G3B

MUF, LUF and propagation paths

Explain the usable frequency window between LUF and MUF, how far one hop reaches, and short versus long path.

12 pool questions1 on the exam

Syllabus: Maximum Usable Frequency; Lowest Usable Frequency; short path and long path propagation; determining propagation conditions; ionospheric refraction

Between the LUF and the MUF, the ionosphere returns your signal

MUF = Maximum Usable Frequency, LUF = Lowest Usable Frequency, each between two specific points. Above the MUF the wave passes through. Below the LUF it is absorbed before it arrives. Absorption keeps falling as frequency rises, so the quietest choice is just below the MUF.

Absorption falls as frequency rises. The LUF is where it becomes tolerable; the MUF is the ceiling.

Operating frequency14 MHzMUF (sunlight, season, path)21 MHzLUF (D-region absorption)7 MHz

Usable window7 to 21 MHz

RememberLUF above MUF means no ordinary skywave path. Aim just under the MUF.

Everything about the Sun, the clock and the path moves the MUF

Factor	Why it changes the MUF
Time of day, season	Sunlight sets how ionized the layers are
Solar radiation	More UV and X-rays, denser layers
Ionospheric disturbances	Storms and flares reshape the layers
Path distance, location	The MUF is for a specific path

RememberAll of them matter. The MUF belongs to one path at one moment.

The higher the layer, the longer the hop

Schematic, same takeoff angle: the higher the layer, the longer the hop. Layer heights not to scale.

A single F2 hop covers up to about 2,500 miles. A single E-region hop reaches about 1,200 miles. Longer paths use several hops.

RememberF2: 2,500 miles. E: 1,200 miles.

Short path and long path arrive together: you hear an echo

The signal can go either way around the globe. When both arrive, the long path lands a little later, so you hear a slightly delayed echo.

Top view of the Earth. Both paths carry the same signal, so you hear it twice.

Station position around the globe100° from you

Echo delay59 ms

RememberShort plus long path equals a delayed echo.

Let receivers tell you; summer lows are noisy, not dead

Method	Does it show your path now?
Automated receiving stations on the internet showing where you are heard	Yes, it is a direct measurement
A-index	No: it describes Earth's magnetic field
Dots and listening for echoes	No

On the lower HF bands in summer, expect high atmospheric noise (static).

RememberCheck who actually hears you. Summer lows mean static.

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