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A layer has a critical frequency of 6 MHz. What is the maximum usable frequency at an incidence whose secant is 3?

  • A.18 MHz
  • B.2 MHz
  • C.9 MHz
  • D.6 MHz, since the critical frequency is a fixed limit

Answer: A

Applying the secant law directly:

\[ f_{MUF} = 6 \times 3 = 18 \]

megahertz, that is 18 MHz. A wave at that frequency would escape if sent vertically but is returned at the shallow angle of a long path. This is why the longest hops support the highest frequencies, and why a station may hear a distant contact on a band while nearer stations are inaudible on it.

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