30 Questions
What is the measure of an antenna's directivity?
Beam width
How is the beam width of an antenna measured?
Between the points on the radiation curve that are 3 dB down from the maximum amplitude
How is antenna gain usually expressed?
In decibels
What is the impedance of a dipole antenna at its center?
73 Ω
What is the purpose of the insulators in a wire dipole antenna?
To support the wire elements
What is the purpose of the transmission line in a dipole antenna?
To provide power to the antenna
What happens when a radio wave encounters a perfect conductor?
Total reflection occurs
Which type of surfaces can cause reflection of radio waves?
Metallic and partially conductive surfaces
What happens to a radio wave when it encounters a good conductor?
Most of the wave is reflected
What causes refraction of a radio wave?
Changes in air density
What is the effect of a poor conductor on a radio wave?
Absorbs some of the wave energy
How does the degree of bending of a radio wave during refraction relate to the medium's index of refraction?
Directly proportional
What is the reason a radio signal is called an electromagnetic wave?
It consists of both electric and magnetic fields.
In a wire antenna, what happens when voltage is applied?
Electric field is set up and current flows, producing a magnetic field.
How are the electric and magnetic fields emitted from the antenna?
At right angles to each other.
What do transmission lines, unlike antennas, not efficiently do?
Radiate radio signals.
What happens to the direction of the electric field between wires in a transmission line during one-half cycle of the ac input?
The direction reverses once per cycle.
How does the strength of a magnetic field produced by a wire antenna relate to the strength of an electric field produced?
They are both expressions of different field strengths.
What is the index of refraction for air?
Very close to 1
What happens when an incident wave from a transmitter travels through air and meets a region of ionized air?
The speed of propagation slows down, and the wave is bent (refracted).
What is Snell's law related to?
The relationship between the angles and the indices of refraction when a wave is refracted.
What is a shadow zone?
A region where some of the signal is blocked by an obstacle between the transmitter and receiver.
What is the main signal path for radio signals in the 30-kHz to 3-MHz range?
Ground waves
What type of polarization is required for ground waves to be propagated from an antenna?
Vertical polarization
Which of the following layers of the ionosphere are weakly ionized and exist only during daylight hours?
D and E layers
What is the primary effect of the F layer on radio signals?
Causing refraction of radio signals when they cross boundaries with different ionization levels
What happens when a radio signal goes into the ionosphere?
It is gradually bent due to different levels of ionization
Which statement is true about radiation directly vertical from the antenna?
It passes through the ionosphere
What happens to radio signals at very high frequencies above about 50 MHz?
Refraction seldom occurs regardless of the angle
What is the result of reflected radio waves being sent back to Earth with minimum signal loss?
The signal is propagated over an extremely long distance
Learn about the basics of antennas in radio and TV engineering, including how radio signals are electromagnetic waves made up of electric and magnetic fields. Understand how the application of voltage to an antenna creates electric and magnetic fields emitted from the antenna.
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