Antenna Basics: Radiation, Directivity, and Gain Quiz Quiz

Explore the essential concepts of antenna radiation patterns, directivity, and gain with this focused quiz. Ideal for those interested in electromagnetic theory and antenna performance, these questions help reinforce key fundamentals of wireless communication.

1. Radiation Pattern Understanding

   Which of the following best describes an antenna’s radiation pattern in free space, such as that of a typical dipole antenna?

   1. It spreads equally in all directions as a perfect sphere.
   2. It radiates only straight upward above the antenna.
   3. It forms a doughnut-shaped pattern around the antenna axis.
   4. It projects a single, narrow beam in one fixed direction.

   Explanation: A dipole antenna in free space typically produces a doughnut-shaped radiation pattern, with maximum radiation perpendicular to its length. The 'perfect sphere' description is for an ideal isotropic radiator, which does not exist in practice. A 'single, narrow beam' describes highly directional antennas, not standard dipoles. 'Straight upward' radiation does not accurately represent standard antenna behavior, as dipoles radiate least along their axis.

2. Directivity Concept

   If Antenna A concentrates energy into a narrow beam while Antenna B radiates energy almost equally in all directions, which statement about their directivities is correct?

   1. Both antennas have the same directivity.
   2. Antenna B has higher directivity than Antenna A.
   3. Directivity cannot be compared without knowing their gains.
   4. Antenna A has higher directivity than Antenna B.

   Explanation: Directivity measures how focused an antenna's radiation is in a particular direction compared to an ideal omnidirectional source. Since Antenna A concentrates energy into a narrow beam, its directivity is higher. Antenna B's nearly equal distribution means lower directivity. Directivity can be compared independently of gain, which also considers efficiency. Having the same directivity is incorrect due to their differing patterns.

3. Interpreting Antenna Gain

   If an antenna is specified to have a gain of 6 dBi, what does this tell you about its performance?

   1. It is exactly twice as efficient as a half-wave dipole antenna.
   2. It adds 6 dB of electrical power to the transmitted signal.
   3. It radiates 6 dB more efficiently than an isotropic radiator in a specific direction.
   4. It performs equally well in every direction around itself.

   Explanation: A gain of 6 dBi means the antenna radiates 6 dB more power in its maximum direction compared to an isotropic (spherical) radiator. Gain does not add power to the signal, but concentrates it directionally. Being 'twice as efficient as a dipole' is misleading, as dBi compares to isotropic, not a dipole. Equal performance in all directions describes an isotropic radiator, not one with gain.

4. Relation of Efficiency and Gain

   Which scenario shows how antenna gain can be less than its directivity?

   1. When the antenna has significant energy loss due to poor efficiency.
   2. When it is used indoors instead of outdoors.
   3. When the operating frequency is slightly mistuned.
   4. When the antenna is physically larger than a wavelength.

   Explanation: Antenna gain equals directivity multiplied by efficiency; so if efficiency is less than 100%, gain will be less than directivity. Indoor versus outdoor use does not change inherent gain. Slight frequency mistuning may affect performance, but primarily impacts matching rather than efficiency. Antenna size relative to wavelength affects pattern, not directly the gain-to-directivity relationship.

5. Beamwidth and Directivity Relationship

   How does reducing the half-power beamwidth of an antenna typically affect its directivity?

   1. It causes the antenna to radiate equally in all directions.
   2. It decreases the antenna's directivity.
   3. It keeps the directivity constant regardless of beamwidth.
   4. It increases the antenna's directivity.

   Explanation: A narrower half-power beamwidth means the antenna concentrates its radiation more, which increases directivity. Decreasing directivity would occur with a wider beamwidth, not a narrower one. Directivity is not constant for different beamwidths, and equal radiation in all directions characterizes an isotropic radiator, not one with a narrow beam.