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Fig 1-2—A Solar Terrestrial Report shows the day-by-day evolution of the Solar Flux, the Sunspot Number and
the Planetary A Index. (Courtesy of Jan Alvestad.)

Cycle 23 (just over 100). Old timers will remember the
spectacular 10-meter signals we enjoyed in 1957-1958 at the
peak of Cycle 19.
2.1.1. The MUF1
The critical frequency is the highest frequency at which
a signal transmitted straight up at a 90º elevation angle is
returned to earth. The critical frequency is continuously mea­
sured in several hundred places around the world by devices
called ionosondes. At frequencies higher than the critical
frequency, all energy will travel through the ionosphere and
be lost in space (Fig 1-5). The critical frequency varies with
sunspot cycle, time of year and day, as well as geographical
location. Typical values are 9 MHz at noon and 5 MHz at
night. During periods with low sunspot activity the critical
frequency can be as low as 3 MHz. During those times we can
witness dead zones on 80 meters at night.
Fig 1-6A shows a world map with the critical frequencies
for midwinter and a low Smoothed Sunspot Number (SSN=65)
at 0000 UTC. Fig 1-6B shows the same map for midsummer
with a high Smoothed Sunspot Number (SSN=240) at 1200
UTC.
At frequencies slightly higher than the critical frequency,
refraction will occur for a relatively high wave angle and all

CH1.PMD

3

lower angles. As we increase the frequency, the maximum
elevation angle at which we have ionospheric refraction will
become lower and lower. At 30 MHz during periods of high
solar activity, such angles can be of the order of 10º and even
as low as 1º (Source: VOACAP by NTIA/ITS; see www.
uwasa.fi/~jpe/voacap/).
The relation between MUF and critical frequency is the
wave elevation angle, where:
MUF = Fcrit / sin(α)
where α = angle of elevation.
Table 1-1 gives an overview of the
multiplication factor (1/sin α, also called
secant α) for a number of elevation angles
Table 1-1
(α). For the situation where the critical
α (°)
1/sin(α)
frequency is as low as 2 MHz it can be
10
5.8
seen that any 3.8-MHz energy radiated at
20
2.9
angles higher than 30º will be lost into
30
2.0
space. This is one reason for using an 40
1.6
50
1.3
antenna with a low radiation angle for the
60
1.2
low bands.
70
1.1
The MUF is the highest frequency at
80
1.0
which reliable radio communications by 90
1.0
ionospheric propagation can be mainPropagation
1-3

2/17/2005, 2:39 PM