate, and the number of elements used determines the amount of gain it will have. The orientation of the individual antenna elements is what determines the polarization of a Yagi antenna. If the elements are oriented vertically, then the antenna will have vertical polarization.
The most ubiquitous example of a parabolic antenna is that of a satellite dish. Parabolic antennas make use of a parabolic reflecting surface (hence the name) to "catch" electromagnetic energy and direct (or focus) it toward a smaller antenna (typically called the feed). The benefit of a parabolic antenna is the extraordinarily large amount of gain achievable. But, of course, there is no such thing as a free lunch! In exchange for the high gain, parabolic antennas are highly directive and in turn must be precisely pointed toward the antenna at the other end of the link. The positioning of parabolic reflectors can be tricky as they tend to be large, and in addition to catching radio waves, they also do a really good job at catching wind as well. Even with these drawbacks, parabolic antennas find extensive use in microwave and millimeter wave applications where high antenna gain is necessary to compensate for increased path loss over long link distances.
Antenna arrays are a method for increasing the effective gain of an antenna. Arrays are constructed by taking several antennas and mounting them near each other in a specific pattern and spacing. Although antenna arrays are more complex than stand-alone antennas, there are applications where the increased amount of gain makes an antenna array a viable solution.
Amplification is the process of taking a weak signal and increasing its amplitude (or power). As a signal progresses through a communications system, the signal must be continuously amplified in order for the data contained within the signal to remain intact. Amplifiers are the components within communications systems whose sole purpose is to amplify a signal. While this may sound like a relatively simple task, the physical limitations of practical amplifiers often make it more of a challenge than you might guess. Because of these performance limitations, there are several different types of amplifiers used, depending on their location in the system.
Low noise amplifiers (LNA) could be described as hearing aids for a radio. The LNA is typically the very first amplifier that a signal will encounter once it is received by the antenna. Just as the human brain can't decipher what someone is saying if the background noise is too high, a receiver can't demodulate an RF signal if the amount of received noise is too high. Noise is everywhere in nature, and electronics are no exception to this.
The RF signal coming from an antenna has two major components: the desired RF signal and noise. The strength of the received RF signal varies depending on many factors including transmitted power level, distance from transmitter, type of antenna being used, and the type of environment between the transmitter and receiver. The amount of noise present in a microwave signal is dominated by thermal noise and is typically at a constant level. The amount noise that is present at the input of a receiver is referred to as the noise floor of the receiver.
A problem quickly arises as the amount of power contained within a received RF signal decreases: At some point, the RF signal power will fall below the noise floor of the receiver, and once this happens, the receiver can no longer detect the signal. In fact, the data contained within a signal is typically lost well before the signal disappears into the noise. The signal to noise ratio (SNR) of a signal describes how much stronger a signal is than the accompanying noise and is calculated as shown in Equations 12a and 12b:
It is impossible for an amplifier to differentiate between an RF signal and a noise signal. For this reason, whenever the RF signal is amplified in the receiver, the noise signal is also amplified by the same amount. Every active component in an amplifier will also contribute an additional noise component to the signal. The end result of all this is a reduction in the signal to noise ratio of the received signal. The first components that a signal has to pass through once it comes from the antenna tend to dominate the degradation in SNR.
Low noise amplifiers are designed to contribute a very small amount of additional noise to the signal which they are amplifying. They are typically one of the very first components in the receive signal path. By providing a healthy amount of gain coupled with a minimal amount of noise, they are able to reduce the negative impact on SNR caused by the subsequent components in the receiver. The standard measurement of the quality of a LNA (and entire receivers) is called noise figure. The noise figure of an amplifier or receiver is expressed in dB and the smaller the number, the better.
Power amplifiers (PA) are the last type of amplifier that a signal passes through on its way to the antenna during transmission. Once an RF signal reaches a certain power level, typically −10 dBm (100 ìW) to 0 dBm (1 mW), it becomes increasingly difficult to amplify it any further using regular amplifiers. Power amplifiers are specifically designed to amplify signals to the high power levels required before the signal reaches the antenna. The power amplifiers used in wireless LAN applications are capable of generating +18 dBm (63 mW) of clean (nondistorted) output power. Newer wireless standards, such as WiMax, require clean power levels of up to +24 dBm (250 mW). Power amplifiers used in very high power applications are capable of generating over +50 dBm (100 Watts) in a single chip that is smaller than your fingernail!
Just as the LNA determines most of the receive characteristics of a radio, the PA is the component that determines the transmit capabilities of a radio. A radio's transmit power is directly determined by the power amplifier inside it. Unlike LNAs, the characteristics of PAs that make them difficult to design and implement are not easily explained at a basic level. PAs have the characteristic of sounding deceivingly simple, but are often tricky to design and implement.
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