What does the maximum usable frequency (MUF) describe?
The highest frequency that works reliably over a given path.
- The lowest frequency that avoids D layer absorption
- The highest frequency that works reliably over a path ✔
- The frequency where an antenna is resonant
Why: NOAA's glossary defines MUF as the highest frequency that allows reliable HF communication over a given ground range by ionospheric refraction. Go above it and your signal passes through into space.
Huh, didn't know that: When the MUF for a path drops after dark, switching to a lower HF band keeps the contact going.
What is the sun's favourite ride at the theme park? The solar coasters.
References
Answer: The highest frequency that works reliably over a path. NOAA's glossary defines MUF as the highest frequency that allows reliable HF communication over a given ground range by ionospheric refraction. Go above it and your signal passes through into space.
- NOAA SWPC Space Weather Glossary www.spaceweather.gov/content/space-weather-glossary
The highest frequency that allows reliable HF radio communication
Checked 2026-10-11. - NCVEC 2024-2028 Amateur Extra Class Question Pool www.ncvec.org/downloads/2024-2028%20Extra%20Class%20Question%20Pool%20and%20Syllabus%20Public%20Release%20with%204th%20Errata%20Feb%204%202026.pdf
B. Switch to a lower frequency HF band
Checked 2026-10-11. - NOAA SWPC Space Weather Glossary www.spaceweather.gov/content/space-weather-glossary
that frequency capable of penetration just to the layer of maximum ionization
Checked 2026-10-11.
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