Chapter 2 | Auto Pilot Flashcards
What is necessary to understand the reasons for certain circuits in radio equipment?
Knowledge of the propagation of radio waves
Understanding radio wave propagation helps explain frequency preferences and performance limitations.
What surrounds a conductor when a current flows through it?
Lines of magnetic force producing a magnetic field
The strength of the magnetic field is proportional to the amount of current.
What is established between the plates of a capacitor when a voltage source is connected?
An electrostatic, or electric, field
This electric field is perpendicular to the plates of the capacitor.
What type of wave is generated when both electric and magnetic fields are present?
Transverse Electro Magnetic wave (TEM wave)
The TEM wave has electric and magnetic fields that are perpendicular to each other.
What is the impedance of free space?
Z = 377 ohms
This value is derived from the parallel plate concept.
What is the velocity of a TEM wave in free space?
Approximately 300 x 10^6 meters/second
This is also the velocity of light in free space.
What happens to the radiation pattern of an antenna when it is used near the earth’s surface?
It assumes a toroidal or doughnut shape
This is due to the interaction with the earth’s surface.
What are the three main groups of waves radiated by an antenna?
- Space Wave
- Surface Wave
- Tropospheric or Sky Wave
What is the effect of diffraction on electromagnetic waves?
Waves follow the edge of the material causing the diffraction
This is most pronounced at low frequencies.
What are the two main causes of wave attenuation?
- Spatial attenuation
- Absorption attenuation
What is the relationship between the conductivity of the surface and wave attenuation?
Attenuation is inversely proportional to the conductivity of the surface
Higher conductivity (like water) results in lower attenuation.
What is the plane of polarisation defined as?
The plane of the E field
A vertically polarised wave is radiated from a vertical dipole antenna.
What is the ionosphere and where does it extend?
A region of ionised gas extending from approximately 30 miles to at least 250 miles above the earth’s surface
Ionisation occurs due to solar radiation.
What happens to a radio wave when it enters an ionised layer?
It is refracted towards the earth
The angle of refraction depends on the angle of incidence and the frequency of the wave.
What is the critical angle in relation to radio wave propagation?
The angle of incidence below which a ray will not be reflected but escape through the ionised layer
Rays that exceed this angle will escape instead of reflecting back.
Fill in the blank: The power radiated by an isotropic radiator is enclosed within a _______.
sphere
True or False: The surface wave is most effective at high frequencies.
False
Diffraction is most pronounced at low frequencies.
What is the CRITICAL ANGLE in ionospheric propagation?
The angle below which the ray will not be reflected but will escape through the layer.
What is the CRITICAL FREQUENCY?
The maximum frequency at which reflection can take place at vertical incidence.
Define MAXIMUM USABLE FREQUENCY (MUF).
The maximum frequency which will be reflected at a given angle of incidence.
What is the OPTIMUM WORKING FREQUENCY (OWF)?
About 85% of the MUF, chosen to ensure reliable communication.
What is SKIP DISTANCE?
The shortest distance at which a frequency will return to earth for a given layer height.
True or False: The skip distance increases as frequency increases.
True.
What happens if the frequency is too low?
Losses in the ionosphere increase, leading to high attenuation.
