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G3C02

RADIO WAVE PROPAGATION · - Ionospheric regions; critical angle and frequency; HF scatter; near vertical incidence skywave (NVIS)

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What is meant by the term "critical frequency" at a given incidence angle?

  1. AThe highest frequency which is refracted back to Earth
  2. BThe lowest frequency which is refracted back to Earth
  3. CThe frequency at which the signal-to-noise ratio approaches unity
  4. DThe frequency at which the signal-to-noise ratio is 6 dB
Why The ionosphere bends (refracts) signals back toward Earth only up to a limit set by its electron density and the angle at which the wave enters. For a given incidence angle, the highest frequency that still gets bent back instead of passing into space is the critical frequency; above it the signal punches through the layer and is lost. Measured straight up (vertical incidence) this is foF2, and the usable frequency rises as the angle gets lower, which is why long, low-angle paths support higher bands than short ones.
Watch out The choice about the lowest frequency refracted back confuses this with the LUF, and the LUF is set by D-layer absorption, not by failure to refract. Lower frequencies are refracted more easily, not less.
Critical = ceiling. Go above it and your signal goes through the ionosphere, not back down.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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The rest of G3C

  1. G3C01 Which ionospheric region is closest to the surface of Earth?
  2. G3C02 What is meant by the term "critical frequency" at a given incidence angle?
  3. G3C03 Why is skip propagation via the F2 region longer than that via the other ionospheric regions?
  4. G3C04 What does the term "critical angle" mean, as applied to radio wave propagation?
  5. G3C05 Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?
  6. G3C06 What is a characteristic of HF scatter?
  7. G3C07 What makes HF scatter signals often sound distorted?
  8. G3C08 Why are HF scatter signals in the skip zone usually weak?
  9. G3C09 What type of propagation allows signals to be heard in the transmitting station's skip zone?
  10. G3C10 What is near vertical incidence skywave (NVIS) propagation?
  11. G3C11 Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?
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