Extra question pool with answers. - antennas

What is included in the total resistance of an antenna system?

Radiation resistance plus ohmic resistance.
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Which of the following describes an isotropic radiator?

A theoretical radiator used to compare other antennas

When is it useful to refer to an isotropic radiator?

When comparing the gains of directional antennas
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How much gain does a 1/2-wavelength dipole have over an isotropic radiator?

About 2.1 dB
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Which of the following antennas has no gain in any direction?
Isotropic radiator
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Which of the following describes the radiation pattern of an isotropic radiator?

A sphere with the antenna in the center
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Why would one need to know the feed point impedance of an antenna?

To match impedances for maximum power transfer
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What factors determine the radiation resistance of an antenna?

Antenna location with respect to nearby objects and the conductors' length/diameter ratio
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What is the term for the ratio of the radiation resistance of an antenna to the total resistance of the system?

Antenna efficiency
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What is included in the total resistance of an antenna system?

Radiation resistance plus ohmic resistance.
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What is a folded dipole antenna?
A dipole whose ends are connected by a one-half wavelength piece of wire
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What is meant by antenna gain?

The numerical ratio relating the radiated signal strength of an antenna to that of another antenna
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What is meant by antenna bandwidth?

The frequency range over which an antenna can be expected to perform well
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How can the approximate beamwidth of a beam antenna be determined?

Note the two points where the signal strength of the antenna is down 3 dB from the maximum signal point and compute the angular difference
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How is antenna efficiency calculated?

(radiation resistance / total resistance) x 100%
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How can the efficiency of an HF grounded vertical antenna be made comparable to that of a half-wave dipole antenna?

By installing a good ground radial system
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What theoretical reference antenna provides a comparison for antenna measurements?
Isotropic radiator
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How much gain does an antenna have over a 1/2-wavelength dipole when it has 6 dB gain over an isotropic radiator?

About 3.9 dB
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Which of the following describes the directivity of an isotropic radiator?

No directivity at all
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What is meant by radiation resistance of an antenna?

The value of resistance that would dissipate the same amount of power as that radiated from an antenna.
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What determines the free-space polarization of an antenna?

The orientation of its electric field (E Field)
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In the free-space H-Field radiation pattern shown in Figure E9-1, what is the 3-dB beamwidth?

50 degrees
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In the free-space H-Field pattern shown in Figure E9-1, what is the front-to-back ratio?

18 dB
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In the free-space H-field pattern shown in Figure E9-1, what is the front-to-side ratio?

14 dB
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What information is needed to accurately evaluate the gain of an antenna?

All of these choices
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Which is NOT an important reason to evaluate a gain antenna across the whole frequency band for which it was designed?

The dielectric constant may vary significantly
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What usually occurs if a Yagi antenna is designed solely for maximum forward gain?

The feed-point impedance becomes very low
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If the boom of a Yagi antenna is lengthened and the elements are properly retuned, what usually occurs?

The gain increases
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What type of computer program is commonly used for modeling antennas?

Method of Moments
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What is the principle of a Method of Moments analysis?

A wire is modeled as a series of segments, each having a distinct value of current
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What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase?
Figure-8 end-fire in line with the antennas
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What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 90 degrees out of phase?

Unidirectional cardioid
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What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed in phase?

Figure-8 broadside to the antennas
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What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 180 degrees out of phase?

Figure-8 end-fire in line with the antennas
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What is the radiation pattern for two 1/4-wavelength vertical antennas spaced 1/8-wavelength apart and fed 180 degrees out of phase?

Figure-8 end-fire in line with the antennas
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What is the radiation pattern for two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed in phase?

Elliptical
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Which of the following is the best description of a resonant rhombic antenna?

Bidirectional; four-sided, each side approximately one wavelength long; open at the end opposite the transmission line connection
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What are the advantages of a terminated rhombic antenna?
Wide frequency range, high gain and high front-to-back ratio
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What are the disadvantages of a terminated rhombic antenna for the HF bands?

A large area and four sturdy supports for proper installation
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What is the effect of a terminating resistor on a rhombic antenna?

It changes the radiation pattern from essentially bidirectional to essentially unidirectional
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What type of antenna pattern over real ground is shown in Figure E9-2?
Elevation pattern
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In the H field antenna radiation pattern shown in Figure E9-2, what is the elevation angle of the peak response?

7.5 degrees
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In the H field antenna radiation pattern shown in Figure E9-2, what is the front-to-back ratio?

28 dB
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In the H field antenna radiation pattern shown in Figure E9-2, how many elevation lobes appear in the forward direction?
4
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How is the far-field elevation pattern of a vertically polarized antenna affected by being mounted over seawater versus rocky ground?

The low-angle radiation increases
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If only a modest on-ground radial system can be used with an eighth-wavelength-high, inductively loaded vertical antenna, what would be the best compromise to minimize near-field losses?

A wire-mesh screen at the antenna base, an eighth-wavelength square
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What is one characteristic of a Beverage antenna?

For best performance it should be longer than one wavelength
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How would the electric field be oriented for a Yagi with three elements mounted parallel to the ground?

Horizontally
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What strongly affects the shape of the far-field, low-angle elevation pattern of a vertically polarized antenna?

