Sub Element G9: Antennas and Feed Lines Flashcards
Which of the following factors determine the characteristic impedance of a parallel conductor antenna feed line?
The distance between the centers of the conductors and the radius of the conductors
What are the typical characteristic impedances of coaxial cables used for antenna feed lines at amateur stations?
50 and 75 ohms
What is the typical characteristic impedance of “window line” parallel transmission line?
450 ohms
What might cause reflected power at the point where a feed line connects to an antenna?
A difference between feed-line impedance and antenna feed-point impedance
How does the attenuation of coaxial cable change as the frequency of the signal it is carrying increases?
Attenuation increases
In what units is RF feed line loss usually expressed?
Decibels per 100 feet
What must be done to prevent standing waves on an antenna feed line?
The antenna feed point impedance must be matched to the characteristic impedance of the feed line
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?
5 to 1
What standing wave ratio will result when connecting a 50 ohm feed line to a non- reactive load having 200 ohm impedance?
4:1
What standing wave ratio will result when connecting a 50 ohm feed line to a non- reactive load having 10 ohm impedance?
5:1
What standing wave ratio will result when connecting a 50 ohm feed line to a non- reactive load having 50 ohm impedance?
1:1
What is the interaction between high standing wave ratio (SWR) and transmission line loss?
If a transmission line is lossy, high SWR will increase the loss
What is the effect of transmission line loss on SWR measured at the input to the line?
The higher the transmission line loss, the more the SWR will read artificially low
What is one disadvantage of a directly fed random-wire HF antenna?
You may experience RF burns when touching metal objects in your station
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?
Slope the radials downward
Which of the following best describes the radiation pattern of a quarter-wave, ground-plane vertical antenna?
Omnidirectional in azimuth
What is the radiation pattern of a dipole antenna in free space in a plane containing the conductor?
It is a figure-eight at right angles to the antenna
How does antenna height affect the horizontal (azimuthal) radiation pattern of a horizontal dipole HF antenna?
If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional
Where should the radial wires of a ground-mounted vertical antenna system be placed?
On the surface of the Earth or buried a few inches below the ground
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?
It steadily decreases
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?
It steadily increases
Which of the following is an advantage of a horizontally polarized as compared to a vertically polarized HF antenna?
Lower ground reflection losses
What is the approximate length for a 1/2 wave dipole antenna cut for 14.250 MHz?
32 feet
What is the approximate length for a 1/2 wave dipole antenna cut for 3.550 MHz?
131 feet
What is the approximate length for a 1/4 wave vertical antenna cut for 28.5 MHz?
8 feet
Which of the following would increase the bandwidth of a Yagi antenna?
Larger-diameter elements
What is the approximate length of the driven element of a Yagi antenna?
1/2 wavelength
How do the lengths of a three-element Yagi reflector and director compare to that of the driven element??
The reflector is longer, and the director is shorter
How does antenna gain stated in dBi compare to gain stated in dBd for the same antenna?
dBi gain figures are 2.15 dB higher than dBd gain figures
How does increasing boom length and adding directors affect a Yagi antenna?
Gain increases
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?
The reflector element must be approximately 5 percent longer than the driven element
What does “front-to-back ratio” mean in reference to a Yagi antenna?
The power radiated in the major radiation lobe compared to that in the opposite direction
What is meant by the “main lobe” of a directive antenna?
The direction of maximum radiated field strength from the antenna
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?
Approximately 3 dB higher
Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna?
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
Which HF antenna would be the best to use for minimizing interference?
A directional antenna
Which of the following is an advantage of using a gamma match with a Yagi antenna?
It does not require that the driven element be insulated from the boom
Approximately how long is each side of the driven element of a quad antenna?
1/4 wavelength
How does the forward gain of a two-element quad antenna compare to the forward gain of a three-element Yagi antenna?
About the same
What is meant by the terms dBi and dBd when referring to antenna gain?
dBi refers to an isotropic antenna, dBd refers to a dipole antenna
What is a beta or hairpin match??
It is a shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching
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 horizontal dipole placed between 1/10 and 1/4 wavelength above the ground
What is the feed-point impedance of an end-fed half-wave antenna?
Very high
In which direction is the maximum radiation from a portable VHF/UHF “halo” antenna?
Omnidirectional in the plane of the halo
What is the primary purpose of antenna traps?
To permit multiband operation
What is an advantage of vertical stacking of horizontally polarized Yagi antennas?
It narrows the main lobe in elevation
Which of the following is an advantage of a log periodic antenna?
Wide bandwidth
Which of the following describes a log periodic antenna?
Element length and spacing vary logarithmically along the boom
How does a “screwdriver” mobile antenna adjust its feed-point impedance?
By varying the base loading inductance
What is the primary use of a Beverage antenna?
Directional receiving for low HF bands
In which direction or directions does an electrically small loop (less than 1/3 wavelength in circumference) have nulls in its radiation pattern?
Broadside to the loop
Which of the following is a disadvantage of multiband antennas?
They have poor harmonic rejection
What is the common name of a dipole with a single central support?
Inverted V
What is the combined vertical and horizontal polarization pattern of a multi- wavelength, horizontal loop antenna?
Virtually omnidirectional with a lower peak vertical radiation angle than a dipole
