34 Chapter 2 ■ Radio Frequency Fundamentals
c02.indd 08/22/2014 Page 34 F I G U R E 2 . 2 A sine wave
An RF electromagnetic signal radiates away from the antenna in a continuous pattern that is governed by certain properties such as wavelength, frequency, amplitude, and phase.
Additionally, electromagnetic signals can travel through mediums of different materials or travel in a perfect vacuum. When an RF signal travels through a vacuum, it moves at the speed of light, which is 299,792,458 meters per second, or 186,000 miles per second.
To simplify mathematical calculations that use the speed of light, it is com- mon to approximate the value by rounding it up to 300,000,000 meters per second. Any references to the speed of light in this book will use the approximate value.
RF electromagnetic signals travel using a variety or combination of movement behaviors.
These movement behaviors are referred to as propagation behaviors. We discuss some of these propagation behaviors, including absorption, refl ection, scattering, refraction, diffrac- tion, amplifi cation, and attenuation, later in this chapter.
Radio Frequency Characteristics
These characteristics, defi ned by the laws of physics, exist in every RF signal:
F I G U R E 2 . 3 Wavelength
Distance (360 degrees)
The Greek symbol λ represents wavelength. Frequency is usually denoted by the Latin letter f. The Latin letter c represents the speed of light in a vacuum. This is derived from celeritas, the Latin word meaning speed.
It is very important to understand that there is an inverse relationship between wave- length and frequency. The three components of this inverse relationship are frequency (f, measured in hertz, or Hz), wavelength (λ, measured in meters, or m), and the speed of light (c, which is a constant value of 300,000,000 m/sec). The following reference formulas illustrate the relationship: λ = c/f and f = c/λ. A simplifi ed explanation is that the higher the frequency of an RF signal, the smaller the wavelength of that signal. The larger the wave- length of an RF signal, the lower the frequency of that signal.
AM radio stations operate at much lower frequencies than WLAN 802.11 radios, while satellite radio transmissions occur at much higher frequencies than WLAN radios. For instance, radio station WSB-AM in Atlanta broadcasts at 750 KHz and has a wavelength of 1,312 feet, or 400 meters. That is quite a distance for one single cycle of an RF signal to travel. In contrast, some radio navigation satellites operate at a very high frequency, near 252 GHz, and a single cycle of the satellite’s signal has a wavelength of less than 0.05 inches, or 1.2 millimeters. Figure 2.4 displays a comparison of these two extremely differ- ent types of RF signals.
As RF signals travel through space and matter, they lose signal strength (attenuate). It is often thought that a higher frequency electromagnetic signal with a smaller wavelength will attenuate faster than a lower frequency signal with a larger wavelength. In reality, the frequency and wavelength properties of an RF signal do not cause attenuation. Distance is the main cause of attenuation. All antennas have an effective area for receiving power
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wavelengths attenuate faster than signals with a larger wavelength. Theoretically, in a vacuum, electromagnetic signals will travel forever. However, as a signal travels through our atmosphere, the signal will attenuate to amplitudes below the receive sensitivity threshold of a receiving radio. Essentially, the signal will arrive at the receiver, but it will be too weak to be detected.
F I G U R E 2 . 4 750 KHz wavelength and 252 GHz wavelength 750 KHz wavelength = 1,312 feet/400 meters
252 GHz wavelength = 0.05 inches/1.2 millimeters Radio tower
(WSB-AM)
Satellite
The perception is that the higher frequency signal with smaller wavelength will not travel as far as the lower frequency signal with larger wavelength. The reality is that the amount of energy that can be captured by the aperture of a high frequency antenna is smaller than the amount of RF energy that can be captured by a low frequency antenna. A good analogy to a receiving radio would be the human ear. The next time you hear a car coming down the street with loud music, notice that the fi rst thing you hear will be the bass (lower frequencies). This practical example demonstrates that the lower frequency signals
with the larger wavelength will be heard from a greater distance than the higher frequency signal with the smaller wavelength.
The majority of wireless LAN (WLAN) radios operate in either the 2.4 GHz frequency range or the 5 GHz range. In Figure 2.5, you see a comparison of a single cycle of the two waves generated by different frequency WLAN radios.
F I G U R E 2 . 5 2.45 GHz wavelength and 5.775 GHz wavelength
4.82 inches (12.24 centimeters) 2.45 GHz
2.04 inches (5.19 centimeters) 5.775 GHz
Higher frequency signals will generally attenuate faster than lower frequency signals as they pass through various physical mediums such as brick walls. This is important for a wireless engineer to know for two reasons. First, the coverage distance is dependent on the attenuation through the air (referred to as free space path loss, discussed later in this chap- ter). Second, the higher the frequency, typically the less the signal will penetrate through obstructions. For example, a 2.4 GHz signal will pass through walls, windows, and doors with greater amplitude than a 5 GHz signal. Think of how much farther you can receive an AM station’s signal (lower frequency) versus an FM station’s signal (higher frequency).
