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Chapter 4: Radio Frequency Signal and Antenna Concepts
Terms in this set (20)
Which of the following refers to the polar chart of an antenna as viewed from above the antenna? (Choose all that apply)
A. Horizontal view
B. Vertical view
E. Elevation chart
F. Azimuth chart
A, C and F. The Azimuth chart is the top-down view of an antenna's radiation pattern, also known as the H-plane, or horizontal. The size view is known as the Elevation chart, vertical view, or E-plane.
The azimuth chart represents a view of an antenna's radiation pattern from which direction?
D. Both top and side
A. The azimuth is the top-down view of an antenna's radiation pattern, also known as the H-plane.
What is the definition of the horizontal beamwidth of an antenna?
A. The measurement of the angle of the main lobe as represented on the azimuth chart.
B. The distance between the two points on the horizontal axis where the signal decreases by a third. This distance is measured in degrees.
C. The distance between the two -3 dB power points on the horizontal axis, measured in degrees.
D. The distance between the peak power and the point where the signal decreases by half. This distance is measured in degrees.
C. The beamwidth is the distance in degrees between the -3 dB (half-power) point on one side of the main signal and the -3 dB point on the other side of the main signal, measured along the horizontal axis. These are sometimes known as half-power points.
Which antennas are highly directional? (Choose all that apply.)
D. Parabolic dish
D and E. A parabolic dish and a grid are highly directional. The rest of the antennas are semidirectional, and the sector antenna is a special type of semidirectional antenna.
Semidirectional antennas are often used for which of the following purposes? (Choose allthat apply.)
A. Providing short-distance point-to-point communications
B. Providing long-distance point-to-point communications
C. Providing unidirectional coverage from an access point to clients in an indoor environment
D. Reducing reflections and the negative effects of multipath
A, C and D. Semidirectional antennas provide too wide of a beamwidth to support long-distance communications but will work for short distances. They are also useful for providing unidirectional coverage from the access point to clients in an indoor environment. They can also minimize reflections and thus the negative effects of multipath.
The Fresnel zone should not be blocked by more than what percentage to maintain a reliable communications link?
A. 20 percent
B. 40 percent
C. 50 percent
D. 60 percent
B. Any more than 40 percent encroachment into the Fresnel zone is likely to make a link unreliable. The clearer the Fresnel zone, the better, and ideally it should not be blocked at all.
The size of the Fresnel zone is controlled by what factors? (Choose all that apply.)
A. Antenna beamwidth
B. RF line of sight
C and D. The distance and frequency determine the size of the Fresnel zone; these are the only variables in the Fresnel zone formula.
When a long-distance point-to-point link is installed, earth bulge should be considered beyond what distance?
A. 5 miles
B. 7 miles
C. 10 miles
D. 30 miles
B. The distance when the curvature of the earth should be considered is 7 miles.
A network administrator replaced some coaxial cabling used in an outdoor bridge deployment after water damaged the cabling. After replacing the cabling, the network administrator noticed that the EIRP increased drastically and is possibly violating the maximum EIRP power regulation mandate. What are the possible causes of the increased amplitude? (Choose all that apply.)
A. The administrator installed a shorter cable.
B. The administrator installed a lower-grade cable.
C. The administrator installed a higher-grade cable.
D. The administrator installed a longer cable.
E. The administrator used a different-color cable.
A and C. Installing a shorter cable of the same grade will result in less loss and thus more amplitude being transmitted out the antenna. A higher-grade cable rated for less dB loss will have the same result.
Which of the following are true for antenna diversity used by 802.11a/b/g access points? (Choose all that apply.)
A. The transceiver combines the signal from both antennas to provide better coverage.
B. Transceivers can transmit from both antennas at the same time.
C. The transceiver samples both antennas and chooses the best received signal from one
D. Transceivers can transmit from only one of the antennas at a time.
C and D. A transceiver using antenna diversity can transmit from only one antenna at a time. If it transmitted from both antennas, the two signals would interfere with each other. A transceiver can also interpret only one signal at a time, so it samples the signals received by both antennas and chooses the better signal to be received.
To establish a 4-mile point-to-point bridge link in the 2.4 GHz ISM band, what factors should be taken under consideration? (Choose all that apply.)
