GROL - General Radiotelephone Operator License Radio Wave Propagation Questions and Answers 1 — Questions and Answers
Question 1: A ship is attempting to communicate with a shore station 40 nautical miles away using a VHF marine radio in the 156-162 MHz range. The signal is blocked by a large coastal mountain range between them. What is the primary propagation path for these signals, and why is communication failing?
- Skywave, because the frequency is too high for ionospheric refraction.
- Groundwave, because the signal's energy is being absorbed by the terrain.
- Space wave (line-of-sight), because VHF signals are blocked by significant physical obstructions. (Correct answer)
- Tropospheric scatter, because the required atmospheric conditions are not present.
Correct answer: Space wave (line-of-sight), because VHF signals are blocked by significant physical obstructions.
VHF and UHF radio waves travel primarily via space wave, which is commonly known as line-of-sight (LOS) propagation. [1, 14, 21] These signals travel in a relatively straight line and are blocked by large physical objects like mountains, buildings, or the curvature of the Earth. [1, 11] Groundwave is effective at lower frequencies (MF/LF), and skywave is used for long-distance HF communication.
Question 2: In HF skywave propagation, what is the term for the region between the farthest reach of the groundwave and the point where the first refracted skywave returns to Earth, where the signal cannot be received?
- The Fresnel zone
- The skip zone (Correct answer)
- The absorption region
- The multipath area
Correct answer: The skip zone
The skip zone, sometimes called the silent zone or dead zone, is the area where a signal is too weak to be received via groundwave and too close to be received via skywave because the skywave overshoots the location. [20, 24, 25] It is the area between the end of groundwave coverage and the first return of the skywave. [19, 24]
Question 3: Which layer of the ionosphere is the highest in altitude, the most heavily ionized, and is primarily responsible for the long-distance propagation of HF radio signals, especially during transcontinental communication?
- The D layer
- The E layer
- The F2 layer (Correct answer)
- The troposphere
Correct answer: The F2 layer
The F layer is the most important for long-haul HF communications. During the day, it splits into the F1 and F2 layers. The F2 layer is the highest and most ionized region, making it the primary refractor of HF signals for long-distance (DX) communication. [18, 22] The D layer primarily absorbs HF energy, and the E layer refracts signals over shorter distances. The troposphere is the lowest part of the atmosphere and is not part of the ionosphere.
Question 4: A radio signal is received via two or more distinct paths, causing the signals to interfere with each other at the receiver. This results in rapid fluctuations in signal strength, sometimes causing deep fades. What is this phenomenon called?
- Multipath fading (Correct answer)
- Doppler shift
- Faraday rotation
- Atmospheric absorption
Correct answer: Multipath fading
Multipath fading occurs when a radio signal reaches the receiving antenna by two or more paths. [5, 6, 10] These multiple signals can arrive out of phase, causing destructive interference that leads to a significant drop in signal strength, known as a fade. [10] This is a common issue in both HF skywave and mobile VHF/UHF communications.
Question 5: Under certain weather conditions, such as a temperature inversion where a layer of warm air sits above cooler air, VHF and UHF signals can become trapped and travel far beyond their normal line-of-sight horizon. What is this propagation mode called?
- Ionospheric skip
- Sporadic E
- Groundwave propagation
- Tropospheric ducting (Correct answer)
Correct answer: Tropospheric ducting
Tropospheric ducting occurs when a temperature inversion creates a boundary between air masses of different temperatures and humidities in the troposphere. [2, 4] This boundary can act as a duct, guiding VHF and UHF radio waves for hundreds or even thousands of miles, far beyond the normal radio horizon. [3, 4, 8]
Question 6: Which of the following frequency bands is most reliant on groundwave propagation for effective communication over seawater for distances up to a few hundred miles?
- UHF (300-3000 MHz)
- VHF (30-300 MHz)
- MF (300-3000 kHz) (Correct answer)
- SHF (3-30 GHz)
Correct answer: MF (300-3000 kHz)
Groundwave propagation is most effective at lower frequencies. [29] The Medium Frequency (MF) band is ideal for this mode, especially over conductive surfaces like saltwater. [22, 29] This allows for reliable communication over the horizon. VHF and UHF signals are quickly attenuated by the ground and primarily use line-of-sight propagation.
GROL - General Radiotelephone Operator License Radio Wave Propagation Questions and Answers 1
A ship is attempting to communicate with a shore station 40 nautical miles away using a VHF marine radio in the 156-162 MHz range.
The signal is blocked by a large coastal mountain range between them.
What is the primary propagation path for these signals, and why is communication failing?