Sub Element G9: Antennas and Feed Lines

Question 1

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

Answer 1

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

Question 2

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

Answer 2

50 and 75 ohms

Question 3

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

Answer 3

450 ohms

Question 4

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

Answer 4

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

Question 5

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

Answer 5

Attenuation increases

Question 6

In what units is RF feed line loss usually expressed?

Answer 6

Decibels per 100 feet

Question 7

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

Answer 7

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

Question 8

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?

Answer 8

5 to 1

Question 9

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

Answer 9

4:1

Question 10

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

Answer 10

5:1

Question 11

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

Answer 11

1:1

Question 12

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

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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?

Answer 15

Slope the radials downward

Question 16

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

Answer 16

Omnidirectional in azimuth

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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?

Answer 20

It steadily decreases

Question 21

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?

Answer 21

It steadily increases

Question 22

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

Answer 22

Lower ground reflection losses

Question 23

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

Answer 23

32 feet

Question 24

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

Answer 24

131 feet

Question 25

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

Answer 25

8 feet

Question 26

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

Answer 26

Larger-diameter elements

Question 27

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

Answer 27

1/2 wavelength

Question 28

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

Answer 28

The reflector is longer, and the director is shorter

Question 29

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

Answer 29

dBi gain figures are 2.15 dB higher than dBd gain figures

Question 30

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

Answer 30

Gain increases

Question 31

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?

Answer 31

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

Question 32

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

Answer 32

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

Question 33

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

Answer 33

The direction of maximum radiated field strength from the antenna

Question 34

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?

Answer 34

Approximately 3 dB higher

Question 35

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

Answer 35

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

Question 36

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

Answer 36

A directional antenna

Question 37

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

Answer 37

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

Question 38

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

Answer 38

1/4 wavelength

Question 39

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

Answer 39

About the same

Question 40

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

Answer 40

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

Question 41

What is a beta or hairpin match??

Answer 41

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

Question 42

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?

Answer 42

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

Question 43

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

Answer 43

Very high

Question 44

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

Answer 44

Omnidirectional in the plane of the halo

Question 45

What is the primary purpose of antenna traps?

Answer 45

To permit multiband operation

Question 46

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

Answer 46

It narrows the main lobe in elevation

Question 47

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

Answer 47

Wide bandwidth

Question 48

Which of the following describes a log periodic antenna?

Answer 48

Element length and spacing vary logarithmically along the boom

Question 49

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

Answer 49

By varying the base loading inductance

Question 50

What is the primary use of a Beverage antenna?

Answer 50

Directional receiving for low HF bands

Question 51

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

Answer 51

Broadside to the loop

Question 52

Which of the following is a disadvantage of multiband antennas?

Answer 52

They have poor harmonic rejection

Question 53

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

Answer 53

Inverted V

Question 54

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

Answer 54

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