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E X E R C I S E 2 . 2 ( c o n t i n u e d )
Two identical, vertically polarized waves are superposed (you might not see both of them because they cover each other). The result is a wave having double the ampli- tude of the component waves.
5. Click Exercise B.
Two identical, 70-degree out-of-phase waves are superposed. The result is a wave with an increased amplitude over the component waves.
6. Click Exercise C.
Two identical, 140-degree out-of-phase waves are superposed. The result is a wave with a decreased amplitude over the component waves.
7. Click on Exercise D.
Two identical, vertically polarized waves are superposed. The result is a cancellation of the two waves.
signal’s amplitude changes over time. The conventional name for a time domain tool is an oscilloscope. Figure 2.16 shows how both of these tools can be used to display amplitude.
It should be noted that spectrum analyzers are often used by WLAN engineers during site surveys. An oscilloscope is rarely if ever used when deploying a WLAN; however, oscillo- scopes are used by RF engineers in laboratory test environments.
F I G U R E 2 .1 6 RF signal measurement tools
Amplitude
Time Time domain tool
Oscilloscope
Amplitude
Frequency Frequency domain tool
Spectrum analyzer
+
−
Summary
This chapter covered the meat and potatoes, the basics, of radio frequency signals. To prop- erly design and administer a WLAN network, it is essential to have a thorough understand- ing of the following principles of RF properties and RF behaviors:
■ Electromagnetic waves and how they are generated
■ The relationship between wavelength, frequency, and the speed of light
■ Signal strength and the various ways in which a signal can either attenuate or amplify
■ The importance of the relationship between two or more signals
■ How a signal moves by bending, bouncing, or absorbing in some manner
When troubleshooting an Ethernet network, the best place to start is always at layer 1, the Physical layer. WLAN troubleshooting should also begin at the Physical layer. Learning the RF fundamentals that exist at layer 1 is an essential step in proper wireless network administration.
Exam Essentials
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Remember all the RF propagation behaviors. Be able to explain the differences between each RF behavior (such as refl ection, diffraction, scattering, and so on) and the various mediums that are associated with each behavior.
Understand what causes attenuation. Loss can occur either on the wire or in the air.
Absorption, free space path loss, and multipath downfade are all causes of attenuation.
Define free space path loss. Despite the lack of any obstructions, electromagnetic waves attenuate in a logarithmic manner as they travel away from the transmitter.
Remember the four possible results of multipath and their relationship to phase. Multipath may cause downfade, upfade, nulling, and data corruption. Understand that the effects of multipath can be either destructive or constructive.
Know the results of intersymbol interference and delay spread. The time differential between a primary signal and refl ected signals may cause corrupted bits and affect through- put and latency due to layer 2 retransmissions.
Explain the difference between active and passive gain. Transceivers and RF amplifi ers are active devices, whereas antennas are passive devices.
Explain the difference between transmit and received amplitude. Transmit amplitude is typically defi ned as the amount of initial amplitude that leaves the radio transmitter.
When a radio receives an RF signal, the received signal strength is most often referred to as received amplitude.
Review Questions
1. What are some results of multipath interference? (Choose all that apply.) A. Scattering delay
B. Upfade
C. Excessive retransmissions D. Absorption
2. What term best defines the linear distance traveled in one positive-to-negative-to-positive oscillation of an electromagnetic signal?
A. Crest B. Frequency C. Trough D. Wavelength
3. Which of the following statements are true about amplification? (Choose all that apply.) A. All antennas require an outside power source.
B. RF amplifiers require an outside power source.
C. Antennas are passive gain amplifiers that focus the energy of a signal.
D. RF amplifiers passively increase signal strength by focusing the AC current of the signal.
4. A standard measurement of frequency is called what?
A. Hertz B. Milliwatt C. Nanosecond D. Decibel E. K-factor
5. When an RF signal bends around an object, this propagation behavior is known as what?
A. Stratification B. Refraction C. Scattering D. Diffraction E. Attenuation
6. When the multiple RF signals arrive at a receiver at the same time and are with the primary wave, the result can be of the primary signal.
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D. 180 degrees out of phase, amplification E. in phase, cancellation
F. 180 degrees out of phase, cancellation
7. Which of the following statements are true? (Choose all that apply.)
A. When upfade occurs, the final received signal will be stronger than the original trans- mitted signal.
B. When downfade occurs, the final received signal will never be stronger than the origi- nal transmitted signal.
C. When upfade occurs, the final received signal will never be stronger than the original transmitted signal.
D. When downfade occurs, the final received signal will be stronger than the original transmitted signal.
8. What is the frequency of an RF signal that cycles 2.4 million times per second?
A. 2.4 hertz B. 2.4 MHz C. 2.4 GHz D. 2.4 kilohertz E. 2.4 KHz
9. What is the best example of a time domain tool that could be used by an RF engineer?
A. Oscilloscope B. Spectroscope C. Spectrum analyzer D. Refractivity gastroscope
10. What are some objects or materials that are common causes of reflection? (Choose all that apply.)
A. Metal B. Trees C. Asphalt road D. Lake
E. Carpet floors
11. Which of these propagation behaviors can result in multipath? (Choose all that apply.) A. Refraction
B. Diffraction C. Reflection D. Scattering E. None of the above
12. Which behavior can be described as an RF signal encountering a chain link fence, causing the signal to bounce into multiple directions?
A. Diffraction B. Scatter C. Reflection D. Refraction E. Multiplexing
13. Which 802.11 radio technologies are most impacted by the destructive effects of multipath?
(Choose all that apply.) A. 802.11a
B. 802.11b C. 802.11g D. 802.11n E. 802.11i
14. Which of the following can cause refraction of an RF signal traveling through it? (Choose all that apply.)
A. Shift in air temperature B. Change in air pressure C. Humidity
D. Smog E. Wind F. Lightning
15. Which of the following statements are true about free space path loss? (Choose all that apply.)
A. RF signals will attenuate as they travel, despite the lack of attenuation caused by obstructions.
B. Path loss occurs at a constant linear rate.
C. Attenuation is caused by obstructions.
D. Path loss occurs at a logarithmic rate.
16. What term is used to describe the time differential between a primary signal and a reflected signal arriving at a receiver?
A. Path delay B. Spread spectrum C. Multipath
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17. What is an example of a frequency domain tool that could be used by an RF engineer?
A. Oscilloscope B. Spectroscope C. Spectrum analyzer D. Refractivity gastroscope
18. Using knowledge of RF characteristics and behaviors, which two options should a WLAN engineer be most concerned about during an indoor site survey? (Choose all that apply.) A. Brick walls
B. Indoor temperature C. Wood-lath plaster walls D. Drywall
19. Which three properties are interrelated?
A. Frequency, wavelength, and the speed of light B. Frequency, amplitude, and the speed of light C. Frequency, phase, and amplitude
D. Amplitude, phase, and the speed of sound
20. Which RF behavior best describes a signal striking a medium and bending in a different direction?
A. Refraction B. Scattering C. Diffusion D. Diffraction
E. Microwave reflection