ndex associated with the maximum electron density, is satisfied. cosa> no (7.91) This results in no sky wave energy being returned to Earth closer to the transmitter than a range referred to as the "skip distance," as shown in Figure 7.26. For any given distance at a particular time, there is a maximum frequency that can be used, such that the signal will arrive at the receiver via the ionosphere. This fre- quency is the Maximum Usable Frequency (MUF). This frequency is also liable to be the one giving the strongest sky wave signal. The MUF is an important parameter in the propagation of short waves (i.e., frequencies from 1.6 to 30 MHz). It is given by equation 7.92. MUF= f.coseca (7.92) The maximum skip distance is about 1700 km for the E layer and 3300 km for the F layer. Transmission can also take place by two or more hops, to be received at much greater distances. Sporadic E Propagation Sometimes waves are returned to Earth by the E layer at frequencies greater than the nominal MUF. This phenomenon is called sporadic E. It is assumed that this is caused by strongly ionized "clouds" in the lower part of the E region. By this means it is possible for propagation to occur over long distances at frequencies as high as about 100 MHz. Ground Waves and Surface Waves Sunspots HF propagation is strongly influenced by sunspot activity. It can cause loss of communica- tion or permit very extensive communication. The presence of sunspots is accompanied by a large increase in ultraviolet radiation. Sunspots vary cyclically in number and size with a periodicity of about 11 years. This activity is approximately quantified by the relative Zurich sunspot number, also called the Wolf number, derived from observations of the number and size of sunspots. Prediction of Field Strengths Received via the lonosphere So far, descriptions have been given of a variety of ionospheric phenomena. This is helpful in qualitative terms, but most engineers require an ability to make quantitative predictions of the field strength likely to be 