The conductivity and dielectric constant of the soil
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Why are elevated-radial counterpoises popular with vertically polarized antennas?

They reduce the near-field ground losses, compared to on-ground radial systems using more radials
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What is a terminated rhombic antenna?

A unidirectional antenna terminated in a resistance equal to its characteristic impedance
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What factors determine the receiving antenna gain required at an amateur satellite station in earth operation?

Height, transmitter power and antennas of satellite
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What factors determine the EIRP required by an amateur satellite station in earth operation?

Satellite antennas and height, satellite receiver sensitivity
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What is the approximate beamwidth of a symmetrical pattern antenna with a gain of 20 dB as compared to an isotropic radiator?

20 degrees
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How does the gain of a parabolic dish antenna change when the operating frequency is doubled?

Gain increases 6 dB
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How is circular polarization produced using linearly polarized antennas?

Arrange two Yagis perpendicular to each other, with the driven elements in the same plane, fed 90 degrees out of phase
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How does the beamwidth of an antenna vary as the gain is increased?

It decreases
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Why does a satellite communications antenna system for earth operation need to have rotators for both azimuth and elevation control?

In order to track the satellite as it orbits the earth
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For a shortened vertical antenna, where should a loading coil be placed to minimize losses and produce the most effective performance?

Near the center of the vertical radiator
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Why should an HF mobile antenna loading coil have a high ratio of reactance to resistance?

To minimize losses
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What is a disadvantage of using a trap antenna?

It will radiate harmonics
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How must the driven element in a 3-element Yagi be tuned to use a hairpin matching system?

The driven element reactance is capacitive
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What is the equivalent lumped-constant network for a hairpin matching system on a 3-element Yagi?

L network
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What happens to the bandwidth of an antenna as it is shortened through the use of loading coils?

It is decreased
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What is an advantage of using top loading in a shortened HF vertical antenna?

Improved radiation efficiency
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What is the approximate input terminal impedance at the center of a folded dipole antenna?

300 ohms
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Why is a loading coil often used with an HF mobile antenna?

To tune out the capacitive reactance
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What is an advantage of using a trap antenna?

It may be used for multi-band operation
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What happens at the base feed-point of a fixed length HF mobile antenna as the frequency of operation is lowered?

The resistance decreases and the capacitive reactance increases
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What is the beamwidth of a symmetrical pattern antenna with a gain of 30 dB as compared to an isotropic radiator?

6.4 degrees
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What is the beamwidth of a symmetrical pattern antenna with a gain of 15 dB as compared to an isotropic radiator?

36 degrees
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What is the beamwidth of a symmetrical pattern antenna with a gain of 12 dB as compared to an isotropic radiator?

51 degrees
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What system matches a high-impedance transmission line to a lower impedance antenna by connecting the line to the driven element in two places, spaced a fraction of a wavelength each side of element center?

The delta matching system
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: What system matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center?

The gamma matching system
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What impedance matching system uses a short perpendicular section of transmission line connected to the feed line near the antenna?

The stub matching system
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What should be the approximate capacitance of the resonating capacitor in a gamma matching circuit on a Yagi beam antenna for the 20-meter band?

140 pF
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What should be the approximate capacitance of the resonating capacitor in a gamma matching circuit on a Yagi beam antenna for the 10-meter band?

70 pF
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What is the velocity factor of a transmission line?

The velocity of the wave on the transmission line divided by the velocity of light in a vacuum
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What determines the velocity factor in a transmission line?

Dielectrics in the line
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Why is the physical length of a coaxial cable transmission line shorter than its electrical length?

RF energy moves slower along the coaxial cable
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What is the typical velocity factor for a coaxial cable with polyethylene dielectric?

0.66
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What would be the physical length of a typical coaxial transmission line that is electrically one-quarter wavelength long at 14.1 MHz? (Assume a velocity factor of 0.66.)

3.5 meters
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What is the physical length of a parallel conductor feed line that is electrically one-half wavelength long at 14.10 MHz? (Assume a velocity factor of 0.95.)

10 meters
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What parameter best describes the interactions at the load end of a mismatched transmission line?

Reflection coefficient
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Which of the following measurements describes a mismatched transmission line?

An SWR greater than 1:1
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What characteristic will 450-ohm ladder line have at 50 MHz, as compared to 0.195-inch-diameter coaxial cable (such as RG-58)?

Lower loss in dB/100 feet
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What is the term for the ratio of the actual velocity at which a signal travels through a transmission line to the speed of light in a vacuum?

Velocity factor
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What would be the physical length of a typical coaxial transmission line that is electrically one-quarter wavelength long at 7.2 MHz? (Assume a velocity factor of 0.66.)

6.9 meters
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What kind of impedance does a 1/8-wavelength transmission line present to a generator when the line is shorted at the far end?

An inductive reactance
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What kind of impedance does a 1/8-wavelength transmission line present to a generator when the line is open at the far end?

A capacitive reactance
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What kind of impedance does a 1/4-wavelength transmission line present to a generator when the line is open at the far end?

A very low impedance
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What kind of impedance does a 1/4-wavelength transmission line present to a generator when the line is shorted at the far end?

A very high impedance
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What kind of impedance does a 1/2-wavelength transmission line present to a generator when the line is shorted at the far end?

A very low impedance
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