Avionics MCQs on “Types of Antenna – 2”.
1. What is the use of loading coil?
a) Correcting resonance to a desired frequency
b) Increasing the antenna gain
c) Sideband suppression
d) Increasing the range of the antenna
Answer: a
Clarification: When a vertical antenna is made less than one-quarter wavelength, the practical effect is a decreased inductance. The antenna no longer resonates at the desired operating frequency, but at a higher frequency. To compensate for this, a series inductor, called a loading coil, is connected in series with the antenna coil. The loading coil brings the antenna back into resonance at the desired frequency.
2. Why is top hat added to antennas?
a) To increase capacitance
b) Increasing the antenna gain
c) Sideband suppression
d) Increasing the range of the antenna
Answer: a
Clarification: A top hat is a structure that increases the capacitance to surrounding items, bringing the antenna back into resonance. Obviously, such an arrangement is too top-heavy and inconvenient for portable and mobile antennas. However, it is sometimes used in larger fixed antennas at lower frequencies.
3. In a parasitic array antenna, the conductors that are not connected to the transmission line is called as?
a) Driven element
b) Parasitic elements
c) Extra elements
d) Array elements
Answer: b
Clarification: A parasitic array consists of a basic antenna connected to a transmission line plus one or more additional conductors that are not connected to the transmission line. These extra conductors are referred to as parasitic elements, and the antenna itself is referred to as the driven element.
4. In a parasitic array the elements are shorted if they are connected to a conducting beam.
a) True
b) False
Answer: b
Clarification: The boom does not have to be an insulator. Because there is a voltage null at the center of a one-half wavelength conductor at the resonant frequency, there is no potential difference between the elements and so they can all be connected to a conducting boom with no undesirable effect. In other words, the elements are not “shorted together.”
5. Parasitic element that is typically about 5 percent longer than the half-wave dipole-driven element is called _______
a) Array element
b) Director element
c) Reflector element
d) Driven element
Answer: c
Clarification: The reflector, a parasitic element that is typically about 5 percent longer than the half-wave dipole-driven element, is spaced from the driven element by a distance of 0.15λ to 0.25λ. When the signal radiated from the dipole reaches the reflector, it induces a voltage into the reflector and the reflector produces some radiation of its own creating a highly focused beam in the direction of the driven element.
6. Why is the boom of the yagi antenna connected to a metal mast and electrical ground?
a) Better signal directivity
b) Increased bandwidth
c) Lightning protection
d) To avoid short circuiting
Answer: c
Clarification: The centers of the parasitic elements are neutral electrically; these elements can be connected directly to the boom. For the best lightning protection, the boom can then be connected to a metal mast and electrical ground.
7. Which of the following is not one of the types of driven array antenna?
a) Rectilinear antenna
b) Broadside antenna
c) End fire antenna
d) log periodic antenna
Answer:a
Clarification: Each element in a driven array antenna receives RF energy from the transmission line, and different arrangements of the elements produce different degrees of directivity and gain. The three basic types of driven arrays are the collinear, the broadside, and the end-i re. A fourth type is the wide-bandwidth log-periodic antenna.
8. What is the length of the shortest element in the yagi antenna?
a) One quarter the wavelength of the highest frequency
b) One quarter the wavelength of the lowest frequency
c) One half the wavelength of the highest frequency
d) One half the wavelength of the lowest frequency
Answer: c
Clarification: The lengths of the driven elements vary from long to short and are related logarithmically. The longest element has a length of one-half wavelength at the lowest frequency to be covered, and the shortest element is one-half wavelength at the higher frequency. The spacing is also variable.
To practice MCQs on all areas of Avionics,