A. Fresnel zone with 40 percent or less blockage
B. Earth bulge calculations
C. Minimum of 16 dBi of passive gain
D. Proper choice of semidirectional antennas
E. Proper choice of highly directional antennas
A and D. Point-to-point bridge links require a minimum Fresnel zone clearance of 60 percent. Semidirectional antennas such as patch antennas or Yagi antennas are used for short-to-medium-distance bridge links. Highly directional antennas are used for long distance bridge links. Compensating for earth bulge is not a factor until 7 miles.
The ratio between the maximum peak voltage and minimum voltage on a line is known as what?
A. Signal flux
B. Return loss
D. Signal incidents
C. Voltage standing wave ratio (VSWR) is the difference between these voltages and is represented as a ratio, such as, for example, 1.5:1.
What are some of the possible negative effects of an impendence mismatch? (Choose all that apply.)
A. Voltage reflection
B. Blockage of the Fresnel zone
C. Erratic signal strength
D. Decreased signal amplitude
E. Amplifier/transmitter failure
A, C, D and E. The reflected voltage caused by an impedance mismatch can result in a decrease in power or amplitude (loss) of the signal that is supposed to be transmitted. If the transmitter is not protected from excessive reflected power or large voltage peaks, it can overheat and fail. Understand that VSWR may cause decreased signal strength, erratic signal strength, or even transmitter failure.
When determining the mounting height of a long-distance point-to-point antenna, which of the following needs to be considered? (Choose all that apply.)
C. Visual line of sight
D. Earth bulge
E. Antenna beamwidth
F. RF line of sight
A, B, D and F. Frequency and distance are needed to determine the Fresnel zone. Visual line of sight is not needed as long as you have RF line of sight. You may not be able to see the antenna because of fog, but the fog will not prevent RF line of sight. Earth bulge will need to be considered. The beamwidth is not needed to determine the height, although it is useful when aiming the antenna.
Which of the following are true about cables? (Choose all that apply.)
A. They cause impedance on the signal.
B. They work regardless of the frequency.
C. Attenuation decreases as frequency increases.
D. They add loss to the signal.
A and D. Cables must be selected that support the frequency you are using. Attenuation actually increases with frequency.
Amplifiers can be purchased with which of the following features? (Choose all that apply.)
A. Bidirectional amplification
B. Unidirectional amplification
C. Fixed gain
D. Fixed output
A, B, C and D. These are all possible capabilities of RF amplifiers.
The signal between the transceiver and the antenna will be reduced by which of the following methods? (Choose all that apply.)
A. Adding an attenuator
B. Increasing the length of the cable
C. Shortening the length of the cable
D. Using cheaper-quality cable
A, B and D. Adding an attenuator is an intentional act to add loss to the signal. Since cable adds loss, increasing the length will add more loss, whereas shortening the length will reduce the loss. Better-quality cables produce less signal loss.
Lightning arrestors will defend against which of the following?
A. Direct lightning strikes
B. Power surges
C. Transient currents
D. Improper common grounding
C. Lightning arrestors will not stand up to a direct lightning strike, only transient currents caused by nearby lightning strikes.
The radius of the second Fresnel zone is . (Choose all that apply.)
A. The area where the signal is out of phase with the point source
B. The area where the signal is in phase with the point source
C. Smaller than the first Fresnel zone
D. Larger than the first Fresnel zone
A and D. The first Fresnel zone is in phase with the point source. The second Fresnel zone begins at the point where the signals transition from being in phase to being out of phase. Because the second Fresnel zone begins where the first Fresnel zone ends, the radius of the second Fresnel zone is larger than the radius of the first Fresnel zone.
While aligning a directional antenna, you notice that the signal drops as you turn the antenna away from the other antenna, but then it increases a little. This increase in signal is cause by what?
A. Signal reflection
B. Frequency harmonic
C. Side band
D. Side lobe
D. Side lobes are areas of coverage (other than the coverage provided by the main signal) that have a stronger signal than would be expected when compared with the areas around them. Side lobes are best seen on an azimuth chart. Side bands and frequency harmonics have nothing to do with antenna coverage.
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