Note that the length of a 2.45 GHz wave is about 4.8 inches, or 12 centime- ters. The length of a 5.775 GHz wave is a distance of only about 2 inches, or 5 centimeters.
As you can see in Figure 2.4 and Figure 2.5, the wavelengths of the different frequency signals are different because, although each signal cycles only one time, the waves travel dissimilar distances. In Figure 2.6, you see the formulas for calculating wavelength distance in either inches or centimeters.
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F I G U R E 2 . 6 Wavelength formulas
Wavelength (inches) = 11.811/frequency (GHz) Wavelength (centimeters) = 30/frequency (GHz)
Throughout this study guide, you will be presented with various formulas.
You will not need to know these formulas for the CWNA certification exam.
The formulas are in this study guide to demonstrate concepts and to be used as reference material.
How Does the Wavelength of a Signal Concern Me?
It is often thought that a higher frequency electromagnetic signal with a smaller wave- length will attenuate faster than a lower frequency signal with a larger wavelength. In reality, the frequency and wavelength properties of an RF signal do not cause attenuation.
Distance is the main cause of attenuation. All antennas have an effective area for receiv- ing power known as the aperture. The amount of RF energy that can be captured by the aperture of an antenna is smaller with higher frequency antennas. Although wavelength and frequency do not cause attenuation, the perception is that higher frequency signals with smaller wavelengths attenuate faster than signals with a larger wavelength. When all other aspects of the wireless link are similar, Wi-Fi equipment using 5 GHz radios will have shorter range and a smaller coverage area than Wi-Fi equipment using 2.4 GHz radios.
Part of the design of the WLAN includes what is called a site survey. The site survey is responsible for determining zones, or cells, of usable received signal coverage in your facilities. If single radio access points are being used, the 2.4 GHz access points can typi- cally provide greater RF footprints (coverage area) for client stations than the higher fre- quency equipment. More 5 GHz access points would have to be installed to provide the same coverage that can be achieved by a lesser number of 2.4 GHz access points. The penetration of these signals will also reduce coverage for 5 GHz more than it will for 2.4 GHz. Most enterprise Wi-Fi vendors sell dual-frequency access points (APs) with both 2.4 GHz and 5 GHz radios. Site survey planning and coverage analysis for dual-frequency APs should initially be based on the higher frequency 5 GHz signal which effectively provides a smaller coverage area.
Frequency
As previously mentioned, an RF signal cycles in an alternating current in the form of an electromagnetic wave. You also know that the distance traveled in one signal cycle is the wavelength. But what about how often an RF signal cycles in a certain time period?
Frequency is the number of times a specifi ed event occurs within a specifi ed time interval.
A standard measurement of frequency is hertz (Hz), which was named after the German physicist Heinrich Rudolf Hertz. An event that occurs once in 1 second has a frequency of 1 Hz. An event that occurs 325 times in 1 second is measured as 325 Hz. The frequency at which electromagnetic waves cycle is also measured in hertz. Thus, the number of times an RF signal cycles in 1 second is the frequency of that signal, as pictured in Figure 2.7.
Different metric prefi xes can be applied to the hertz (Hz) measurement of radio frequen- cies to make working with very large frequencies easier:
1 hertz (Hz) = 1 cycle per second
1 kilohertz (KHz) = 1,000 cycles per second
1 megahertz (MHz) = 1,000,000 (million) cycles per second 1 gigahertz (GHz) = 1,000,000,000 (billion) cycles per second F I G U R E 2 . 7 Frequency
High frequency radio waves
Low frequency radio waves 1 second
So when we are talking about 2.4 GHz WLAN radios, the RF signal is oscillating 2.4 billion times per second!
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c02.indd 08/22/2014 Page 40 Inverse Relationship
Remember that there is an inverse relationship between wavelength and frequency. The three components of this inverse relationship are frequency (f, measured in hertz, or Hz), wavelength (λ, measured in meters, or m), and the speed of light (c, which is a constant value of 300,000,000 m/sec). The following reference formulas illustrate the relationship:
λ = c/f and f = c/λ. A simplifi ed explanation is that the higher the frequency of an RF sig- nal, the shorter the wavelength will be of that signal. The longer the wavelength of an RF signal, the lower the frequency will be of that signal.