G3C02 - What is meant by the term “critical frequency” at a given incidence angle?

General Questions
1

Question

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

Answer Options

  • A) The highest frequency which is refracted back to Earth
  • B) The lowest frequency which is refracted back to Earth
  • C) The frequency at which the signal-to-noise ratio approaches unity
  • D) The frequency at which the signal-to-noise ratio is 6 dB

Correct Answer: A

Explanation

The term critical frequency, in the context of radio propagation, refers to the highest frequency at which a radio wave, transmitted straight up (vertical incidence), will be refracted back to Earth by an ionospheric layer. If a signal’s frequency exceeds this critical frequency, it will pass through the ionosphere into space.

For signals transmitted at shallow angles (oblique incidence), the concept is generalized to the Maximum Usable Frequency (MUF). The MUF is always higher than the critical frequency (by a factor known as the Secant Law) because a shallow ray angle spends more time in the ionosphere and is easier to refract.

This topic was automatically created to facilitate community discussion about this exam question. Feel free to share study tips, memory tricks, or additional explanations!

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I don’t understand why the question includes the qualifying phrase “at a given incidence angle.” Isn’t the incidence angle always vertical? If the angle is anything other than vertical (90 degrees), wouldn’t we be talking about MUF? Seems like the question should ask: “What is meant by the term ‘critical frequency’ at a vertical incidence angle”?

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As far as I understand it, you are right. I get the impression that the NCVEC would actually appreciate more (thoughtful, constructive, like this) feedback on the question pool. They noted when the new Technician pool was released that they often get feedback too late in the edit cycle to act on it. So maybe now is an excellent time to provide feedback for the General pool?

Here’s course material as a reference that supports our shared understanding:

If there is another definition that is being assumed by the test pool authors, I’m not aware of it and would like to learn. :slightly_smiling_face:

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I sent them an email and can update here if they reply.

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@mcdanlj Indeed, they responded positively:

You contacted the NCVEC at just the right time regarding this G3C02, as the Question Pool Committee is working on the General Pool revisions now.

I see your point, and we are changing the question to make it clear that the term “critical frequency” refers to waves with vertical incidence to the ionosphere. I imagine that the reason for the additional phrase regarding the angle was to recognize that the highest frequency reflected DOES depend on the angle of incidence. But, as you point out, the specific term “critical frequency” is universally applied to the vertical incidence condition.

BTW, that is a nice reference you included. I found that very interesting reading.

I gave you credit for the suggestion and reference and pointed them to the forum, since I thought it would be of interest.

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You definitely get credit since I just nodded past the redundancy as a gloss and didn’t stop to think how it could instead be confusing. This is the community impact I hoped this forum would help foster!

And this is a wonderful opportunity to see that the NCVEC members are actually a thoughtful and responsive part of our ham community, not some faceless bureaucracy. :heart: