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Radio Wave Propagation. Ionosphere Layers. Radio Waves are Bent by the Ionosphere. F1 and F2 Ionosphere Layers Combine at Night. Critical Angle. Ground Wave, Sky Wave and Skip Zone. Great Circle Route. MUF is the highest frequency that allows a radio wave to reach the desired destination.

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MUF is the highest frequency that allows a radio wave to reach the desired destination.

Stay below the Maximum Usable Frequency for the best chance for a successful contact.

MUF is affected by all of these:

Path, distance and locations

Time of day and seasons


MUF: Maximum Usable Frequency

LUF: The lower limit to the range of frequencies that provide useful communications between two stations.

All signals below LUF are absorbed.

When LUF exceeds MUF – No HF radio frequency will support communications along an ionosphere signal path.

LUF: Lowest Usable Frequency

11 Year Cycle – Variation in the sunspot number.

We are at the bottom of Cycle 23.

Higher bands like 15 meters and 10 meters are not very good.

Sunspot Number: Daily index of sunspot activity.


Solar Activity

Higher Sunspot Numbers = Better HF Propagation

Above 100 is good

Solar Flux
  • SFI (the Solar Flux Index) is radio energy emitted by the Sun.
  • The higher the SFI, the better the propagation.
Three Numbers:

SFI= Solar Flux Index .

“A” Index = A daily measurement of the state of activity of the Earth’s magnetic field. Based on yesterday’s K Index.

“K” Index = Readings of the Earth’s geomagnetic field updated every 3 hours.

Solar Flux

sudden ionosphere disturbance
Caused by a solar flare – ionizes the D Layer.

Disrupts lower HF frequencies first.

Most sensitive above 45° latitude.

So, try a higher frequency.

It takes about 8 minutes for UV and X radiation (traveling at speed of light) to reach Earth.

Sudden Ionosphere Disturbance
geomagnetic disturbance
A dramatic change in the Earth’s magnetic field that occurs over a short time.

Sometimes caused by a coronal mass ejection. These particles can take 20 to 40 hours to reach Earth.

Geomagnetic Disturbance
geomagnetic disturbance1
Degrades high latitude HF propagation, particularly those latitudes greater than 45 degrees.

Sometimes creates a visible aurora.

Coronal holes also disrupt HF communication.

Geomagnetic Disturbance
Scatter – Forward Scatter and Back Scatter
  • Scatter signals fall between ground wave and skip zone.
  • HF Scatter signals have a wavering sound.
  • Occur most often above the MUF.
  • (Also used on 6 meters with meteors!)