General G3C: Ionospheric regions, scatter and NVIS
These are the 11 official questions in group G3C of the 2023–2027 General question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- G3C01Question 1
Which ionospheric region is closest to the surface of Earth?
- AThe D region Correct
- BThe E region
- CThe F1 region
- DThe F2 region
Why: The D region is the lowest ionospheric region.
- G3C02Question 2
What is meant by the term “critical frequency” at a given incidence angle?
- AThe highest frequency which is refracted back to Earth Correct
- BThe lowest frequency which is refracted back to Earth
- CThe frequency at which the signal-to-noise ratio approaches unity
- DThe frequency at which the signal-to-noise ratio is 6 dB
Why: The critical frequency is the highest frequency refracted back to Earth at that angle.
- G3C03Question 3
Why is skip propagation via the F2 region longer than that via the other ionospheric regions?
- ABecause it is the densest
- BBecause of the Doppler effect
- CBecause it is the highest Correct
- DBecause of temperature inversions
Why: The F2 region is the highest, so its skip covers the longest distance.
- G3C04Question 4
What does the term “critical angle” mean, as applied to radio wave propagation?
- AThe long path azimuth of a distant station
- BThe short path azimuth of a distant station
- CThe lowest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
- DThe highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions Correct
Why: The critical angle is the highest takeoff angle that still returns to Earth.
- G3C05Question 5
Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?
- AThe F region absorbs signals at these frequencies during daylight hours
- BThe F region is unstable during daylight hours
- CThe D region absorbs signals at these frequencies during daylight hours Correct
- DThe E region is unstable during daylight hours
Why: The D region absorbs these lower frequencies during daylight.
- G3C06Question 6
What is a characteristic of HF scatter?
- APhone signals have high intelligibility
- BSignals have a fluttering sound Correct
- CThere are very large, sudden swings in signal strength
- DScatter propagation occurs only at night
Why: HF scatter signals have a fluttering sound.
- G3C07Question 7
What makes HF scatter signals often sound distorted?
- AThe ionospheric region involved is unstable
- BGround waves are absorbing much of the signal
- CThe E region is not present
- DEnergy is scattered into the skip zone through several different paths Correct
Why: Energy scattered into the skip zone arrives by several paths, so it sounds distorted.
- G3C08Question 8
Why are HF scatter signals in the skip zone usually weak?
- AOnly a small part of the signal energy is scattered into the skip zone Correct
- BSignals are scattered from the magnetosphere, which is not a good reflector
- CPropagation is via ground waves, which absorb most of the signal energy
- DPropagation is via ducts in the F region, which absorb most of the energy
Why: Only a small part of the signal is scattered into the skip zone, so it's weak.
- G3C09Question 9
What type of propagation allows signals to be heard in the transmitting station’s skip zone?
- AFaraday rotation
- BScatter Correct
- CChordal hop
- DShort-path
Why: Scatter propagation lets you hear stations in your own skip zone.
- G3C10Question 10
What is near vertical incidence skywave (NVIS) propagation?
- APropagation near the MUF
- BShort distance MF or HF propagation at high elevation angles Correct
- CLong path HF propagation at sunrise and sunset
- DDouble hop propagation near the LUF
Why: NVIS is short-distance MF or HF propagation using high elevation angles.
- G3C11Question 11
Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?
- AThe F2 region
- BThe F1 region
- CThe E region
- DThe D region Correct
Why: The D region absorbs signals below 10 MHz most during daylight.