Correct answer: b) Reflection from the ionosphere
Sky wave propagation occurs when radio waves are reflected (actually refracted) back to Earth by ionized layers in the ionosphere. This allows long-distance communication beyond the horizon.
Correct answer: d) F2 layer
The F2 layer is the highest and most ionized layer during daytime, making it most important for long-distance HF communication. It persists day and night (though weaker at night) and can refract higher frequency signals than other layers.
Correct answer: a) 3-30 MHz (HF band)
The HF (High Frequency) band is most effective for sky wave propagation. Lower frequencies may be absorbed by the D layer, while higher frequencies tend to pass through the ionosphere into space.
Correct answer: a) The highest frequency that returns to Earth after refraction
The critical frequency is the highest frequency at which a radio wave, when sent vertically upwards, will be reflected back to Earth. Frequencies above this will pass through the ionosphere.
Correct answer: a) The highest frequency that allows communication between two specific points
The MUF is the highest frequency that can be used for transmission between two points via sky wave propagation. It's always higher than the critical frequency because oblique incidence requires less refraction to return to Earth.
Correct answer: d) All of the above
Fading in sky wave communication can result from several factors: interference between sky wave and ground wave components, Faraday rotation (polarization changes) in the ionosphere, and multipath interference where waves arrive via different paths with different delays.
Correct answer: a) The minimum distance at which the sky wave first returns to Earth
Skip distance is the minimum distance from the transmitter where the sky wave returns to Earth. Closer to the transmitter, only ground wave propagation is possible, creating a "skip zone" where no signal is received.
Correct answer: a) Increased solar activity generally improves HF propagation
Increased solar activity (more sunspots) leads to greater ionization of the ionosphere, which generally improves HF propagation by allowing higher frequencies to be reflected. However, extreme solar activity can also cause disruptions.
Correct answer: a) The D layer disappears, reducing absorption
At night, the D layer (which absorbs HF signals) disappears due to lack of solar radiation, allowing signals to reach higher layers. However, the remaining layers become less dense and predictable, making propagation less reliable despite longer distances being possible.
Correct answer: b) Short-range regional communication (typically 0-500 km)
NVIS uses high-angle radiation (nearly straight up) that reflects off the ionosphere to cover a regional area, filling in the "skip zone" that normally exists in sky wave propagation. It's particularly useful in mountainous terrain or emergency communications.