Maximum Usable Frequency (MUF) | Amateur Radio Tools

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MUF numbers in the chart are in MHz.

Source: prop.kc2g.com – click for full-screen image = which is made possible by funding from WWROF and data from ionosonde operators around the world, distributed through GIRO.

What is the MUF?

The Maximum Usable Frequency (MUF) is an important concept for amateur radio operators, particularly those of us engaging in long-distance communication (also known as “DXing”).

MUF represents the highest possible frequency (HPF) at which a radio wave can be transmitted between two points on Earth and still be refracted back down to the surface by the ionosphere, at a specified time and independent of transmitter power.

The ionosphere mostly passes frequencies above the MUF and loses them to space, making contact difficult or impossible.

Because the ionosphere’s properties are constantly changing due to solar activity, time of day, and season, the MUF is not a fixed value but a dynamic prediction.

For an amateur radio operator, understanding the MUF is a useful skill when selecting the best frequency band for a given contact. By checking a real-time MUF map or using propagation prediction software, an operator can determine which bands are “open” to a specific geographic area.

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If the MUF at your location is predicted to be 35 MHz, and the destination point in another country is predicted to be 18 MHz, you can surmise that the bands 10m (28 MHz), 12m (24 MHz) and 15m (21 MHz) will likely not work for a long-distance contact due to the frequencies of these bands being higher than the lowest MUF frequency of the two points: 35 MHz and 18 MHz.
With this example MUF being predicted to be 18 MHz, the 17m band (18 MHz) or 20m band (14 MHz) would be the optimal choice (i.e. the band closest to the MUF without being higher – or not much higher – than the MUF). Every band below the MUF (aka 20m, 30m, 40m, 60m, 80m and 160m) could be an option too, as all of their frequencies are below the MUF.

Knowledge of the MUF at the transmission and destination points saves time and enables operators to maximise their chances of making successful DX contacts at receiving stations in other countries.

UK Band Plan Frequencies

Referencing the MUF chart, locate the numbers in both your QTH and your destination; then, using the lower of the two numbers, find a match within the Lower and Upper limit MHz columns. This will be the ideal band for your DX transmission…

Band (m)	Lower limit (MHz)	Upper limit (MHz)
160m	1.810	2.000
80m	3.500	3.800
60m	5.2585	5.4065
40m	7.000	7.200
30m	10.100	10.150
20m	14.000	14.350
17m	18.068	18.168
15m	21.000	21.450
12m	24.890	24.990
10m	28.000	29.700

Planning a Path

Knowledge of MUF helps operators choose the ideal operating frequency for a particular path.

Generally, the optimal operating frequency (FOT) is about 80-90% of the MUF.

Example

A MUF of 25 would suggest the 15m band to be a good choice:

25 * 0.85 = 21.25MHz == 15m band.

Longer Skip(s)

By operating near the MUF, a radio signal can travel through the ionosphere at a shallower angle, which results in a longer “skip” or hop distance.

This knowledge is useful for planning single-hop contacts across long distances, as well as reducing the number of hops required for multi-hop pathways (which reduces signal loss due to absorption in the lower D-layer of the ionosphere and reflections off the Earth’s surface).