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General License Ham Radio > Sub Element G9: Antennas and Feed Lines > Flashcards

Sub Element G9: Antennas and Feed Lines Flashcards

1
Q

Which of the following factors determine the characteristic impedance of a parallel conductor antenna feed line?

A

The distance between the centers of the conductors and the radius of the conductors

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2
Q

What are the typical characteristic impedances of coaxial cables used for antenna feed lines at amateur stations?

A

50 and 75 ohms

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3
Q

What is the typical characteristic impedance of “window line” parallel transmission line?

A

450 ohms

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4
Q

What might cause reflected power at the point where a feed line connects to an antenna?

A

A difference between feed-line impedance and antenna feed-point impedance

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5
Q

How does the attenuation of coaxial cable change as the frequency of the signal it is carrying increases?

A

Attenuation increases

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6
Q

In what units is RF feed line loss usually expressed?

A

Decibels per 100 feet

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7
Q

What must be done to prevent standing waves on an antenna feed line?

A

The antenna feed point impedance must be matched to the characteristic impedance of the feed line

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8
Q

If the SWR on an antenna feed line is 5 to 1, and a matching network at the transmitter end of the feed line is adjusted to 1 to 1 SWR, what is the resulting SWR on the feed line?

A

5 to 1

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9
Q

What standing wave ratio will result when connecting a 50 ohm feed line to a non- reactive load having 200 ohm impedance?

A

4:1

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10
Q

What standing wave ratio will result when connecting a 50 ohm feed line to a non- reactive load having 10 ohm impedance?

A

5:1

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11
Q

What standing wave ratio will result when connecting a 50 ohm feed line to a non- reactive load having 50 ohm impedance?

A

1:1

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12
Q

What is the interaction between high standing wave ratio (SWR) and transmission line loss?

A

If a transmission line is lossy, high SWR will increase the loss

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13
Q

What is the effect of transmission line loss on SWR measured at the input to the line?

A

The higher the transmission line loss, the more the SWR will read artificially low

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14
Q

What is one disadvantage of a directly fed random-wire HF antenna?

A

You may experience RF burns when touching metal objects in your station

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15
Q

Which of the following is a common way to adjust the feed-point impedance of a quarter wave ground-plane vertical antenna to be approximately 50 ohms?

A

Slope the radials downward

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16
Q

Which of the following best describes the radiation pattern of a quarter-wave, ground-plane vertical antenna?

A

Omnidirectional in azimuth

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17
Q

What is the radiation pattern of a dipole antenna in free space in a plane containing the conductor?

A

It is a figure-eight at right angles to the antenna

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18
Q

How does antenna height affect the horizontal (azimuthal) radiation pattern of a horizontal dipole HF antenna?

A

If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional

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19
Q

Where should the radial wires of a ground-mounted vertical antenna system be placed?

A

On the surface of the Earth or buried a few inches below the ground

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20
Q

How does the feed-point impedance of a 1/2 wave dipole antenna change as the antenna is lowered below 1/4 wave above ground?

A

It steadily decreases

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21
Q

How does the feed point impedance of a 1/2 wave dipole change as the feed point is moved from the center toward the ends?

A

It steadily increases

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22
Q

Which of the following is an advantage of a horizontally polarized as compared to a vertically polarized HF antenna?

A

Lower ground reflection losses

23
Q

What is the approximate length for a 1/2 wave dipole antenna cut for 14.250 MHz?

A

32 feet

24
Q

What is the approximate length for a 1/2 wave dipole antenna cut for 3.550 MHz?

A

131 feet

25
Q

What is the approximate length for a 1/4 wave vertical antenna cut for 28.5 MHz?

A

8 feet

26
Q

Which of the following would increase the bandwidth of a Yagi antenna?

A

Larger-diameter elements

27
Q

What is the approximate length of the driven element of a Yagi antenna?

A

1/2 wavelength

28
Q

How do the lengths of a three-element Yagi reflector and director compare to that of the driven element??

A

The reflector is longer, and the director is shorter

29
Q

How does antenna gain stated in dBi compare to gain stated in dBd for the same antenna?

A

dBi gain figures are 2.15 dB higher than dBd gain figures

30
Q

How does increasing boom length and adding directors affect a Yagi antenna?

A

Gain increases

31
Q

What configuration of the loops of a two-element quad antenna must be used for the antenna to operate as a beam antenna, assuming one of the elements is used as a reflector?

A

The reflector element must be approximately 5 percent longer than the driven element

32
Q

What does “front-to-back ratio” mean in reference to a Yagi antenna?

A

The power radiated in the major radiation lobe compared to that in the opposite direction

33
Q

What is meant by the “main lobe” of a directive antenna?

A

The direction of maximum radiated field strength from the antenna

34
Q

How does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three- element Yagi?

A

Approximately 3 dB higher

35
Q

Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna?

A

The physical length of the boom
The number of elements on the boom
The spacing of each element along the boom
All these choices are correct

36
Q

Which HF antenna would be the best to use for minimizing interference?

A

A directional antenna

37
Q

Which of the following is an advantage of using a gamma match with a Yagi antenna?

A

It does not require that the driven element be insulated from the boom

38
Q

Approximately how long is each side of the driven element of a quad antenna?

A

1/4 wavelength

39
Q

How does the forward gain of a two-element quad antenna compare to the forward gain of a three-element Yagi antenna?

A

About the same

40
Q

What is meant by the terms dBi and dBd when referring to antenna gain?

A

dBi refers to an isotropic antenna, dBd refers to a dipole antenna

41
Q

What is a beta or hairpin match??

A

It is a shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching

42
Q

Which of the following antenna types will be most effective as a Near Vertical Incidence Skywave (NVIS) antenna for short-skip communications on 40 meters during the day?

A

A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground

43
Q

What is the feed-point impedance of an end-fed half-wave antenna?

A

Very high

44
Q

In which direction is the maximum radiation from a portable VHF/UHF “halo” antenna?

A

Omnidirectional in the plane of the halo

45
Q

What is the primary purpose of antenna traps?

A

To permit multiband operation

46
Q

What is an advantage of vertical stacking of horizontally polarized Yagi antennas?

A

It narrows the main lobe in elevation

47
Q

Which of the following is an advantage of a log periodic antenna?

A

Wide bandwidth

48
Q

Which of the following describes a log periodic antenna?

A

Element length and spacing vary logarithmically along the boom

49
Q

How does a “screwdriver” mobile antenna adjust its feed-point impedance?

A

By varying the base loading inductance

50
Q

What is the primary use of a Beverage antenna?

A

Directional receiving for low HF bands

51
Q

In which direction or directions does an electrically small loop (less than 1/3 wavelength in circumference) have nulls in its radiation pattern?

A

Broadside to the loop

52
Q

Which of the following is a disadvantage of multiband antennas?

A

They have poor harmonic rejection

53
Q

What is the common name of a dipole with a single central support?

A

Inverted V

54
Q

What is the combined vertical and horizontal polarization pattern of a multi- wavelength, horizontal loop antenna?

A

Virtually omnidirectional with a lower peak vertical radiation angle than a dipole

General License Ham Radio (10 decks)

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