This question has Statement-1 and Statement-2. Of the four choices given after the statements, choose the one that best describes the two statements.
Statement-1 : Sky wave signals are used for long distance radio communication. These signals are in general, less stable than ground wave signals.
Statement -2 : The state of ionosphere varies from hour to hour, day to day and season to season.
Sky waves are radio waves reflected by the ionosphere for long-distance communication. Their stability is indeed lower than ground waves, which follow the Earth's surface and are more stable. Therefore, Statement-1 is true.
The ionosphere's state changes due to solar radiation, varying hourly, daily, and seasonally, which affects sky wave propagation stability. This variation directly explains why sky waves are less stable, making Statement-2 true and the correct explanation for Statement-1.
Final Answer: Statement-1 is true, Statement-2 is true and Statement-2 is the correct explanation of Statement-1.
This question involves analyzing two statements about radio wave propagation and determining their truth values and relationship.
Sky Waves: These are radio waves (typically in the HF band, 3-30 MHz) that are reflected back to Earth by the ionosphere. This allows them to travel beyond the horizon, making them suitable for long-distance communication.
Ground Waves: These are radio waves (typically in the MF and lower frequencies) that travel along the Earth's surface. Their range is limited to the visual horizon (or slightly beyond) but they provide very stable and reliable communication within that range because they are not affected by atmospheric changes.
The ionosphere is a layer of the Earth's atmosphere that is ionized by solar radiation. Its density, composition, and ionization level are not constant. They change:
These variations cause fluctuations in the ionosphere's ability to reflect sky waves. This makes sky wave signals less stable compared to ground wave signals, which propagate along the stable and predictable surface of the Earth.
Statement-1: "Sky wave signals are used for long distance radio communication. These signals are in general, less stable than ground wave signals."
This statement is true. As explained, sky waves are used for long distances, and their stability is indeed lower due to their dependence on the variable ionosphere.
Statement-2: "The state of ionosphere varies from hour to hour, day to day and season to season."
This statement is also true. The ionosphere is highly dynamic.
Relationship: Statement-2 provides the direct reason why sky waves are less stable (Statement-1). The variability of the ionosphere (Statement-2) is the correct explanation for the instability of sky wave signals (Statement-1).
Therefore, the correct choice is: Statement-1 is true, Statement-2 is true and Statement-2 is the correct explanation of Statement-1.
Key Topic: Propagation of Electromagnetic Waves (1192)
Theory: The maximum usable frequency (MUF) for skywave communication is given by: where is the critical frequency and is the angle of incidence. This frequency changes with the state of the ionosphere.